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Large Intestine, Defecation, Feces

Anatomy. The terminal end of the gastrointestinal tract includes the large intestine (cecum and colon, ca. 1.3 m in length) and rectum. The large intestinal mucosa has characteristic pits (crypts), most of which are lined with mucus-forming cells (goblet cells). Some of the surface cells are equipped with a brush border membrane and reabsorb ions and water.

The large intestine has two main functions: (1) It serves as a reservoir for the intestinal contents (cecum, ascending colon, rectum). (2) It absorbs water and electrolytes (^ p. 262), so theca. 500-1500 mL of chyme that reaches the large intestine can be reduced to about 100200 mL. The large intestine is not an essential organ; therefore, large segments of the intestine can be removed—e.g., for treatment of cancer.

Water instilled into the rectum via an enema is reabsorbed. Anallydelivered drugs (suppositories) also diffuse through the intestinal wall into the bloodstream. Substances administered by this route bypass the liver and also escape the effects of gastric acid and digestive enzymes.

Motility. Different local mixing movements of the large intestine can be distinguished, e.g., powerful segmentation contractions associated with pouch formation (haustration) and anterograde or retrograde peristaltic waves (pacemaker located in transverse colon). Thus, stool from the colon can also be transported to the cecum. Mass movements occur 2-3 times daily (^ A). They are generally stimulated by a meal and are caused by the gastrocolic reflex and gastrointestinal hormones.

The typical sequence of mass movement can be observed on X-ray films after administration of a barium meal, as shown in the diagrams (^ A1 -8). A1, barium meal administered at 7:00 a.m. A2,12 noon: the barium mass is already visible in the last loop of the ileum and in the cecum. Lunch accelerates the emptying of the ileum. A3, about 5 minutes later, the tip of the barium mass is choked off. A4, shortly afterwards, the barium mass fills the transverse colon. A5, haustration divides the barium mass in the transverse colon, thereby mixing its contents. A6-8, a few minutes later (still during the meal), the transverse colon suddenly contracts around the leading end of the intestinal contents and rapidly propels them to the sigmoid colon.

Intestinal bacteria. The intestinal tract is initially sterile at birth, but later becomes colonized with orally introduced anaerobic Despopoulos, Color Atlas of Physiology © All rights reserved. Usage subject to terms bacteria during the first few weeks of life. The large intestine of a healthy adult contains 1011 to 1012 bacteria per mL of intestinal contents; the corresponding figure for the ileum is roughly 106/mL. The low pH inside the stomach is an important barrier against pathogens. Consequently, there are virtually no bacteria in the upper part of the small intestine (0-104/ mL). Intestinal bacteria increase the activity of intestinal immune defenses ("physiological inflammation"), and their metabolic activity is useful for the host. The bacteria synthesize vitamin K and convert indigestible substances (e.g. cellulose) or partially digested sac-charides (e.g. lactose) into absorbable short-chain fatty acids and gases (methane, H2, CO2).

The anus is normally closed. Anal closure is regulated by Kohlrausch's valve (transverse rectal fold), the puborectal muscles, the (involuntary) internal and (voluntary) external anal sphincter muscles, and a venous spongy body. Both sphincters contract tonically, the internal sphincter (smooth muscle) intrinsically or stimulated by sympathetic neurons (L1, L2) via a-adrenoceptors, the external sphincter muscle (striated muscle) by the pudendal nerve.

Defecation. Filling of the upper portion of the rectum (rectal ampulla) with intestinal contents stimulates the rectal stretch receptors (^ B2), causing reflex relaxation of the internal sphincter (accommodation via VIP neurons), constriction ofthe external sphincter, and an urge to defecate. If the (generally voluntary) decision to defecate is made, the rectum shortens, the puborectal and external anal sphincter muscles relax, and (by a spinal parasympathetic reflex via S2-S4) annular contractions of the circular muscles of the descending colon, sigmoid colon and rectum—assisted by increased abdominal pressure—propel the feces out of the body (^ B). The normal frequency of bowel evacuation can range from 3 times a day to 3 times a week, depending on the dietary content of indigestible fiber (e.g. cellulose, lig-nin). Frequent passage of watery stools (diarrhea) or infrequent stool passage (constipation) can lead to various disorders.

Stool (feces; ^ C). The average adult excretes 60-80 g of feces/day. Diarrhea can raise this over 200 g/d. Roughly '/< of the feces is composed of dry matter, about 1/3 is attribut-2003Thi^miria from the large intestine. and conditions of license.

I— A. Mass movement in large intestine

Kohlrausch Fold

(After Hertz & Newton)

(After Hertz & Newton)

r- B. Anal closure and defecation

Rectal ampulla-

Kohlrausch's fold

Rectal ampulla-

Kohlrausch's fold

Kohlrausch Folds

Rectum shortens

Puborectal muscles relaxed

Rectum shortens

Puborectal muscles relaxed

1 Anus closed

2 Urge to defecate 3 Defecation

I— C. Composition of feces

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