Porth's Essentials of Pathophysiology, 4e

688

Gastrointestinal and Hepatobiliary Function

U N I T 8

replaced as a result of the high mitotic index of the epi- thelial stem cells in the crypts. The epithelium is entirely replaced approximately every 4 to 5 days. Paneth cells , which are present in the bottom of the crypts, secrete antibacterial substances and are involved in the host defense system of the small intestine. The large intestine secretes mainly mucus and bicar- bonate. Mucus not only protects the mucosa of the colon, but also facilitates compaction of the feces. Bicarbonate adheres to the mucus and acts as a buffer, protecting the mucosa from acid by-products of bacte- rial metabolism within the feces. ANS activity strongly influences mucus production in the bowel, as in other parts of the digestive tract. During intense parasympathetic stimulation, mucus secretion may increase to the point that the stool contains large amounts of obvious mucus. Gastrointestinal Hormones The GI tract is the largest endocrine organ in the body. It produces hormones that act locally, pass into the gen- eral circulation for distribution to more distant sites, and interact with the CNS through the enteric and auto- nomic nervous systems. These hormones influence appe- tite, GI motility, enzyme activity, electrolyte levels, and the secretion and actions of hormones such as growth hormone, insulin, and glucagon. The actions of many of these hormones overlap: two or more GI hormones may affect the same process in the same direction, or they may inhibit each other. The GI tract hormones and their functions are summarized in Table 28-2. The stomach is the source of two important GI hor- mones: gastrin and ghrelin. Gastrin is produced by G cells, located predominantly in the antrum of the

Enterocyte being extruded from a villus

Enterocyte

Vein

Artery Lacteal

Crypt of Lieberkühn

Paneth cells

FIGURE 28-9. A single villus from the small intestine.

and water. The second type of secretion consists of surface enzymes that aid absorption. These enzymes include pep- tidases, which separate amino acids, and disaccharidases, which split sugars. Also present in intestinal secretions are mucins secreted by goblet cells. The mucins, which are glycoproteins high in carbohydrates, form a gel that serves to lubricate the mucosal surface and protect it from mechanical injury from solid food particles. The crypts of Lieberkühn also function to replace epi- thelial cells. The epithelial cells lining the small intestine are constantly being sloughed off into the lumen and

TABLE 28-2 Selected Gastrointestinal Hormones andTheir Actions Hormone Site of Secretion Stimulus for Secretion

Action

Cholecystokinin

Duodenum, jejunum Products of protein digestion and long-chain fatty acids

Stimulates contraction of gallbladder and secretion of pancreatic enzymes; slows gastric emptying; inhibits food intake Stimulates secretion of gastric acid and pepsinogen; increases gastric blood flow; stimulates gastric smooth muscle contractions; stimulates growth of gastric and intestinal mucosal cells Stimulates secretion of growth hormone; acts as an appetite-stimulating signal from stomach when an increase in metabolic efficiency is necessary Augments insulin release; suppresses glucagon release; slows gastric emptying; decreases appetite and body weight

Gastrin

Antrum of stomach, duodenum

Vagal stimulation; epinephrine; neutral amino acids; calcium- containing foods such as milk; alcohol Secretion inhibited by acid content of stomach antrum (pH < 2.5)

Ghrelin

Fundus of stomach Nutritional (fasting) and hormonal (decreased levels of growth hormone)

Glucagon-like

Distal small intestine High-carbohydrate meal

peptide-1 (GLP-1)

Glucose-dependent insulinotropic polypeptide (GIP)

Small intestine,

High-carbohydrate meal

Augments insulin release

mainly jejunum

Secretin

Duodenum

Acid pH or chyme entering duodenum (pH < 3.0)

Stimulates secretion of bicarbonate- containing fluids by pancreas and liver