The Digestive System

The digestive system is like a long tube that runs from the mouth to the anus and includes organs that help the body digest and absorb food and nutrients. The organs that make up the digestive system include the mouth, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. Inside these organs is a lining called the mucosa. The digestive tract also contains a layer of smooth muscle. The muscle moves food through the tract in a wavelike movement called peristalsis, helping to mix it and break it down.

The liver and the pancreas also belong to the digestive system. They produce digestive juices that empty into the small intestine through ducts. The gallbladder stores the liver's digestive juices.

Digestion begins in the mouth. Chewed food is swallowed and moves into the pharynx, and then is pushed down the esophagus into the stomach. There, food is churned by smooth muscle contractions and mixed with digestive juices produced by the stomach. The stomach slowly empties its contents into the small intestine, where food dissolves into juices secreted from the pancreas, liver, and intestine. As the digested food moves through the small intestine, its nutrients are absorbed into the intestinal wall and into the bloodstream, where it is transported throughout the body for distribution. The colon removes water from the digested matter and converts it into a mostly solid mass of waste material, consisting largely of indigestible elements of the food, called fiber, and older cells that have been shed from the mucosa. Waste material is then expelled as feces.

The Colon

The small intestine joins the large intestine, or colon, in the right lower abdomen. The colon is a muscular tube about 5 feet long. It's made up of four sections: the ascending colon, which extends upward on the right side of the abdomen; the transverse colon, which goes across the body from the right to the left side in the upper abdomen; the descending colon, which continues downward on the left side; and the sigmoid colon (called this because of its "S" or "sigmoid" shape). The sigmoid colon attaches to the rectum, which ends in the anus.

The colon is composed of a number of different layers of tissue:

• The mucosa, the innermost layer, includes a single layer of epithelial cells, a layer of connective tissue, and a thin muscle layer. It is lined with goblet cells, glands that secrete mucous to help the passage of material through the colon.
• The submucosa is a layer of connective tissue beneath the mucosa.
• The circular muscle is a band of muscle that wraps around the entire colon and helps move waste material through it.
• The longitudinal muscle runs lengthwise along the colon. It works in conjunction with the circular muscle to create the wavelike motion of peristalsis.
• The serosa is the outermost layer of the colon.

Colon Function

The major function of the colon is to extract water, salts, and nutrients from partially digested food, and to propel the residue to the rectum and anus for expulsion. When the contents of the small intestine reach the colon, they're liquid. By the time they are expelled as feces, they're solid. About 2 pints of liquid matter enter the colon each day; stool volume is about a third of a pint. The difference between these two amounts is what the colon has absorbed in the course of digestion. If the digested matter lacks enough water, the colon can also add water to it to soften the stool. Maintaining this balance is one of the colon's most important functions.

By the time the partially digested food enters the colon, most of its nutrients have been absorbed by the small intestine. However, the process of digestion isn't over. The colon contains large numbers of bacteria. The most important of these are the lactobacilli and bifidus bacteria, which live in a symbiotic (mutually beneficial) relationship with the colon. They ferment the soluble fiber in food, forming valuable short-chain fatty acids that nourish intestinal cells, help regulate production of cholesterol, and are thought to help prevent a variety of diseases (including cancer). Beneficial bacteria also help to keep harmful bacteria in check. In addition, intestinal bacteria synthesize vitamin K, important in blood clot formation.

Figure 23.21 Large Intestine The large intestine includes the cecum, colon, and rectum.

The large intestine is about 5 feet long in adults and absorbs water and any remaining nutrients from partially digested food passed from the small intestine. The large intestine then changes waste from liquid to a solid matter called stool.

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, form feces, and eliminate feces from the body.

Structure

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.

Cecum

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.

Colon

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 (image). 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.

Rectum

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.

Anal Canal

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.

Histology

There are several notable differences between the walls of the large and small intestines (image). 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.

Anatomy

Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages (image). 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 = haustrum), 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.

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”). 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.

The entire colon is about 5 feet (150 cm) long, and is divided into five major segments. The rectum is the last anatomic segment before the anus.

The ascending and descending colon are supported by peritoneal folds called mesentery.

The right colon consists of the cecum, ascending colon, hepatic flexure and the right half of the transverse colon. The left colon consists of the left half of the transverse colon, splenic flexure, descending colon, and sigmoid.

Parts of Colon and Rectum

• Cecum (proximal right colon)
  6 x 9 cm pouch covered with peritoneum
• Appendix
  A vermiform (wormlike) diverticulum located in the lower cecum
• Ascending colon
  20-25 cm long, located behind the peritoneum
• Hepatic flexure
  Lies under right lobe of liver
• Transverse colon
  Lies anterior in abdomen, attached to gastrocolic ligament
• Splenic flexure
  Near tail of pancreas and spleen
• Descending colon
  10-15 cm long, located behind the peritoneum
• Sigmoid colon
  Loop extending distally from border of left posterior major psoas muscle
• Rectosigmoid segment
  Between 10 and 15 cm from anal verge
• Rectum
  12 cm long; upper third covered by peritoneum; no peritoneum on lower third which is also called the rectal ampulla. About 10 cm of the rectum lies below the lower edge of the peritoneum (below the peritoneal reflection), outside the peritoneal cavity
• Anal canal
  Most distal 4-5 cm to anal verge

• Lumen (interior surface of colon "tube")
• Mucosa
• Surface epithelium
• Lamina propria or basement membrane—dividing line between in situ and invasive lesions
• Muscularis mucosae
• Submucosa—lymphatics; potential for metastases increases
• Muscularis propria
• Circular layer
• Longitudinal layer—in three bands called taenia coli
• Subserosa—sometimes called pericolic fat or subserosal fat
• Serosa—present on ascending, transverse, sigmoid only (also called the visceral peritoneum)
• Retroperitoneal fat (also called pericolic fat)
• Mesenteric fat (also called pericolic fat)

A pouch that forms the first part of the large intestine. It connects the small intestine to the colon, which is part of the large intestine.

The valve, at the junction of the CECUM with the COLON, that guards the opening where the ILEUM enters the LARGE INTESTINE.

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.

Figure 16.7 The stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

Extends upward from the cecum on the right side of the abdomen to the under surface of the right lobe of the liver where it turns to the left forming the hepatic flexure.

Begins at the hpetic flexure passing horizontally across the abdomen, below the liver and stomach and above the small intestine. On the left side of the abdomen near the spleen, the colon turns downward at the junction of the transverse and descending colon forming the splenic flexure. The main function of the transverse colon is to absorb water and salts.

Starts at the splenic flexure and passes downward until it turns toward the midline at the rim of the pelvis and continues downward to become the sigmoid colon.

This part of the colon is curved like the letter S. The lower part of the curve joins the rectum at the rectosigmoid junction.

There are between 100 and 150 lymph nodes in the mesentery of the colon. Regional lymph nodes are the nodes along the colon, plus the nodes along the major arteries that supply blood to that particular colon segment.

Lymph nodes along a "named vascular trunk" (as defined by the fourth edition of the AJCC staging manual) are those along a vein or artery that carries blood to a specific part of the colon, for example, the inferior and superior mesenteric arteries, sigmoidal artery, left or right colic artery. In the fifth and sixth editions, the location of the nodes does not affect assignment of the N category.