Although intestinal chyme slightly stimulates gastric secretion during the early intestinal phase of stomach secretion, it paradoxically inhibits gastric secretion at other times. This inhibition results from at least two influences.
Phases of gastric secretion and their regulation.
1. The presence of food in the small intestine initiates a reverse enterogastric reflex, transmitted through the myenteric nervous system as well as through extrinsic sympathetic and vagus nerves, that inhibits stomach secretion. This reflex can be initiated by distending the small bowel, by the presence of acid in the upper intestine, by the presence of protein breakdown products, or by irritation of the mucosa. This is part of the complex mechanism discussed in Chapter 63 for slowing stomach emptying when the intestines are already filled.
2. The presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any irritating factor in the upper small intestine causes release of several intestinal hormones. One of these is secretin, which is especially important for control of pancreatic secretion. However, secretin opposes stomach secretion. Three other hormones—gastric inhibitory peptide, vasoactive intestinal polypeptide, and somatostatin—also have slight to moderate effects in inhibiting gastric secretion.
The functional purpose of inhibitory gastric secretion by intestinal factors is presumably to slow passage of chyme from the stomach when the small intestine is already filled or already overactive. In fact, the entero-gastric inhibitory reflexes plus inhibitory hormones usually also reduce stomach motility at the same time that they reduce gastric secretion, as was discussed in Chapter 63.
Gastric Secretion During the Interdigestive Period. The stomach secretes a few milliliters of gastric juice each hour during the "interdigestive period," when little or no digestion is occurring anywhere in the gut. The secretion that does occur usually is almost entirely of the nonoxyntic type, composed mainly of mucus but little pepsin and almost no acid.
Unfortunately, emotional stimuli frequently increase interdigestive gastric secretion (highly peptic and acidic) to 50 milliliters or more per hour, in very much the same way that the cephalic phase of gastric secretion excites secretion at the onset of a meal. This increase of secretion in response to emotional stimuli is believed to be one of the causative factors in development of peptic ulcers, as discussed in Chapter 66.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.