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Updated: 03/13/08 12:17 PM |
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C. Gastric Emptying (Figure 5)
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Stomach acts as reservoir. "Receptive Relaxation" Food bolus → stretch stomach → vagal relaxation of smooth muscle → no ↑ in pressure |
The proximal stomach acts as a reservoir for the boluses of food and drink from the esophagus. Filling of the stomach with food or fluid causes an increase in gastric volume without increasing intragastric pressure. This "receptive relaxation" is mediated by the vagus nerves and involves inhibiting gastric smooth muscle activity. Slow, sustained fundal contractions exert a steady pressure which presses the proximal gastric contents towards the distal stomach. Liquids pass readily through the distal stomach into the duodenum so that the pressure exerted by the proximal stomach controls the gastric emptying rate of liquids. Some vagal fibers stimulate proximal gastric contractions while others inhibit contractions thereby allowing relaxation and storage.
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Figure 5 |
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Distal stomach mixes and grinds solids
<1 mm → goes through pylorus Indigestible solids >2 mm remain in stomach until phase III of MMC. |
The distal stomach acts as a mixer and grinder for solids. This grinding process is under control of a pacemaker, a group of muscle cells located in the mid-body along the greater curvature. The pacemaker generates three electrical cycles per minute. As the cycles sweep distally through the gastric wall, superimposed spike (action) potentials result in peristaltic contractions. In contrast to liquids, solid particles more than 1 mm in size do not pass through the pylorus. Instead they are ground together and then squeezed backward toward the proximal antrum only to be thrust forward again by the next peristaltic wave. When solid food is broken down to about 1 mm in size the particles pass into the duodenum. Large (>2 mm) non-dispersible, indigestible solids are retained by the stomach during the digestive phase.
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| (Migrating motor complex) |
They are emptied later during fasting by a special sequence of cyclically recurring contractions, which move from the stomach to the terminal ileum at about 5 cm per minute (see Chapter 5, Section D).
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| ↑ distension → ↑ rate of emptying. Acidic, hypertonic, & fatty chyme in duodenum → ↓ rate of emptying. |
Gastric distension tends to increase the rate of gastric emptying, especially of liquids. Acidic, hypertonic, and fatty chyme in the proximal small intestine inhibit antral motility. This inhibition is mediated by hormones released from small intestinal mucosa and by mucosal receptors which trigger vagal afferent fibers. A fatty meal leaves the stomach over 4 6 hours so that the upper small intestine is burdened by only about 10 g of fat an hour. The stomach's chief role in digestion is to regulate the flow of gastric contents into the small intestine.
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| Effects of various vagotomies on gastric emptying. |
Various vagotomies have differing effects upon gastric emptying. 1) Cutting the vagal fibers to the proximal stomach impairs gastric receptive relaxation for boluses of food and drink, and speeds gastric emptying of liquids. 2) Vagotomy of the distal stomach weakens peristaltic contractions, and slows gastric emptying of solids. 3) Truncal vagotomy causes rapid emptying of liquids (especially when the patient is upright), slows emptying of solids, and prolongs retention of indigestible solids. Moreover, reflexes from small intestine that normally slow gastric emptying are disrupted. You can understand why a drainage procedure (antrectomy, or pyloroplasty, or gastro-jejunostomy) must be performed when the vagal nerves to the distal stomach are cut (as occurs in bilateral truncal vagotomy).
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| Effects of proximal & distal gastrectomy on gastric emptying | Surgical removal of the proximal stomach reduces the capacity of the gastric reservoir so that emptying rate of liquids is increased. Resection of the distal stomach and pylorus impairs gastric grinding so that solids are emptied before maceration is completed. | |
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Next Section (D): Gastric Acid Secretion and Its Control » |
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