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. 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.

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. The appendix or vermiform appendix is a winding tube that attaches to the cecum. Although the 7. 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 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. Each year, approximately , Americans are diagnosed with colorectal cancer, and another 49, 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 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. 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.

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.

Figure 5. a The histologies of the large intestine and small intestine not shown are adapted for the digestive functions of each organ. LM x credit b: Micrograph provided by the Regents of University of Michigan Medical School © Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages Figure 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.

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.

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. 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.

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 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. 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.

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.

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. 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 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. The small intestine absorbs about 90 percent of the water you ingest either as liquid or within solid food.

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 mL 17 ounces of food residue that enters the cecum each day, about mL 5 ounces become feces. Feces are eliminated through contractions of the rectal muscles. 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 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 fold, including circular folds, villi, and microvilli.

There are around 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. brush border: fuzzy appearance of the small intestinal mucosa created by microvilli. circular fold: also, plica circulare deep fold in the mucosa and submucosa of the small intestine.

descending colon: part of the colon between the transverse colon and the sigmoid colon. duodenum: first part of the small intestine, which starts at the pyloric sphincter and ends at the jejunum. epiploic appendage: small sac of fat-filled visceral peritoneum attached to teniae coli.

gastrocolic reflex: propulsive movement in the colon activated by the presence of food in the stomach. gastroileal reflex: long reflex that increases the strength of segmentation in the ileum.

hepatopancreatic ampulla: also, ampulla of Vater bulb-like point in the wall of the duodenum where the bile duct and main pancreatic duct unite. hepatopancreatic sphincter: also, sphincter of Oddi sphincter regulating the flow of bile and pancreatic juice into the duodenum.

ileocecal sphincter: sphincter located where the small intestine joins with the large intestine. intestinal gland: also, crypt of Lieberkühn gland in the small intestinal mucosa that secretes intestinal juice. intestinal juice: mixture of water and mucus that helps absorb nutrients from chyme.

left colic flexure: also, splenic flexure point where the transverse colon curves below the inferior end of the spleen. main pancreatic duct: also, duct of Wirsung duct through which pancreatic juice drains from the pancreas. major duodenal papilla: point at which the hepatopancreatic ampulla opens into the duodenum.

microvillus: small projection of the plasma membrane of the absorptive cells of the small intestinal mucosa. pectinate line: horizontal line that runs like a ring, perpendicular to the inferior margins of the anal sinuses.

rectal valve: one of three transverse folds in the rectum where feces is separated from flatus. Ingested food is chewed, swallowed, and passes through the esophagus into the stomach where it is broken down into a liquid called chyme. Chyme passes from the stomach into the duodenum.

There it mixes with bile and pancreatic juices that further break down nutrients. Finger-like projections called villi line the interior wall of the small intestine and absorb most of the nutrients. The remaining chyme and water pass to the large intestine, which completes absorption and eliminates waste.

Villi that line the walls of the small intestine absorb nutrients into capillaries of the circulatory system and lacteals of the lymphatic system. Villi contain capillary beds, as well as lymphatic vessels called lacteals. Fatty acids absorbed from broken-down chyme pass into the lacteals.

Other absorbed nutrients enter the bloodstream through the capillary beds and are taken directly to the liver, via the hepatic vein, for processing. Chyme passes from the small intestine through the ileocecal valve and into the cecum of the large intestine. Any remaining nutrients and some water are absorbed as peristaltic waves move the chyme into the ascending and transverse colons.

This dehydration, combined with peristaltic waves, helps compact the chyme. The solid waste formed is called feces. It continues to move through the descending and sigmoid colons. The large intestine temporarily stores the feces prior to elimination. The body expels waste products from digestion through the rectum and anus.

This process, called defecation, involves contraction of rectal muscles, relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external anal sphincter. The defecation reflex is mostly involuntary, under the command of the autonomic nervous system.

But the somatic nervous system also plays a role to control the timing of elimination. Download Digestive System Lab Manual. Study: Immune system promotes digestive health from Science Daily. Visible Body Web Suite provides in-depth coverage of each body system in a guided, visually stunning presentation.