The stomach is anatomically and functionally divided into the fundus, body, and antrum. The fundus and body are highly distensible and act as reservoir for the ingested meal. A 1.5 L volume increase causes only a small increase in pressure in the lumen of the stomach. The esophageal swallowing reflex promotes release of NO and VIP in the wall of the stomach, causing relaxation of the fundus and body. This relaxation allows the stomach to accommodate the increased volume.
Food is stored unmixed in the fundus and body of the stomach for up to 1 hour. During this period, there is separation of the ingested food according to density, with fats rising to the surface of the gastric contents. Liquids can flow around the solid food, and they accumulate at the bottom. This separation leads to a sequence of gastric emptying into the duodenum: first liquids, then solids, and finally lipids.The ingested food mixes with gastric secretions, and the luminal contents are now called “chyme” during passage through the GI tract.
Slow waves, which in the stomach do not generate spike potentials, initiate gastric contractions. Slow waves occur at a rate of three per minute and begin at the pacemaker region in the middle of the body of the stomach. Contractions increase in force and velocity as they approach the antrum. Acetylcholine increases the amplitude and duration of depolarization and therefore contraction. Norepinephrine reduces the amplitude and duration of depolarization.
The antrum has strong contractions, fragmenting the food into smaller particles. The contractions also mix chyme with gastric secretions to initiate digestion. The strength of antral contractions is enhanced when slow waves are augmented by action potentials. The pylorus and terminal end of the antrum contract almost simultaneously; this is called systolic contraction of the antrum. This coordinated contraction allows only a small amount of the antral contents to enter the duodenum before contraction of the pylorus closes the stomach/intestinal opening. Antral contraction continues, but because the pylorus is now closed, the contraction only causes further mixing and grinding of contents (retropulsion).
After a meal, the rate of gastric contractions is equal to the rate of gastric slow waves, about three per minute. During fasting, the antrum is mostly quiescent for 1 to 2 hours, with intervening periods of contraction. The contractions are due to a migrating myoelectric complex, which initiates 10 to 20 minutes of intense contractions against an open pylorus.
The gastric contents empty into the duodenal bulb at a controlled rate, which is modulated by neural reflexes and hormonal reflexes originating in the duodenum (Fig. 12-4). The rate of gastric emptying is modulated by the enteric nervous system and autonomic nervous system. Vagal cholinergics stimulate motility, and sympathetic adrenergics inhibit motility. The enteric nervous system coordinates these external influences and the numerous internal reflexes. The rate of gastric emptying is also under hormonal control. Gastric emptying is increased by gastrin and decreased by secretin. Sensory gastric afferents play a role in satiety, and they are activated by intragastric pressure, gastric distention, and intragastric pH.
When the antrum contracts, the duodenum relaxes, allowing a small volume of gastric chyme to enter the duodenum. The pylorus functions as a sphincter, but anatomically it does not qualify, since the muscles across the pylorus are not electrically connected by gap junctions. The rate of gastric emptying is matched to duodenal buffering ability, preventing acid damage to the duodenal mucosa and subsequent duodenal ulcers. The pylorus also prevents regurgitation of duodenal contents back into the stomach, preventing bile damage to the gastric mucosa, which can lead to gastric ulcers.
Negative feedback control balances the rate of chyme entry into the duodenum to the capability of the duodenum to digest and absorb the diet. Gastric chyme is acidic and often hyperosmolar. Within the duodenum, both acidity and hypertonicity initiate reflexes that inhibit gastric motility and therefore the further entry of gastric chyme. This allows the duodenum to process the luminal contents before any new gastric chyme is allowed to enter.
Duodenal acidity (pH < 3.5) decreases the rate of gastric emptying through multiple routes. Acidity initiates a neural reflex that reduces gastric emptying. Duodenal acidity also stimulates secretin release, which increases HCO3– buffer secretion from ductal epithelia of the pancreatic and hepatic ducts. HCO3– secretion acts to neutralize the chyme present in the duodenum.
Duodenal hypertonicity also decreases the rate of gastric emptying. Chyme becomes more hypertonic as digestion progresses. Duodenal hypertonicity decreases gastric emptying by a neural reflex and an unidentified hormonal component. The duodenal tonicity decreases with time as the digested components of the diet are absorbed across the duodenum. In addition, contraction of the pylorus is increased by unabsorbed products of digestion in the duodenum. Monoglycerides increase contraction of the pylorus and slow gastric emptying. Amino acids, especially tryptophan, and peptides in the duodenum also slow gastric emptying.
Fat content and fatty acids (especially long chain, unsaturated, or both) decrease the rate of gastric emptying. Fats stimulate the release of cholecystokinin (CCK) from the duodenum and jejunum, which contracts the pylorus and relaxes the stomach fundus. Gastric inhibitory peptide (GIP) may also have a role in limiting gastric emptying.