MECHANICAL FUNCTIONS OF THE STOMACH AND SMALL INTESTINE
1. How do the stomach parts serve their mechanical functions?
2. How do the parasympathetics increase the number of contractions?
3. Does a hyperosmotic solution in the stomach become isotonic by withdrawal of water from the blood?
4.
What factors delay gastric emptying?5. What animals vomit easily? Is there a vomiting center? What interferes with vomiting in the horse? Why is vomiting not observed in cattle?
6. Why must the flow of contents in the small intestine be controlled? Where does the greatest delay occur?
The most important functions of the stomach are storage of ingested food, mixing of the food with secretions, and control of the emptying of its contents. The parts of the stomach mentioned above (fundus, body, and pyloric antrum) are suited to these functions. The fundus receives and stores contents by adapting its volume so that excessive pressure does not develop. The body serves as the mixing vat for saliva, food, and gastric secretions. The pyloric antrum serves as the pump by regulating the propulsion of food past the pyloric sphincter into the duodenum. The pyloric antrum contractions, together with a contracted pyloric sphincter, cause the contents to return to the body for additional mixing. Liquid leaves the stomach at a faster rate than solid materials, so adequate time is given for required solubilization and beginning digestion of solid materials.
The maximum number of contractions of the pyloric antrum is controlled by the slow waves. For the stomach, these occur at the rate of four or five each minute. The slow waves do not, however, necessarily result in contraction. Contraction depends on the superimposition of spikes that are created when the stomach is distended with food. Stomach distention causes receptors in the stomach wall to be activated; this in turn increases vagal tone (parasympathetic tone) so that the slow waves are closer to threshold and spike more readily.
The spikes (action potentials) are followed by contraction waves.Delay of Gastric Emptying
Inhibition to emptying is produced through a neural mechanism (enterogastric reflex) and an endocrine mechanism (enterogastrone reflex). The receptors for these mechanisms are present in the duodenum. The osmoreceptor, an important receptor in this regard, monitors the osmotic pressure of the material entering the duodenum. The gastric contents could be hyperosmotic and, if emptied into the duodenum, would result in fluid being withdrawn from the blood to achieve osmotic equilibrium of the contents. This does not occur in the stomach because of its lower permeability to water. The osmoreceptors detect the hypertonicity and inhibit gastric emptying via a neural mechanism so that slow emptying occurs and rapid loss of water from the blood is prevented. Excess protein or carbohydrate is also effective in inhibiting gastric emptying. It is believed that their influence is mediated through the osmoreceptor neural mechanism. Other receptors respond to high hydrogen ion concentrations and cause delays in gastric emptying until the gastric content previously emptied into the duodenum has been neutralized by secretions from the pancreas and liver. These two reflexes are mediated by a neural mechanism. A hormonally mediated delay to gastric emptying occurs in response to lipids entering the duodenum. Cholecystokinin is released in response to the presence of lipids and delayed emptying provides sufficient time for fat digestion. Another hormone, gastric inhibitory polypeptide (GIP), is secreted by the jejunal mucosa in response to the presence of lipids and carbohydrate and it also delays gastric emptying.
The following list summarizes the factors that delay gastric emptying and thus permit time for adequate digestion:
1. Enterogastric reflexes (neural mechanisms)
a. Osmoreceptors in the duodenum respond to hypertonic content (hypertonicity can be caused by the presence of products of protein and carbohydrate digestion as well as electrolytes).
b. Hydrogen ion receptors in the duodenum respond to high hydrogen ion concentration.
2. Enterogastrone reflexes (endocrine mechanisms)
a. Cholecystokinin released from duodenal mucosa in response to lipids
b. GIP released from jejunal mucosa in response to lipids and carbohydrate
Emesis
Emesis (vomiting) is an emptying of the cranial part of the duodenum and stomach in an orad (toward the mouth) direction. A series of reflexes is involved to initiate antiperistalsis and closure of the glottis and nasal cavity. Swine, dogs, and cats vomit easily. Vomiting is a protective mechanism that helps prevent absorption of noxious substances. Vomiting in ruminants occurs as an ejection of abomasal content into the forestomachs; thus, ejection from the mouth does not occur. Vomiting in the horse is rare because of the difficulty in opening the cardia from a reverse direction. Dilatation of the horse’s stomach because of pressure from attempted vomiting can occur to the point of rupture. The reflexes of vomiting are controlled by a vomiting center in the brain.
Mechanical Functions of the Small Intestine
The small intestine provides movements that both mix the contents and propel the contents aborally as digestion proceeds. The flow of contents must be controlled for two major reasons: (1) to provide proper mixing of luminal contents with pancreatic enzymes and bile and (2) to provide time for luminal digestion of carbohydrates, fat, and proteins and for maximum exposure of digested nutrients to the mucosa of the small intestine. One means of delaying transport is to delay transit time in the ileum. This can occur because of the greater number of segmental contractions and fewer peristaltic contractions at that location.
Small intestine activity can be increased or decreased by parasympathetic and sympathetic stimulation, respectively. Also, the hormone secretin inhibits and cholecystokinin and gastrin stimulate small intestine motility. These hormones control the rate of passage of intestinal content.
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