The Pharynx and Esophagus
From the soft palate area, the bolus moves into the pharynx, a commom passageway for food and air. As the bolus is forced to the back of the mouth, the soft palate moves up and closes off the nasal cavities. At the same time, a flap of tissue called the epiglotis seals off the trachea, or wind-pipe. The larynx, or voice box, at the bottom of the epiglottis. Food can then pass quickly through the pharynx, across the trachea, and into the stomach. If a person attempts to breathe while swallowing, food gets into the trachea.
Until food enters the pharynx, voluntary muscles control the process of mechanical digestion. Once a swallow has started, however, it cannot be stopped because involuntary muscles take over the pharynx. A strong contraction of a muscle around the pharynx propels food into the esophagus. This contraction starts a wavelike motion called peristalsis, which moves food along. Peristalsis results from the action of the two layers of involuntary muscles that form the walls of most of the digestive tract. One layer of muscles wraps around the tract, and the second layer runs along its length. While the circular muscles squeeze, the parallel ones relax. The squeezing and contraction of the two sets of muscles always occurs above the bolus or liquid in the digestive tract.
At the end of the esophagus is a muscular valve called the cardiac sphincter. A sphinkter is a muscle that controls a circular opening of the body. This valve prevents food from reentering the esophagus. Food passes through the valve into the stomach, a J-shaped, baglike organ with a capacity of 2 or 4 L (2.1 to 4.2 qt.). Both mechanical and chemical digestion continue in the stomach. In addition to the layers of involuntary muscle, the stomach has a third, diagonal layer of muscle. Through the action of these three muscle layersa, the stomach can actually grind food.
Chemical digestion of protein begins in the stomach. The stomach contains about 35 million glands that produce mucus and gastric secretions. The chief gastric secretions produced by the stomach are hydrochloric acid and an enzyme called pepsin. Pepsin is active only in a highly acidic environment. This enzyme splits protein into smaller groups of amino acids called polypeptides. Hydrochloric acid also dissolves minerals and kills bacteria. There is a muscular valve that controls the passage of food out of the stomach. This valve is known as the pyloric sphincter.
What prevents the stomach from digesting itself? Mucussecreting cells line the surface of the stomach. The mucus helps protect the stomach lining hydrochloric acid. About 500,000 cells of the stomach lining are shed every minute. As a result of this process, the cell layer replaces itself every three days. Occasionally too little mucus or too much acid exists in the stomach. An open, painful sore called an ulcer may than form in the stomach lining. Ulcers may sometimes bleed severely. They can also eat completely through the stomach wall, leading to much more serious conditions.
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