The Digestive

System

Chapter 23

Anatomy of the Digestive System – Part 4

Large Intestine

• Frames the small intestine on 3 sides

• Extends from the ileocecal valve to the anus.

• Lumen diameter ~7cm (2 ¾ in)

• Length 1.5 meter (5 ft)

Large Intestine

• Major function is to absorb most of the remaining water from the indigestible food, stores the residue temporarily, and then eliminate them from the body in the form of feces.

• Because the colon doesn’t secrete digestive enzymes, chemical digestion officially ends in the small intestine.

Large Intestine

• Cecum (“blind pouch”) – lies below the ileocecal valve. First part of the large intestine.

Large Intestine • Appendix – attached to the cecum. Contains

lymphoid tissue and plays an important role in body immunity

Large Intestine

• Ascending colon – travels up the right side of the abdominal cavity

Large Intestine

• Transverse colon – travels across the abdominal cavity

Large Intestine

• Descending colon – travels down the left side of the abdominal cavity

Large Intestine

• Sigmoid colon – when colon enters the pelvis, s-shaped, travels transverse

Large Intestine

• Rectum – travels down, just in front of the sacrum

Large Intestine • Anal canal – last portion of large intestine,

about 3 cm long. Has the sphincters that open and close the anus during defecation

Large Intestine

• Since most food is absorbed before reaching the large intestine, there are no circular folds, no villi, and virtually no cells that secrete digestive enzymes.

Large Intestine • The mucosa is thicker, and abundant crypts

are deeper and there are lots of goblet cells that produce mucus. The mucus eases the passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria

Large Intestine - Bacterial Flora

• Most bacteria entering from the small intestine are dead, but some are still alive. These and the bacteria that enter the GI tract from the anus, these constitute the bacterial flora of the large intestine over 700 species!

Large Intestine - Bacterial Flora

• Metabolize some proteins and ferment some indigestible carbohydrates (cellulose, xylan, and others), releasing irritating acids and a mixture of gases (including dimethyl sulfide, H2, N2, CH4, and CO2). Some are quite odorous (dimethyl sulfide)!

Large Intestine - Bacterial Flora

• About 500 ml of gas (flatus) is produced each day. More when certain carbohydrate rich foods (beans) are eaten.

Large Intestine - Bacterial Flora

• Bacteria also synthesize B complex vitamins and most of the vitamin K that the liver needs to make some of the clotting proteins.

• Also found in the large intestine are viruses and protozoans. Of these at least 20 are known pathogens.

Large Intestine - Bacterial Flora • Most enteric bacteria are

commensals (no harm and no benefit to the host) that coexist peacefully with their host as long as they remain in the lumen.

• Bacteria break down and metabolize the residual complex carbohydrates further, adding much to their own nutrition but essentially nothing to ours.

Large Intestine - Motility

• Haustral contractions

– Slow segmenting movements mainly occuring in the transverse and descending colon.

– Last about 1 minute and occur every 30 minutes or so

Large Intestine - Motility

• Mass movements

– Long, slow-moving, powerful contractile waves that move over large areas of the colon 3-4 times daily and force the contents towards the rectum.

– Typically occurs during or just after eating.

Homeostatic Imbalance - Diverticulosis • Bulk, or fiber, in the diet increases the

strength of colon contraction and softens the stool, allowing the colon to act like a well-oiled machine.

• When the diet lacks bulk, and the volume of the residues in the colon is small, the colon narrows and its contractions become more powerful, increasing the pressure on its walls. This promotes the formation of diverticula, small herniations of the mucosa through the colon walls.

• Most commonly occurs in the sigmoid colon. Affects over half of people over the age of 70.

Defecation

• The semisolid product delivered to the rectum is called feces or stool, contains undigested food residues, mucus, sloughed-off epithelial cells, millions of bacteria, and just enough water to allow for smooth passage.

• The rectum is usually empty, but when mass movements force the feces into it, the defecation reflex is triggered.

Homeostatic Imbalance - Diarrhea

• Watery stools

• Results from any condition that rushes food residue through the large intestine before that organ has had sufficient time to absorb the remaining water.

• Prolonged diarrhea may result in dehydration and electrolyte imbalance.

Homeostatic Imbalance - Constipation

• When food remains in the colon for extend periods, too much water is absorbed and the stool becomes hard and difficult to pass.

• May result from a lack of fiber in the diet, improper bowel habits (holding it), lack of exercise, emotional upset, or laxative abuse