Page 245 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Gastrointestinal (GI) Tract: Overview, The large intestine is the last stop for water
Immune Defense, Blood Flow and ion absorption. It is colonized by bacteria
and contains storage areas for feces (cecum,
Food covering the body’s energy and nutrient rectum).
requirements (! p. 228ff.) must be swallowed, Immune defense. The large internal surface
2
processed and broken down (digestion) before area of the GI tract (roughly 100 m ) requires a
it can be absorbed from the intestines. The very effective immune defense system. Saliva
three-layered GI musculature ensures that the contains mucins, immunoglobulin A (IgA) and
GI contents are properly mixed and trans- lysozyme that prevent the penetration of
ported. The passage time through the different pathogens. Gastric juice has a bactericidal ef-
GI segments varies and is largely dependent on fect. Peyer’s patches supply the GI tract with
the composition of the food (see A for mean immunocompetent lymph tissue. M cells
Nutrition and Digestion petent substances (see below) and enzymes. patches. Together with macrophages, the
(special membranous cells) in the mucosal
passage times).
Solid food is chewed and mixed with saliva,
epithelium allow antigens to enter Peyer’s
which lubricates it and contains immunocom-
Peyer’s patches can elicit immune responses
The esophagus rapidly transports the food
by secreting IgA (! p. 98). IgA is transported to
bolus to the stomach. The lower esophageal
the intestinal lumen by transcytosis (! p. 30).
sphincter opens only briefly to allow the food
In the epithelium, IgA binds to a secretory
to pass. The proximal stomach mainly serves
tive enzymes. Mucosal epithelium also con-
at which food passes to the distal stomach,
tains intraepithelial lymphocytes (IEL) that
10 as a food reservoir. Its tone determines the rate component, thereby protecting it from diges-
where it is further processed (chyme forma-
function like T killer cells (! p. 98). Transmit-
tion) and its proteins are partly broken down. ter substances permit reciprocal communica-
The distal stomach (including the pylorus) is tion between IEL and neighboring enterocytes.
also responsible for portioning chyme delivery Macrophages of the hepatic sinusoids (Kupf-
to the small intestine. The stomach also fer’s cells) are additional bastions of immune
secretes intrinsic factor (! p. 90). defense. The physiological colonies of intesti-
In the small intestine, enzymes from the nal flora in the large intestine prevent the
pancreas and small intestinal mucosa break spread of pathogens. IgA from breast milk pro-
down the nutrients into absorbable com- tects the GI mucosa of neonates.
–
ponents. HCO 3 in pancreatic juices neutralizes Blood flow to the stomach, gut, liver, pan-
the acidic chyme. Bile salts in bile are essential creas and spleen (roughly 30% of cardiac out-
for fat digestion. The products of digestion put) is supplied by the three main branches of
(monosaccharides, amino acids, dipeptides, the abdominal aorta. The intestinal circulation
monoglycerides and free fatty acids) as well as is regulated by local reflexes, the autonomic
water and vitamins are absorbed in the small nervous system, and hormones. Moreover, it is
intestine. autoregulatory, i.e., largely independent of sys-
Waste products (e.g. bilirubin) to be ex- temic blood pressure fluctuations. Blood flow
creted reach the feces via bile secreted by the to the intestines rises sharply after meals
liver. The liver has various other metabolic (acetylcholine, vasoactive intestinal peptide
functions. It serves, for example, as an obliga- VIP, etc. function as vasodilatory transmitters)
tory relay station for metabolism and distribu- and falls during physical activity (transmit-
tion of substances reabsorbed from the in- ters: norepinephrine, etc.). The venous blood
testine (via the portal vein, see below), synthe- carries substances reabsorbed from the in-
sizes plasma proteins (incl. albumin, globulins, testinal tract and enters the liver via the portal
clotting factors, apolipoproteins etc.) and vein. Some components of reabsorbed fat are
detoxifies foreign substances (biotransforma- absorbed by the intestinal lymph, which trans-
tion) and metabolic products (e.g., ammonia) ports them to the greater circulation while by-
before they are excreted. passing the liver.
232
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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