Page 277 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Large Intestine, Defecation, Feces bacteria during the first few weeks of life. The
large intestine of a healthy adult contains 10 11
Anatomy. The terminal end of the gastrointestinal to 10 12 bacteria per mL of intestinal contents;
tract includes the large intestine (cecum and colon, ca. the corresponding figure for the ileum is
1.3 m in length) and rectum. The large intestinal mu- roughly 10 /mL. The low pH inside the stom-
6
cosa has characteristic pits (crypts), most of which ach is an important barrier against pathogens.
are lined with mucus-forming cells (goblet cells). Consequently, there are virtually no bacteria in
Some of the surface cells are equipped with a brush 4
border membrane and reabsorb ions and water. the upper part of the small intestine (0–10 /
mL). Intestinal bacteria increase the activity of
The large intestine has two main functions: (1) intestinal immune defenses (“physiological in-
It serves as a reservoir for the intestinal con- flammation”), and their metabolic activity is
tents (cecum, ascending colon, rectum). (2) It useful for the host. The bacteria synthesize vi-
absorbs water and electrolytes (! p. 262), so
Nutrition and Digestion large intestine can be reduced to about 100– charides (e.g. lactose) into absorbable short-
tamin K and convert indigestible substances
the ca. 500–1500 mL of chyme that reaches the
(e.g. cellulose) or partially digested sac-
200 mL. The large intestine is not an essential
chain fatty acids and gases (methane, H 2, CO 2).
organ; therefore, large segments of the in-
The anus is normally closed. Anal closure is
testine can be removed—e.g., for treatment of
regulated by Kohlrausch’s valve (transverse
cancer.
rectal fold), the puborectal muscles, the (invol-
Water instilled into the rectum via an enema is reab-
sphincter muscles, and a venous spongy body.
fuse through the intestinal wall into the blood-
Both sphincters contract tonically, the internal
10 sorbed. Anally delivered drugs (suppositories) also dif- untary) internal and (voluntary) external anal
stream. Substances administered by this route by-
pass the liver and also escape the effects of gastric sphincter (smooth muscle) intrinsically or
acid and digestive enzymes. stimulated by sympathetic neurons (L 1, L 2) via
α-adrenoceptors, the external sphincter
Motility. Different local mixing movements of muscle (striated muscle) by the pudendal
the large intestine can be distinguished, e.g., nerve.
powerful segmentation contractions as- Defecation. Filling of the upper portion of
sociated with pouch formation (haustration) the rectum (rectal ampulla) with intestinal
and anterograde or retrograde peristaltic waves contents stimulates the rectal stretch recep-
(pacemaker located in transverse colon). Thus, tors (! B2), causing reflex relaxation of the in-
stool from the colon can also be transported to ternal sphincter (accommodation via VIP neu-
the cecum. Mass movements occur 2–3 times rons), constriction of the external sphincter, and
daily (! A). They are generally stimulated by a an urge to defecate. If the (generally voluntary)
meal and are caused by the gastrocolic reflex decision to defecate is made, the rectum short-
and gastrointestinal hormones.
ens, the puborectal and external anal sphincter
The typical sequence of mass movement can be ob- muscles relax, and (by a spinal parasympa-
served on X-ray films after administration of a bar- thetic reflex via S 2–S 4) annular contractions of
ium meal, as shown in the diagrams (! A1 – 8). A1, the circular muscles of the descending colon,
barium meal administered at 7:00 a.m. A2, 12 noon:
the barium mass is already visible in the last loop of sigmoid colon and rectum—assisted by in-
the ileum and in the cecum. Lunch accelerates the creased abdominal pressure—propel the feces
emptying of the ileum. A3, about 5 minutes later, the out of the body (! B). The normal frequency of
tip of the barium mass is choked off. A4, shortly af- bowel evacuation can range from 3 times a day
terwards, the barium mass fills the transverse colon. to 3 times a week, depending on the dietary
A5, haustration divides the barium mass in the trans- content of indigestible fiber (e.g. cellulose, lig-
verse colon, thereby mixing its contents. A6–8, a few nin). Frequent passage of watery stools (diar-
minutes later (still during the meal), the transverse rhea) or infrequent stool passage (constipa-
colon suddenly contracts around the leading end of
the intestinal contents and rapidly propels them to tion) can lead to various disorders.
the sigmoid colon. Stool (feces; ! C). The average adult ex-
cretes 60–80 g of feces/day. Diarrhea can raise
264 Intestinal bacteria. The intestinal tract is ini- this over 200 g/d. Roughly /4 of the feces is
1
tially sterile at birth, but later becomes 1
colonized with orally introduced anaerobic composed of dry matter, about /3 is attribut-
able to bacteria from the large intestine.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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