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Biology Article
Alimentary Canal Anatomy

Alimentary Canal -Anatomy

The process of breaking down of large food particles into smaller and water-soluble particles, which can be easily absorbed by the blood plasma is termed as digestion. All parts of the body are involved in the uptake and digestion of food along with the elimination of undigested material.

prepare a presentation about various diseases related with alimentary canal

What is the Alimentary Canal?

The alimentary canal is mainly referred to as the pathway by which food enters our body and moves out through the anus after digestion. It is a tube-like structure which starts from the mouth and ends in the anus. The alimentary canal plays a primary role in human digestion and is also termed as the digestive tract.

Organs of the Alimentary Canal

The main organs of the alimentary canal are:

The Mouth and Oral cavity.
Oesophagus.
Small intestine.
Large intestine.

The structure and functions of these organs are discussed below.

Human digestive system comprises the alimentary canal and various digestive glands. The alimentary canal is a muscular tube, which extends from the mouth to the anus. The human digestive system comprises mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and anus.

Let’s learn in detail about various parts of the human digestive system.

The mouth is the first part of our digestive system. Food is ingested through the mouth.

Oral cavity

The oral cavity comprises the palate, tongue and teeth.

Palate – The roof of the oral cavity.

Tongue – Muscular and glandular structure attached to the base of the oral cavity. The upper surface of the tongue has tiny projections known as lingual papillae. Lingual papillae are of three types: circumvallate, fungiform and filiform.

Small Intestine

It is the longest part of the alimentary canal and comprises three parts- Duodenum, Jejunum, and Ileum.

Duodenum – It is C- shaped. The pancreatic, bile and hepatic secretions are added to the food by hepatopancreatic duct.
Jejunum – Middle part of the small intestine.
Ileum – It is highly coiled and opens into the large intestine.

Large Intestine

The small intestine leads into the large intestine . It has three parts- Caecum, Colon, and Rectum.

Caecum – It is a small sac-like structure containing symbiotic microorganisms. The vermiform appendix (vestigial organ) is attached to it.
Colon – It is divided into four regions- ascending, transverse, sigmoid and descending.
Rectum – It opens into the anus.

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154 23.1 Overview of the Digestive System

Learning objectives.

By the end of this section, you will be able to:

Identify the organs of the alimentary canal from proximal to distal, and briefly state their function
Identify the accessory digestive organs and briefly state their function
Describe the four fundamental tissue layers of the alimentary canal
Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning
Explain how the peritoneum anchors the digestive organs

The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process ( Figure 1 ).

This diagram shows the digestive system of a human being, with the major organs labeled.

As is the case with all body systems, the digestive system does not work in isolation; it functions cooperatively with the other systems of the body. Consider for example, the interrelationship between the digestive and cardiovascular systems. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood is diverted to the liver where its nutrients are off-loaded for processing before blood completes its circuit back to the heart. At the same time, the digestive system provides nutrients to the heart muscle and vascular tissue to support their functioning. The interrelationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism. In turn, the digestive system provides the nutrients to fuel endocrine function. Table 1 gives a quick glimpse at how these other systems contribute to the functioning of the digestive system.

Digestive System Organs

The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system.

Alimentary Canal Organs

Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”

Accessory Structures

Each accessory digestive organ aids in the breakdown of food ( Figure 2 ). Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

Histology of the Alimentary Canal

Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery (see Figure 2 ).

This image shows the cross section of the alimentary canal. The different layers of the alimentary canal are shown as concentric cylinders with major muscles and veins labeled.

The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and the lamina propria, a layer of connective tissue analogous to the dermis. In addition, the mucosa has a thin, smooth muscle layer, called the muscularis mucosa (not to be confused with the muscularis layer, described below).

Epithelium —In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs.

Lamina propria —In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it.

Muscularis mucosa —This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.

As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of dense connective tissue, it connects the overlying mucosa to the underlying muscularis. It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below.

The third layer of the alimentary canal is the muscalaris (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing and defecation. The basic two-layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube.

The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column.

Nerve Supply

As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves.

Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which runs from the esophagus to the anus, and contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system). These enteric neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility , especially the rhythm and force of the contractions of the muscularis. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food (see [link] ).

Extrinsic innervations of the alimentary canal are provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation (the fight-or-flight response) restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation (the rest-and-digest response) increases GI secretion and motility by stimulating neurons of the enteric nervous system.

Blood Supply

The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes.

Specifically, the more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta. The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines.

The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the alimentary canal viscera circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines.

The Peritoneum

The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs ( Figure 3 ). The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum.

Digestive System: Peritonitis

Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the alimentary canal is breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease (infection of uterine tubes, usually by sexually transmitted bacteria). Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy. When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate still ranges from 30 to 40 percent.

The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall. Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in Table 2 . Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal .

By clicking on this link you can watch a short video of what happens to the food you eat, as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food?

Chapter Review

The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa. The enteric nervous system provides intrinsic innervation, and the autonomic nervous system provides extrinsic innervation.

Interactive Link Questions

By clicking on this link , you can watch a short video of what happens to the food you eat as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food?

Answers may vary.

Review Questions

1. Which of these organs is not considered an accessory digestive structure?

salivary glands

2. Which of the following organs is supported by a layer of adventitia rather than serosa?

small intestine
large intestine

3. Which of the following membranes covers the stomach?

falciform ligament
parietal peritoneum
visceral peritoneum

Critical Thinking Questions

1. Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?

2. What layer of the alimentary canal tissue is capable of helping to protect the body against disease, and through what mechanism?

Answers for Review Questions

Answers for Critical Thinking Questions

The enteric nervous system helps regulate alimentary canal motility and the secretion of digestive juices, thus facilitating digestion. If a person becomes overly anxious, sympathetic innervation of the alimentary canal is stimulated, which can result in a slowing of digestive activity.
The lamina propria of the mucosa contains lymphoid tissue that makes up the MALT and responds to pathogens encountered in the alimentary canal.

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Module 7: The Digestive System

Overview of the digestive system, learning objectives.

By the end of this section, you will be able to:

Identify the organs of the alimentary canal from proximal to distal, and briefly state their function
Identify the accessory digestive organs and briefly state their function
Describe the four fundamental tissue layers of the alimentary canal
Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning
Explain how the peritoneum anchors the digestive organs

Figure 1. All digestive organs play integral roles in the life-sustaining process of digestion.

Digestive System Organs

Alimentary Canal Organs

Accessory Structures

Each accessory digestive organ aids in the breakdown of food. Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

Histology of the Alimentary Canal

Figure 2. The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.

Epithelium —In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs.
Lamina propria —In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it.
Muscularis mucosa —This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.

Nerve Supply

Blood Supply

The Peritoneum

The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs. The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum.

Figure 3. A cross-section of the abdomen shows the relationship between abdominal organs and the peritoneum (darker lines).

Disorders of the Digestive System: Peritonitis

The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall. Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in Table 2. Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal .

Practice Question

Watch this short video of what happens to the food you eat , as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food?

Chapter Review

Critical Thinking Questions

Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?
What layer of the alimentary canal tissue is capable of helping to protect the body against disease, and through what mechanism?
The enteric nervous system helps regulate alimentary canal motility and the secretion of digestive juices, thus facilitating digestion. If a person becomes overly anxious, sympathetic innervation of the alimentary canal is stimulated, which can result in a slowing of digestive activity.
The lamina propria of the mucosa contains lymphoid tissue that makes up the MALT and responds to pathogens encountered in the alimentary canal.

accessory digestive organ: includes teeth, tongue, salivary glands, gallbladder, liver, and pancreas

alimentary canal: continuous muscular digestive tube that extends from the mouth to the anus

motility: movement of food through the GI tract

mucosa: innermost lining of the alimentary canal

muscularis: muscle (skeletal or smooth) layer of the alimentary canal wall

myenteric plexus: (plexus of Auerbach) major nerve supply to alimentary canal wall; controls motility

retroperitoneal: located posterior to the peritoneum

serosa: outermost layer of the alimentary canal wall present in regions within the abdominal cavity

submucosa: layer of dense connective tissue in the alimentary canal wall that binds the overlying mucosa to the underlying muscularis

submucosal plexus: (plexus of Meissner) nerve supply that regulates activity of glands and smooth muscle

Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

Introduction to the Digestive System

The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract , consists of a long continuous tube that extends from the mouth to the anus . It includes the mouth, pharynx , esophagus , stomach , small intestine , and large intestine . The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver , gallbladder , and pancreas are major accessory organs that have a role in digestion . These organs secrete fluids into the digestive tract.

Food undergoes three types of processes in the body:

Elimination

Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism .

The digestive system prepares nutrients for utilization by body cells through six activities, or functions.

The first activity of the digestive system is to take in food through the mouth. This process, called ingestion , has to take place before anything else can happen.

Mechanical Digestion

The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes . This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.

Chemical Digestion

The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical digestion, through a process called hydrolysis , uses water and digestive enzymes to break down the complex molecules. Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.

After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction . These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids. The movements that propel the food particles through the digestive tract are called peristalsis . These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.

The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries . This process is called absorption.

The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces , is defecation or elimination.

23.2 Digestive System Processes and Regulation

Learning objectives.

By the end of this section, you will be able to:

Discuss six fundamental activities of the digestive system, giving an example of each
Compare and contrast the neural and hormonal controls involved in digestion

The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Table 23.3 provides an overview of the basic functions of the digestive organs.

Interactive Link

Visit this site for an overview of digestion of food in different regions of the digestive tract. Note the route of non-fat nutrients from the small intestine to their release as nutrients to the body.

Digestive Processes

The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.

The first of these processes, ingestion , refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.

Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion , which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along ( Figure 23.5 ). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication , or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme . Segmentation , which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.

In chemical digestion , starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine.

Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption , which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream (the subclavian veins near the heart). The details of these processes will be discussed later.

In defecation , the final step in digestion, undigested materials are removed from the body as feces.

Aging and the...

Digestive system: from appetite suppression to constipation.

Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system. Taste buds become less sensitive, so food isn’t as appetizing as it once was. A slice of pizza is a challenge, not a treat, when you have lost teeth, your gums are diseased, and your salivary glands aren’t producing enough saliva. Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue. Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones.

Pathologies that affect the digestive organs—such as hiatal hernia, gastritis, and peptic ulcer disease—can occur at greater frequencies as you age. Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include jaundice, acute pancreatitis, cirrhosis, and gallstones.

In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal ( Figure 23.6 ).

Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

Regulatory Mechanisms

Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically.

Neural Controls

The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions. These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively. For example, these receptors can sense when the presence of food has caused the stomach to expand, whether food particles have been sufficiently broken down, how much liquid is present, and the type of nutrients in the food (lipids, carbohydrates, and/or proteins). Stimulation of these receptors provokes an appropriate reflex that furthers the process of digestion. This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract.

The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones. These interactions prompt several types of reflexes. Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system. Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall. These two plexuses and their connections were introduced earlier as the enteric nervous system. Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions. For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata. The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food. In contrast, food that distends the stomach initiates short reflexes that cause cells in the stomach wall to increase their secretion of digestive juices.

Hormonal Controls

A variety of hormones are involved in the digestive process. The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa. Other GI hormones are produced and act upon the gut and its accessory organs. Hormones produced by the duodenum include secretin, which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin (CCK), which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits gastric secretion and slows gastric emptying and motility. These GI hormones are secreted by specialized epithelial cells, called endocrinocytes, located in the mucosal epithelium of the stomach and small intestine. These hormones then enter the bloodstream, through which they can reach their target organs.

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Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
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Book title: Anatomy and Physiology 2e
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20.2: Histology of the Alimentary Canal

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Rosanna Hartline
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Histology of the Alimentary Canal

The organs of the GI tract are made from four layers, the inner lining or mucosa , the submucosa containing blood vessels and lymphatics, the muscularis externa or smooth muscle layer, and the outermost layer or serosa/adventitia . Each tissue layer plays a vital role in the digestive system ranging in their capacity to form a protective barrier from the highly acidic contents of the stomach to supplying hormones, producing muscular contractions and draining lymph. For example, specialized cells found the in epithelium lining the inside of the stomach are supporting cells which produce a protective layer of mucus and gastric acid for digestion. Additionally, other supporting cells, in the stomach and the acinar cells of the pancreas, release zymogens , inactive forms of digestive enzymes that become triggered to become active digestive enzymes.

Above: Tissue layers of the organs of the alimentary canal.

MALT = mucosa-associated lymphatic tissue; immune functions

lacteal = fat-absorbing lymphatic vessels in the small intestine

Above: The major diagnostic features of the components of the tubular digestive tract are highlighted in these drawings. All images show the tubular digestive organs cut in cross section, except for the small intestine illustrated in the two smaller panels located in the lower left quadrant. These small intestine sections depict longitudinal sections of this organ. Mucosae are pink, submucosae are orange, muscularis externa are red, and serosa / adventitia are blue-gray.

The esophagus is lined by stratified squamous moist epithelium. Scattered glands are present in lamina propria of the upper and lower esophagus and in the submucosa. Muscularis externa begins as skeletal muscle, transitions to skeletal plus smooth muscle and finally to only smooth muscle in the lower portion. An adventitia surrounds the organ.

The stomach is characterized by a thick mucosa lined by a surface sheet gland. The lamina propria is filled with gastric glands, each opening into a gastric pit. The muscularis externa is very thick with three, rather than the usual two, subdivisions. The stomach is covered by a serosa.

Villi are prominent features of small intestine. Intestinal glands open at the bases of villi and extend to muscularis mucosae. Glands in submucosa distinguish duodenum from the remainder of small intestine. Most of the organ is covered by a serosa, although a portion of duodenum is retroperitoneal and possesses an adventitia.

The large intestine (colon) lacks villi and possesses very straight intestinal glands in lamina propria. No submucosal glands are present. Most of the outer longitudinal subdivision of muscularis externa is relegated to three longitudinal strips termed the taeniae coli. The large intestine is covered by either an adventitia or a serosa, depending on its location.

Attributions

"Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs , Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0
"Human Anatomy Lab Manual" by Malgosia Wilk-Blaszczak , Mavs Open Press , University of Texas at Arlington is licensed under CC BY 4.0
"Mucosa.jpg" by National Cancer Institute is in the Public Domain
"Peristalsis.gif" by Auawise is licensed under CC BY-SA 4.0

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

146 The Alimentary Canal

The mouth receives and mechanically breaks down food, produces saliva, and is the first portion of the alimentary canal.

Learning Objectives

Describe the features of the mouth that play a role in digestion

Key Takeaways

The mouth is also known as the oral cavity. Its purpose is to mechanically break down food, moisten it with saliva, and swallow the food into the esophagus and the stomach.
While vocal sounds are primarily produced in the throat, the tongue, lips, and jaw are also needed to produce the range of sounds included in human language.
Saliva is produced by three main pairs of salivary glands: the parotid, the submandibular, and sublingual. When food is chewed and mixed with this saliva, the resulting wad is known as a bolus.
mastication : The process of physical and mechanical breakdown of food; chewing.
mucous membrane : A membrane that secretes mucus. It forms the lining of various body passages that communicate with the air, such as the respiratory, genitourinary, and alimentary tracts.
mouth : The opening of a organism through which food is ingested.
saliva : A clear, slightly alkaline liquid secreted into the mouth by the salivary glands and mucous glands that consists of water, mucin, protein, and enzymes. It moistens the mouth, lubricates ingested food, and begins the breakdown of starches.
uvula : A soft, punching-bag-like piece of tissue that hangs at the back of the mouth and functions in closing the air passages during swallowing, in conjunction with the epiglottis of the trachea.
hard palate : The bony roof of the mouth, located ventrally to the soft palate.
alimentary canal : The organs of a human or a non-human animal through which food passes.
alveolar arch : The part of the upper or lower jawbones in which the teeth are set.

The mouth has a variety of roles in human anatomy and sociology. While its primary function is to begin the process of mechanically and chemically digesting food, the mouth is also the beginning of the alimentary canal—a larger digestive tube. Without the human mouth, expressions of the lips and language of the tongue and throat would be impossible.

The mouth is the first portion of the alimentary canal. It receives food and moistens the food with saliva, while the food is mechanically processed (mastication) by the teeth. The mouth is also known as the oral cavity, and within the oral cavity sits the tongue, the soft and hard palate, the uvula, and numerous salivary glands.

The oral mucosa is the mucous membrane epithelial tissue that lines the inside of the mouth. This membrane maintains a moist and lubricated environment within the mouth to prepare the digestive system for the entry of food.

The Mouth as a Communication and Breathing Tool

This is an illustration of the inside of a human mouth. The cheeks have been omitted in the drawing and the lips pulled back for an unobstructed view of the teeth, tongue, jaw bones, uvula, and alimentary canal.

Inside of the mouth : An illustration of the inside of a human mouth. The cheeks have been omitted in the drawing and the lips pulled back for an unobstructed view of the teeth, tongue, jaw bones, uvula, and alimentary canal.

In addition to its primary function as the beginning of the digestive system, the mouth also plays a significant role in human communication and breathing. The primary features of human voice are produced in the throat, but the tongue, lips, and jaw also work together to produce the range of sounds we see in human language.

Air is drawn in through the mouth to the trachea and lungs, and the lips and tongue form words. The lips mark the transition from the mucous membrane to the outer epithelial skin that covers most of the body. Lips are remarkably sensitive and often serve as an infant’s second hands with which to explore the world.

Mechanical Food Breakdown by Teeth

In the digestive process, the mouth’s purpose is to prepare food for further digestion in the stomach and the small intestine. This process begins with the mechanical breakdown of food by the teeth, which fit into the alveolar arches. The front teeth (incisors and canines) are used to cut and tear food, while the teeth further back (bicuspids and molars) crush and grind.

Food Lubrication and Chemical Digestion By Saliva

Saliva is projected from three main pairs of salivary glands: the large parotid glands near the cheeks, the submandibular glands beneath the mandible, and the sublingual glands beneath the tongue.

Saliva keeps the mouth moist and lubricates the food, helping the tongue form the food into a soft wad, called a bolus. The fluid of saliva also contains several enzymes, notably lysozyme—an antibacterial agent—and amylase, which catalyzes large starch molecules into simpler sugars via hydrolysis.

This is a cross-section drawing of the head and neck in mid-sagittal view. It shows the structures of the mouth and throat. The lips, jaw, nasal cavity, palate, tongue, oral cavity, pharyhnix, epiglotis, larynx opening into pharynx, larynx, and esophagus are labeled.

Cross section of the head and neck : A cross section of the head and neck in mid-sagittal view, showing the structures of the mouth and throat.

Once properly chewed and lubricated, food and drink are swallowed into the esophagus, the tube that leads to the stomach.

The Structures of the Lips and External Mouth

Infant humans are born with an instinctual sucking reflex, by which they know how to gain nourishment using their lips and jaw. The philtrum, or bow of the lip, is the vertical groove or dip just below the nose.

The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.

The pharynx is part of the digestive and respiratory systems and consists of three main parts: the nasopharynx, oropharynx, and laryngopharynx.

Describe the pharynx’s role in digestion

The human pharynx (plural: pharynges) is the part of the throat situated immediately inferior to (below) the mouth and nasal cavity, and superior to the esophagus and larynx.

The human pharynx is conventionally divided into three sections: the nasopharynx (epipharynx), the oropharynx (mesopharynx), and the laryngopharynx (hypopharynx).
The nasopharynx extends from the base of the skull to the upper surface of the soft palate. Adenoids are lymphoid tissue structures located in the posterior wall of the nasopharynx. The nasopharynx communicates with the middle ear, nasal cavities, and auditory tube.
The oropharynx lies behind the oral cavity and extends from the uvula to the level of the hyoid bone. A flap of connective tissue called the epiglottis closes over the glottis when food is swallowed to prevent aspiration.
The laryngopharynx is the caudal part of the pharynx; it is the part of the throat that connects to the esophagus. It includes three major sites: the pyriform sinus, the postcricoid area, and the posterior pharyngeal wall.
adenoid : One of the two folds of lymphatic tissue covered by ciliated epithelium. They are found in the roof and posterior wall of the nasopharynx at the back of the throat behind the uvula. They may obstruct normal breathing and make speech difficult when swollen—a condition often called adenitis.
epiglottis : A cartilaginous organ in the throat of terrestrial vertebrates that covers the glottis when swallowing to prevent food and liquid from entering the trachea. In Homo sapiens it is also a speech organ.
uvula : The fleshy appendage that hangs from the back of the palate and closes the nasopharynx during swallowing.
pharynx : The part of the alimentary canal that extends from the mouth and nasal cavities to the larynx, where it becomes continuous with the esophagus.

This is a drawing of an overview of the head and neck. The human pharynx is seen situated immediately below the mouth and nasal cavity, and above the esophagus and larynx.

Head and neck overview : The human pharynx is situated immediately below the mouth and nasal cavity, and above the esophagus and larynx.

The human pharynx is conventionally divided into three sections: the nasopharynx (epipharynx), the oropharynx (mesopharynx), and the laryngopharynx (hypopharynx). The pharynx is part of the digestive system and also the respiratory system, as well as an important part in vocalization.

Nasopharynx

The nasopharynx is the most cephalad (toward the head) portion of the pharynx. It extends from the base of the skull to the upper surface of the soft palate. It includes the space between the internal nares and the soft palate, and lies superior to the oral cavity.

The pharyngeal tonsils, more commonly referred to as the adenoids, are lymphoid tissue structures located in the posterior wall of the nasopharynx. Polyps or mucus can obstruct the nasopharynx, as can congestion due to an upper respiratory infection.

The eustachian tubes connect the middle ear to the pharynx and open into the nasopharynx. The opening and closing of the eustachian tubes serves to equalize the barometric pressure in the middle ear with that of the ambient atmosphere.

The anterior portion of the nasopharynx connects with the nasal cavities through openings known as choanae. The nasopharynx and its associated nasal tissues are lined with ciliated pseudostratified columnar epithelium, which is excellent for sweeping debris from the nasal passages.

The Pharyngeal Ostia

On the lateral walls of the nasopharynx are the pharyngeal ostia of the auditory tube—triangle-shaped openings bound from behind by a firm prominence called the torus tubarius or cushion.

This binding is formed by a cartilaginous tube-like opening. Two folds arise from the cartilaginous opening:

The salpingopharyngeal fold, a vertical fold of mucous membrane that extends from the inferior part of the torus.
The salpingopalatine fold, a smaller fold that extends from the superior part of the torus to the palate.

Behind the ostia of the auditory tube is a deep recess known as the pharyngeal recess (or fossa of Rosenmüller).

The posterior wall of the nasopharynx holds the pharyngeal tonsils, which can be especially marked in childhood. Superior to the pharyngeal tonsil, in the midline, an irregular flask-shaped depression of the mucous membrane sometimes extends upward; it is known as the pharyngeal bursa.

The oropharynx or mesopharynx lies behind the oral cavity and extends from the uvula to the level of the hyoid bone. It opens anteriorly, through the isthmus faucium, into the mouth, and contains the palatine tonsil—another grouping of adenoid tissue.

The anterior wall consists of the base of the tongue and the the epiglottis tissue. The lateral walls are made up of the tonsil and associated tonsilar tissues. The superior wall consists of the inferior surface of the soft palate and the uvula.

Because both food and air pass through the pharynx, a flap of connective tissue called the epiglottis closes over the glottis (tracheal opening) when food is swallowed to prevent accidental inhalation. The oropharynx is lined by non-keratinized stratified squamous epithelium.

Laryngopharynx

The hypopharynx or laryngopharynx is the caudal (most inferior) part of the pharynx; it is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to the location where this common pathway diverges into the respiratory (larynx) and digestive (esophagus) pathways.

At that point, the laryngopharynx is continuous with the esophagus posteriorly. The esophagus conducts food and fluids to the stomach; air enters the larynx anteriorly. During swallowing, food has the right of way and air passage temporarily stops.

The laryngopharynx includes three major sites:

The pyriform sinus.
The postcricoid area.
The posterior pharyngeal wall.

Like the oropharynx above it, the laryngopharynx serves as a passageway for food and air and is lined with a stratified squamous epithelium.

The esophagus is a muscular tube that moves food from the pharynx to the stomach via peristalsis.

Describe the role of the esophagus in digestion

The esophagus is the muscular tube that moves food material from the pharynx to the stomach via waves of muscle movement known as peristalsis. The junction between the esophagus and the stomach is known as the gastroesophageal junction or GE junction.
The entry to the esophagus opens only when swallowing or vomiting due to specialized muscles that control the opening.
esophagus : The esophagus is an organ in vertebrates that consists of a muscular tube through which food passes from the pharynx to the stomach.
peristalsis : The rhythmic, wave-like contraction of both longitudinal and circular smooth muscle fibers within the digestive tract that forces food through it.
mucus : A slippery secretion from the lining of the mucous membranes.

Swallowing is a voluntary act that utilizes the muscles of the mouth and tongue to push food into the esophagus. Once food material is pushed into the throat, or pharynx, the trachea (windpipe) is blocked by a flap of tissue known as the epiglottis to prevent the aspiration of food. Food then moves down the esophageal tube through waves of muscle movement, or peristalsis, until it reaches the stomach.

The esophagus is an organ in vertebrates that consists of a muscular tube through which food passes from the pharynx to the stomach. It is a major component of the upper digestive system.

The major organs of the human gastrointestinal system are identified in this drawing. The upper gastrointestinal tract consists of the esophagus, stomach, and duodenum. The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. According to some sources, it also includes the anus.

The word esophagus is derived from the Latin œsophagus, which derives from the Greek word oisophagos, meaning entrance for eating. It is lined with mucus to aid in the passage of food.

Length and Location

In humans the esophagus is continuous with the laryngeal part of the pharynx within the neck, and it passes through the thorax diaphragm and into abdomen to reach the cardiac orifice of the stomach. It is usually about 10–50 cm long depending on an individual’s height. Due to the inferior pharyngeal constrictor muscle, the entry to the esophagus opens only when swallowing or vomiting.

Layers of Tissue

The esophageal tube in humans is comprised of two main layers of smooth muscle, though striated muscle comprises the tube near the pharynx. This combination of muscle tissue allows peristalsis to push food downward, and aids in regurgitation at the pharynx.

The innermost layer of smooth muscle is arranged in a series of concentric rings, while the outermost layer is arranged longitudinally.

In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus) from the pharynx to the stomach.

The Gastroesophageal Junction

The junction between the esophagus and the stomach (the gastroesophageal junction or GE junction) is not actually considered a valve in the anatomical sense, although it is sometimes called the cardiac sphincter.

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23.6 The Small and Large Intestines

Learning objectives.

By the end of this section, you will be able to:

Describe the functional anatomy of the small and large intestines
Identify three main adaptations of the small intestine wall that increase its absorptive capacity
Describe the mechanical and chemical digestion of chyme upon its release into the small intestine
Describe any absorption that happens in the small and large intestines
List three features unique to the wall of the large intestine and identify their contributions to its function
Identify the beneficial roles of the bacterial flora in digestive system functioning
Trace the pathway of food waste from its point of entry into the large intestine through its exit from the body as feces

The word intestine is derived from a Latin root meaning “internal,” and indeed, the two organs together nearly fill the interior of the abdominal cavity. In addition, called the small and large bowel, or colloquially the “guts,” they constitute the greatest mass and length of the alimentary canal and, with the exception of ingestion, perform all digestive system functions.

The Small Intestine

Chyme released from the stomach enters the small intestine , which is the primary digestive organ in the body. Not only is this where most digestion occurs, it is also where practically all absorption occurs. The longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Since this makes it about five times longer than the large intestine, you might wonder why it is called “small.” In fact, its name derives from its relatively smaller diameter of only about 2.54 cm (1 in), compared with 7.62 cm (3 in) for the large intestine. As we’ll see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m 2 , more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it.

The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum ( Figure 23.6.1 ).

The shortest region is the 25.4-cm (10-in) duodenum , which begins at the pyloric sphincter. Just past the pyloric sphincter, it bends posteriorly behind the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve around the head of the pancreas before ascending anteriorly again to return to the peritoneal cavity and join the jejunum. The duodenum can therefore be subdivided into four segments: the superior, descending, horizontal, and ascending duodenum.

Of particular interest is the hepatopancreatic ampulla (ampulla of Vater). Located in the duodenal wall, the ampulla marks the transition from the anterior portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the main pancreatic duct (through which pancreatic juice passes from the pancreas) join. This ampulla opens into the duodenum at a tiny volcano-shaped structure called the major duodenal papilla . The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum.

This diagram shows the small intestine. The different parts of the small intestine are labeled.

The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum. Jejunum means “empty” in Latin and supposedly was so named by the ancient Greeks who noticed it was always empty at death. No clear demarcation exists between the jejunum and the final segment of the small intestine, the ileum.

The ileum is the longest part of the small intestine, measuring about 1.8 meters (6 feet) in length. It is thicker, more vascular, and has more developed mucosal folds than the jejunum. The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve). The jejunum and ileum are tethered to the posterior abdominal wall by the mesentery. The large intestine frames these three parts of the small intestine.

Parasympathetic nerve fibers from the vagus nerve and sympathetic nerve fibers from the thoracic splanchnic nerve provide extrinsic innervation to the small intestine. The superior mesenteric artery is its main arterial supply. Veins run parallel to the arteries and drain into the superior mesenteric vein. Nutrient-rich blood from the small intestine is then carried to the liver via the hepatic portal vein.

The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli ( Figure 23.6.2 ). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs.

Illustration (a) shows the histological cross-section of the small intestine. The left panel shows a small region of the small intestine, along with the blood vessels and the muscle layers. The middle panel shows a magnified view of a small region of the small intestine, highlighting the absorptive cells, the lacteal and the goblet cells. The right panel shows a further magnified view of the epithelial cells including the microvilli. Illustrations (b) shows a micrograph of the circular folds, and illustration (c) shows a micrograph of the villi. Illustration (d) shows an electron micrograph of the microvilli.

Circular folds

Also called plica circulares, or circular folds, are deep ridges in the mucosa and submucosa. Beginning near the proximal part of the duodenum and ending near the middle of the ileum, these folds facilitate absorption. Their shape causes the chyme to spiral, rather than move in a straight line, through the small intestine. Spiraling slows the movement of chyme and provides the time needed for nutrients to be fully absorbed.

Within the circular folds are small (0.5–1 mm long) hairlike vascularized projections called villi (singular = villus) that give the mucosa a furry texture. There are about 20 to 40 villi per square millimeter, increasing the surface area of the epithelium tremendously. The mucosal epithelium, primarily composed of absorptive cells, covers the villi. In addition to muscle and connective tissue to support its structure, each villus contains a capillary bed composed of one arteriole and one venule, as well as a lymphatic capillary called a lacteal . The breakdown products of carbohydrates and proteins (sugars and amino acids) can enter the bloodstream directly, but lipid breakdown products are absorbed by the lacteals and transported to the bloodstream via the lymphatic system.

As their name suggests, microvilli (singular = microvillus) are much smaller (1 µ m) than villi. They are cylindrical apical surface extensions of the plasma membrane of the mucosa’s epithelial cells, and are supported by microfilaments within those cells. Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border . Fixed to the surface of the microvilli membranes are enzymes that finish digesting carbohydrates and proteins. There are an estimated 200 million microvilli per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing absorption.

Intestinal Glands

In addition to the three specialized absorptive features just discussed, the mucosa between the villi is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkühn), which is formed by cells that line the crevices (see Figure 23.6.2 ). These produce intestinal juice , a slightly alkaline (pH 7.4 to 7.8) mixture of water and mucus. Each day, about 0.95 to 1.9 liters (1 to 2 quarts) are secreted in response to the distention of the small intestine or the irritating effects of chyme on the intestinal mucosa.

The submucosa of the duodenum is the only site of the complex mucus-secreting duodenal glands (Brunner’s glands), which produce a bicarbonate-rich alkaline mucus that buffers the acidic chyme as it enters from the stomach.

The roles of the cells in the small intestinal mucosa are detailed in Table 23.7 .

Intestinal MALT

The lamina propria of the small intestine mucosa is studded with quite a bit of MALT. In addition to solitary lymphatic nodules, aggregations of intestinal MALT, which are typically referred to as Peyer’s patches, are concentrated in the distal ileum, and serve to keep bacteria from entering the bloodstream. Peyer’s patches are most prominent in young people and become less distinct as you age, which coincides with the general activity of our immune system.

External Website

Watch this animation that depicts the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats?

Mechanical Digestion in the Small Intestine

The movement of intestinal smooth muscles includes both segmentation and a form of peristalsis called migrating motility complexes. The kind of peristaltic mixing waves seen in the stomach are not observed here.

If you could see into the small intestine when it was going through segmentation, it would look as if the contents were being shoved incrementally back and forth, as the rings of smooth muscle repeatedly contract and then relax. Segmentation in the small intestine does not force chyme through the tract. Instead, it combines the chyme with digestive juices and pushes food particles against the mucosa to be absorbed. The duodenum is where the most rapid segmentation occurs, at a rate of about 12 times per minute. In the ileum, segmentations are only about eight times per minute ( Figure 23.6.3 ).

This diagram shows the process of segmentation in the intestines. The left panel shows the separation of chime, the middle panel shows the remixing of the chime by pushing it back together and the right panel indicates that the chime is being digested and absorbed.

When most of the chyme has been absorbed, the small intestinal wall becomes less distended. At this point, the localized segmentation process is replaced by transport movements. The duodenal mucosa secretes the hormone motilin , which initiates peristalsis in the form of a migrating motility complex . These complexes, which begin in the duodenum, force chyme through a short section of the small intestine and then stop. The next contraction begins a little bit farther down than the first, forces chyme a bit farther through the small intestine, then stops. These complexes move slowly down the small intestine, forcing chyme on the way, taking around 90 to 120 minutes to finally reach the end of the ileum. At this point, the process is repeated, starting in the duodenum.

The ileocecal valve, a sphincter, is usually in a constricted state, but when motility in the ileum increases, this sphincter relaxes, allowing food residue to enter the first portion of the large intestine, the cecum. Relaxation of the ileocecal sphincter is controlled by both nerves and hormones. First, digestive activity in the stomach provokes the gastroileal reflex , which increases the force of ileal segmentation. Second, the stomach releases the hormone gastrin, which enhances ileal motility, thus relaxing the ileocecal sphincter. After chyme passes through, backward pressure helps close the sphincter, preventing backflow into the ileum. Because of this reflex, your lunch is completely emptied from your stomach and small intestine by the time you eat your dinner. It takes about 3 to 5 hours for all chyme to leave the small intestine.

Chemical Digestion in the Small Intestine

The digestion of proteins and carbohydrates, which partially occurs in the stomach, is completed in the small intestine with the aid of intestinal and pancreatic juices. Lipids arrive in the intestine largely undigested, so much of the focus here is on lipid digestion, which is facilitated by bile and the enzyme pancreatic lipase.

Moreover, intestinal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. The intestine is also where most water is absorbed, via osmosis. The small intestine’s absorptive cells also synthesize digestive enzymes and then place them in the plasma membranes of the microvilli. This distinguishes the small intestine from the stomach; that is, enzymatic digestion occurs not only in the lumen, but also on the luminal surfaces of the mucosal cells.

For optimal chemical digestion, chyme must be delivered from the stomach slowly and in small amounts. This is because chyme from the stomach is typically hypertonic, and if large quantities were forced all at once into the small intestine, the resulting osmotic water loss from the blood into the intestinal lumen would result in potentially life-threatening low blood volume. In addition, continued digestion requires an upward adjustment of the low pH of stomach chyme, along with rigorous mixing of the chyme with bile and pancreatic juices. Both processes take time, so the pumping action of the pylorus must be carefully controlled to prevent the duodenum from being overwhelmed with chyme.

Lactose intolerance is a condition characterized by indigestion caused by dairy products. It occurs when the absorptive cells of the small intestine do not produce enough lactase, the enzyme that digests the milk sugar lactose. In most mammals, lactose intolerance increases with age. In contrast, some human populations, most notably Caucasians, are able to maintain the ability to produce lactase as adults.

In people with lactose intolerance, the lactose in chyme is not digested. Bacteria in the large intestine ferment the undigested lactose, a process that produces gas. In addition to gas, symptoms include abdominal cramps, bloating, and diarrhea. Symptom severity ranges from mild discomfort to severe pain; however, symptoms resolve once the lactose is eliminated in feces.

The hydrogen breath test is used to help diagnose lactose intolerance. Lactose-tolerant people have very little hydrogen in their breath. Those with lactose intolerance exhale hydrogen, which is one of the gases produced by the bacterial fermentation of lactose in the colon. After the hydrogen is absorbed from the intestine, it is transported through blood vessels into the lungs. There are a number of lactose-free dairy products available in grocery stores. In addition, dietary supplements are available. Taken with food, they provide lactase to help digest lactose.

The Large Intestine

The large intestine is the terminal part of the alimentary canal. The primary function of this organ is to finish absorption of nutrients and water, synthesize certain vitamins, as well as to form, store, and eliminate feces from the body.

The large intestine runs from the appendix to the anus. It frames the small intestine on three sides. Despite its being about one-half as long as the small intestine, it is called large because it is more than twice the diameter of the small intestine, about 3 inches.

Subdivisions

The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve, located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine.

The first part of the large intestine is the cecum , a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial. However, at least one recent report postulates a survival advantage conferred by the appendix: In diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric bacteria for those surviving the initial phases of the illness. Moreover, its twisted anatomy provides a haven for the accumulation and multiplication of enteric bacteria. The mesoappendix , the mesentery of the appendix, tethers it to the mesentery of the ileum.

The cecum blends seamlessly with the colon . Upon entering the colon, the food residue first travels up the ascending colon on the right side of the abdomen. At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon . The region defined as hindgut begins with the last third of the transverse colon and continues on. Food residue passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen, at the left colic flexure (splenic flexure). From there, food residue passes through the descending colon , which runs down the left side of the posterior abdominal wall. After entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon , which extends medially to the midline ( Figure 23.6.4 ). The ascending and descending colon, and the rectum (discussed next) are located in the retroperitoneum. The transverse and sigmoid colon are tethered to the posterior abdominal wall by the mesocolon.

This image shows the large intestine; the major parts of the large intestine are labeled.

Each year, approximately 140,000 Americans are diagnosed with colorectal cancer, and another 49,000 die from it, making it one of the most deadly malignancies. People with a family history of colorectal cancer are at increased risk. Smoking, excessive alcohol consumption, and a diet high in animal fat and protein also increase the risk. Despite popular opinion to the contrary, studies support the conclusion that dietary fiber and calcium do not reduce the risk of colorectal cancer.

Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Bleeding from the rectum may be either obvious or occult (hidden in feces). Since most colon cancers arise from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy not only allows identification of precancerous polyps, the procedure also enables them to be removed before they become malignant. Screening for fecal occult blood tests and colonoscopy is recommended for those over 50 years of age.

Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. Even though rectum is Latin for “straight,” this structure follows the curved contour of the sacrum and has three lateral bends that create a trio of internal transverse folds called the rectal valves . These valves help separate the feces from gas to prevent the simultaneous passage of feces and gas.

Finally, food residue reaches the last part of the large intestine, the anal canal , which is located in the perineum, completely outside of the abdominopelvic cavity. This 3.8–5 cm (1.5–2 in) long structure opens to the exterior of the body at the anus. The anal canal includes two sphincters. The internal anal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control. Except when defecating, both usually remain closed.

There are several notable differences between the walls of the large and small intestines ( Figure 23.6.5 ). For example, few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria.

This image shows the histological cross section of the large intestine. The left panel shows a small region of the large intestine. The center panel shows a magnified view of this region, highlighting the openings of the intestinal glands. The right panel shows a further magnified view, with the microvilli and goblet cells.

Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages ( Figure 23.6.6 ). The teniae coli are three bands of smooth muscle that make up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end. Tonic contractions of the teniae coli bunch up the colon into a succession of pouches called haustra (singular = hostrum), which are responsible for the wrinkled appearance of the colon. Attached to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages . The purpose of these is unknown. Although the rectum and anal canal have neither teniae coli nor haustra, they do have well-developed layers of muscularis that create the strong contractions needed for defecation.

This image shows the Taenia Coli, haustra and epiploic appendages, which are parts of the large intestine.

The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. The anal canal’s mucous membrane is organized into longitudinal folds, each called an anal column , which house a grid of arteries and veins. Two superficial venous plexuses are found in the anal canal: one within the anal columns and one at the anus.

Depressions between the anal columns, each called an anal sinus , secrete mucus that facilitates defecation. The pectinate line (or dentate line) is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin. The mucosa above this line is fairly insensitive, whereas the area below is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated by somatic sensory fibers.

Bacterial Flora

Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora . Most of the more than 700 species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Some are linked to increased immune response. A refined system prevents these bacteria from crossing the mucosal barrier. First, peptidoglycan, a component of bacterial cell walls, activates the release of chemicals by the mucosa’s epithelial cells, which draft immune cells, especially dendritic cells, into the mucosa. Dendritic cells open the tight junctions between epithelial cells and extend probes into the lumen to evaluate the microbial antigens. The dendritic cells with antigens then travel to neighboring lymphoid follicles in the mucosa where T cells inspect for antigens. This process triggers an IgA-mediated response, if warranted, in the lumen that blocks the commensal organisms from infiltrating the mucosa and setting off a far greater, widespread systematic reaction.

Digestive Functions of the Large Intestine

The residue of chyme that enters the large intestine contains few nutrients except water, which is reabsorbed as the residue lingers in the large intestine, typically for 12 to 24 hours. Thus, it may not surprise you that the large intestine can be completely removed without significantly affecting digestive functioning. For example, in severe cases of inflammatory bowel disease, the large intestine can be removed by a procedure known as a colectomy. Often, a new fecal pouch can be crafted from the small intestine and sutured to the anus, but if not, an ileostomy can be created by bringing the distal ileum through the abdominal wall, allowing the watery chyme to be collected in a bag-like adhesive appliance.

Mechanical Digestion

In the large intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activity regulated by the ileocecal sphincter. Right after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen. Once chyme enters the cecum, colon movements begin.

Mechanical digestion in the large intestine includes a combination of three types of movements. The presence of food residues in the colon stimulates a slow-moving haustral contraction . This type of movement involves sluggish segmentation, primarily in the transverse and descending colons. When a haustrum is distended with chyme, its muscle contracts, pushing the residue into the next haustrum. These contractions occur about every 30 minutes, and each last about 1 minute. These movements also mix the food residue, which helps the large intestine absorb water. The second type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The third type is a mass movement . These strong waves start midway through the transverse colon and quickly force the contents toward the rectum. Mass movements usually occur three or four times per day, either while you eat or immediately afterward. Distension in the stomach and the breakdown products of digestion in the small intestine provoke the gastrocolic reflex , which increases motility, including mass movements, in the colon. Fiber in the diet both softens the stool and increases the power of colonic contractions, optimizing the activities of the colon.

Chemical Digestion

Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation , bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus (gas) in the colon; flatulence is excessive flatus. Each day, up to 1500 mL of flatus is produced in the colon. More is produced when you eat foods such as beans, which are rich in otherwise indigestible sugars and complex carbohydrates like soluble dietary fiber.

Absorption, Feces Formation, and Defecation

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces (“stool”). The large intestine also absorbs B vitamins, vitamin K, and sodium under the influence of the hormone aldosterone. Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Of every 500 mL (17 ounces) of food residue that enters the cecum each day, about 150 mL (5 ounces) become feces.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva’s maneuver , in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.

The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate.

If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can be caused by the ingestion of foodborne pathogens. In general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to three or four per week.

By watching this animation you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?

Chapter Review

The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including circular folds, villi, and microvilli. There are around 200 million microvilli per square millimeter of small intestine, which contain brush border enzymes that complete the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme back and forth, increasing mixing and opportunities for absorption. Migrating motility complexes propel the residual chyme toward the large intestine.

The main regions of the large intestine are the cecum, the colon, and the rectum. The large intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break down additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex.

Interactive Link Questions

Answers may vary.

By watching this animation , you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?

Review Questions

Critical thinking questions.

1. Explain how nutrients absorbed in the small intestine pass into the general circulation.

2. Why is it important that chyme from the stomach is delivered to the small intestine slowly and in small amounts?

3. Describe three of the differences between the walls of the large and small intestines.

American Cancer Society (US). Cancer facts and figures: colorectal cancer: 2011–2013 [Internet]. c2013 [cited 2013 Apr 3]. Available from: http://www.cancer.org/Research/CancerFactsFigures/ColorectalCancerFactsFigures/colorectal-cancer-facts-figures-2011-2013-page .

The Nutrition Source. Fiber and colon cancer: following the scientific trail [Internet]. Boston (MA): Harvard School of Public Health; c2012 [cited 2013 Apr 3]. Available from: http://www.hsph.harvard.edu/nutritionsource/nutrition-news/fiber-and-colon-cancer/index.html .

Centers for Disease Control and Prevention (US). Morbidity and mortality weekly report: notifiable diseases and mortality tables [Internet]. Atlanta (GA); [cited 2013 Apr 3]. Available from: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6101md.htm?s_cid=mm6101md_w .

Answers for Critical Thinking Questions

Nutrients from the breakdown of carbohydrates and proteins are absorbed through a capillary bed in the villi of the small intestine. Lipid breakdown products are absorbed into a lacteal in the villi, and transported via the lymphatic system to the bloodstream.
If large quantities of chyme were forced into the small intestine, it would result in osmotic water loss from the blood into the intestinal lumen that could cause potentially life-threatening low blood volume and erosion of the duodenum.
The mucosa of the small intestine includes circular folds, villi, and microvilli. The wall of the large intestine has a thick mucosal layer, and deeper and more abundant mucus-secreting glands that facilitate the smooth passage of feces. There are three features that are unique to the large intestine: teniae coli, haustra, and epiploic appendages.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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22.4: The Alimentary Canal

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Principles of Alimentary Tract Dysfunction. The primary functions of the alimentary tract are the prehension, digestion and absorption of food and water, and the maintenance of the internal environment by modification of the amount and nature of the materials absorbed.. The primary functions can be divided into four major modes and, correspondingly, there are four major modes of alimentary ...
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An anal fistula - again, a structural disease - often follows drainage of an abscess. It's an abnormal, tube-like passageway from the anal canal to a hole in the skin near the opening of your anus. Body wastes traveling through your anal canal are diverted through this tiny channel and out through the skin, causing itching and irritation.
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The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa.
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The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus.The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines.Food taken in through the mouth is digested to extract nutrients and absorb energy, and ...
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The chemical breakdown of food into small organic fragments for absorption by digestive epithelium. The Digestive Tract. Secretion. Is the release of water, acids, enzymes, buffers, and salts. By epithelium of digestive tract. By glandular organs. Absorption. Movement of organic substrates, electrolytes, vitamins, and water.
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Chapter Review. The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
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The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa.
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The alimentary canal is a continuous passage starting from the mouth and ending at the anus, which carries food through different parts of the digestive system and allows waste to exit the body. The alimentary canal varies widely in organism, but is only seen in organism which are bilaterally symmetrical. Various sections of the alimentary ...
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The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa.
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Digestive Processes. The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation. The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which ...
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symptoms ; the irritation subsides, the patient appears to be in a slate of. extreme exhaustion, and the pulse becomes intermitting and extremely weak ; the suiface cold, anil the skin collapsed ...
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Histology of the Alimentary Canal. The organs of the GI tract are made from four layers, the inner lining or mucosa, the submucosa containing blood vessels and lymphatics, the muscularis externa or smooth muscle layer, and the outermost layer or serosa/adventitia.Each tissue layer plays a vital role in the digestive system ranging in their capacity to form a protective barrier from the highly ...
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The Esophagus. Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = "to nourish") is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary ...
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Body Fluid-Related Diseases and Disorders. 218. Diseases and Disorders of the Reproductive System. 219. ... It forms the lining of various body passages that communicate with the air, such as the respiratory, genitourinary, and alimentary tracts. ... alimentary canal: The organs of a human or a non-human animal through which food passes.
23.6 The Small and Large Intestines
The longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Since this makes it about five times longer than the large intestine, you might wonder why it is called "small."
22.4: The Alimentary Canal
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Diseases of the Alimentary Canal
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Alimentary Canal -Anatomy

What is the Alimentary Canal?

Organs of the Alimentary Canal

Oral cavity

Small Intestine

Large Intestine

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154 23.1 Overview of the Digestive System

Digestive System Organs

Alimentary Canal Organs

Accessory Structures

Histology of the Alimentary Canal

Nerve Supply

Blood Supply

The Peritoneum

Chapter Review

Interactive Link Questions

Review Questions

Critical Thinking Questions

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Module 7: The Digestive System

Digestive System Organs

Alimentary Canal Organs

Accessory Structures

Histology of the Alimentary Canal

Nerve Supply

Blood Supply

The Peritoneum

Disorders of the Digestive System: Peritonitis

Practice Question

Chapter Review

Critical Thinking Questions

Introduction to the Digestive System

Elimination

Mechanical Digestion

Chemical Digestion

23.2 Digestive System Processes and Regulation

Interactive Link

Digestive Processes

Aging and the...

Regulatory Mechanisms

Neural Controls

Hormonal Controls

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20.2: Histology of the Alimentary Canal

Histology of the Alimentary Canal

Attributions

146 The Alimentary Canal

Learning Objectives

Key Takeaways

The Mouth as a Communication and Breathing Tool

Mechanical Food Breakdown by Teeth

Food Lubrication and Chemical Digestion By Saliva

The Structures of the Lips and External Mouth

Nasopharynx

The Pharyngeal Ostia

Laryngopharynx

Length and Location

Layers of Tissue

The Gastroesophageal Junction

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23.6 The Small and Large Intestines

The Small Intestine

Circular folds

Intestinal Glands

Intestinal MALT

External Website

Mechanical Digestion in the Small Intestine

Chemical Digestion in the Small Intestine

The Large Intestine

Subdivisions

Bacterial Flora

Digestive Functions of the Large Intestine

Mechanical Digestion

Chemical Digestion

Absorption, Feces Formation, and Defecation

Chapter Review

Interactive Link Questions

Review Questions