activities Flashcards
Secretion of gastric juice is regulated by both neural and hormonal factors.
The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the gastric (stomach) glands. In addition, the presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin. Gastrin prods the gastric glands to produce still more of the protein-digesting enzymes (such as pepsinogen), mucus, and hydrochloric acid. Under normal conditions, 2 to 3 liters of gastric juice are produced every day.
Hydrochloric acid makes the stomach
very acidic. This can be dangerous because both hydrochloric acid and the protein-digesting enzymes have the ability to digest the stomach itself, causing ulcers (see “A Closer Look”). However, as long as enough mucus is made, the stomach is protected.
heartburn
Occasionally, the cardioesophageal sphincter fails to close tightly and gastric juice backs up into the esophagus, which has little mucus protection. This results in a characteristic pain known as heartburn, which, if uncorrected, leads to inflammation of the esophagus (esophagitis [ĕ-sof′ah-ji′tis]) and perhaps even to ulceration of the esophagus. A common cause is a hiatal hernia, a structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm. Because the diaphragm no longer reinforces the relatively weak cardioesophageal sphincter, gastric juice flows into the unprotected esophagus. Conservative treatment involves restricting food intake after the evening meal, taking antacids, and sleeping with the head elevated.
why is the extremly acidic enviorment hydrochloric acid provides necessary
The extremely acidic environment that hydrochloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily on milk protein and converts it to a substance that looks like sour milk. Many parents mistakenly think that the curdy substance infants spit up after feeding is milk that has soured in their stomach. Rennin, produced in large amounts in infants, is the same enzyme used to make milk curdle into cheese. It is not believed to be produced in adults.
little chemical digestion occurs in stomach
Other than the beginning of protein digestion, little chemical digestion occurs in the stomach. Except for aspirin and alcohol (which seem somehow to have a “special pass”), virtually no absorption occurs through the stomach walls.
food breakdown in stomach
As food enters and fills the stomach, its wall begins to stretch (at the same time the gastric juices are being secreted, as just described). Then the three muscle layers of the stomach wall become active. They compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme-containing gastric juice so that the thick fluid chyme is formed. The process looks something like the preparation of a cake mix, in which the floury mixture is repeatedly folded on itself and mixed with the liquid until it reaches a uniform texture.
Food Propulsion
Once the food has been well mixed, a rippling peristalsis begins in the upper half of the stomach (Figure 14.15, ). The contractions increase in force as the food approaches the pyloric valve, grinding the food into chyme. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter. Because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine (Figure 14.15, ). The contraction also closes the valve, so the rest of the chyme (about 27 ml) is propelled backward into the stomach for more mixing, a process called retropulsion (Figure 14.15, ).
When the duodenum is filled with chyme and its wall is stretched, a nervous reflex, the enterogastric (en″ter-o-gas′trik) reflex, occurs
This reflex “puts the brakes on” gastric activity. It slows the emptying of the stomach by inhibiting the vagus nerve and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up. Generally, it takes about 4 hours for the stomach to empty completely after the person eats a well-balanced meal and 6 hours or more if the meal has a high fat content.
how does the mendulla oblangata play a role in this.
Local irritation of the stomach, such as occurs with bacterial food poisoning, may activate the emetic (ē-met′ik) center in the brain (medulla oblongata). The emetic center, in turn, causes vomiting, or emesis. Vomiting is essentially a reverse peristalsis occurring in the stomach (and perhaps the small intestine), accompanied by contraction of the abdominal muscles and the diaphragm, which increases the pressure on the abdominal organs. The emetic center may also be activated through other pathways; disturbance of the equilibrium apparatus of the inner ear during a boat ride on rough water is one example.
