Gastrointestinal (GI) Tract: Overview, Immune Defense, Blood Flow

Food covering the body's energy and nutrient requirements p. 228ff.) must be swallowed, processed and broken down (digestion) before it can be absorbed from the intestines. The three-layered GI musculature ensures that the GI contents are properly mixed and transported. The passage time through the different GI segments varies and is largely dependent on the composition of the food (see A for mean passage times).

Solid food is chewed and mixed with saliva, which lubricates it and contains immunocompetent substances (see below) and enzymes. The esophagus rapidly transports the food bolus to the stomach. The lower esophageal sphincter opens only briefly to allow the food to pass. The proximal stomach mainly serves as a food reservoir. Its tone determines the rate at which food passes to the distal stomach, where it is further processed (chyme formation) and its proteins are partly broken down. The distal stomach (including the pylorus) is also responsible for portioning chyme delivery to the small intestine. The stomach also secretes intrinsic factor (^ p. 90).

In the small intestine, enzymes from the pancreas and small intestinal mucosa break down the nutrients into absorbable components. HCO3- in pancreaticjuices neutralizes the acidic chyme. Bile salts in bile are essential for fat digestion. The products of digestion (monosaccharides, amino acids, dipeptides, monoglycerides and free fatty acids) as well as water and vitamins are absorbed in the small intestine.

Waste products (e.g. bilirubin) to be excreted reach the feces via bile secreted by the liver. The liver has various other metabolic functions. It serves, for example, as an obligatory relay station for metabolism and distribution of substances reabsorbed from the intestine (via the portal vein, see below), synthesizes plasma proteins (incl. albumin, globulins, clotting factors, apolipoproteins etc.) and detoxifies foreign substances (biotransformation) and metabolic products (e.g., ammonia) before they are excreted.

The large intestine is the last stop for water and ion absorption. It is colonized by bacteria and contains storage areas for feces (cecum, rectum).

Immune defense. The large internal surface area of the GI tract (roughly 100 m2) requires a very effective immune defense system. Saliva contains mucins, immunoglobulin A (IgA) and lysozyme that prevent the penetration of pathogens. Gastric juice has a bactericidal effect. Peyer's patches supply the GI tract with immunocompetent lymph tissue. M cells (special membranous cells) in the mucosal epithelium allow antigens to enter Peyer's patches. Together with macrophages, the Peyer's patches can elicit immune responses by secreting IgA (^ p. 98). IgA is transported to the intestinal lumen by transcytosis (^ p. 30). In the epithelium, IgA binds to a secretory component, thereby protecting it from digestive enzymes. Mucosal epithelium also contains intraepithelial lymphocytes (IEL) that function like T killer cells (^ p. 98). Transmitter substances permit reciprocal communication between 1EL and neighboring enterocytes. Macrophages of the hepatic sinusoids (Kupf-fer's cells) are additional bastions of immune defense. The physiological colonies of intestinal flora in the large intestine prevent the spread of pathogens. IgA from breast milk protects the GI mucosa of neonates.

Blood flow to the stomach, gut, liver, pancreas and spleen (roughly 30% of cardiac output) is supplied by the three main branches of the abdominal aorta. The intestinal circulation is regulated by local reflexes, the autonomic nervous system, and hormones. Moreover, it is autoregulatory, i.e., largely independent of systemic blood pressure fluctuations. Blood flow to the intestines rises sharply after meals (acetylcholine, vasoactive intestinal peptide VIP, etc. function as vasodilatory transmitters) and falls during physical activity (transmitters: norepinephrine, etc.). The venous blood carries substances reabsorbed from the intestinal tract and enters the liver via the portal vein. Some components of reabsorbed fat are absorbed by the intestinal lymph, which transports them to the greater circulation while bypassing the liver.

i— A. Function of gastrointestinal organs


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