Wechat logo 50 41 d4bae92fc2b9067675bfee08d2b00834882415940f50456376420c2e77a679ac
返回

digestive

digestive
【形容词】1. 消化的;助消化的 2. 【化学】浸煮的; 【名词】消化剂

Definitions

pertaining to digestion. digestive system the organs that have as their particular function the ingestion, digestion, and absorption of food or nutritive elements. They include the mouth, teeth, tongue, pharynx, esophagus, stomach, and intestines. The accessory organs of digestion, which contribute secretions important to digestion, include the salivary glands, pancreas, liver, and gallbladder. ( Mouth. The mouth is the entrance to the alimentary canal; in it the teeth, tongue, and jaws begin the process of digestion by mastication. saliva is secreted into the mouth by three separate pairs of glands (the salivary glands) located under the tongue, inside the lower jaw, and in the cheek. Saliva softens and lubricates the food, and dissolves some of it; it also contains an enzyme called ptyalin that begins the conversion of starches into sugar. Saliva also moistens the inside of the mouth, the tongue, and the teeth, and rinses them after the food has departed on the next stage of its journey. Four passageways meet at the back of the throat: the oral and nasal passages, the larynx, and the esophagus. In the act of swallowing, the entrances to the nasal passages and the larynx are each sealed off momentarily by the soft palate and the epiglottis, so that the food can pass into the esophagus without straying into the respiratory tract. Stomach. Propelled by rhythmic muscular contractions called peristalsis, the food moves rapidly through the esophagus, past the cardiac sphincter (a circular muscle at the base of the esophagus) and into the stomach. Here the peristaltic motions are stronger and more frequent, occurring at the rate of three per minute, churning, liquefying, and mixing the foods with the gastric juice. In the juice are the enzymes pepsin and lipase and, in infants, rennin; a secretion called mucin, which coats and protects the stomach lining; and hydrochloric acid. Together the pepsin and hydrochloric acid begin the splitting of the proteins in the food. The lipase in the stomach is a rather weak fat-splitting enzyme, able to act only on fats that are already emulsified, such as those in cream and the yolk of egg; the intestine has a stronger lipase, and it is there that most fats are digested. The average adult stomach holds about 1.5 liters. The stomach reaches its peak of digestive activity nearly 2 hours after a meal and may empty in 3 to 4½ hours; a heavy meal may take as long as 6 hours to pass into the small intestine. Small Intestine. The food leaves the stomach in the form of chyme, a thick, liquid mixture. It passes through the pylorus, a sphincter muscle opening from the lower part of the stomach into the duodenum. This sphincter is closed most of the time, opening each time a peristaltic wave passes over it. The stomach is much wider than the rest of the canal and also has a J-shaped curve at its bottom, so that the passage of food through the pylorus is automatically slowed until the food is of the right consistency to flow through the narrow opening into the intestine. The small intestine is about 6 meters (20 feet) long. The lining of the small intestine has deep folds and fingerlike projections called villi that give it a surface of about 9 square meters (100 square feet) through which absorption of food can take place.The duodenum, a C-shaped curve with a length of about 25 cm (10 in), is the first and widest part of the small intestine. Into it flows the pancreatic juice, with enzymes that break down starch, protein, and fats. The common bile duct also empties into the duodenum. The bile emulsifies fats for the action of the fat-splitting enzymes.Just below the duodenum is the jejunum, the longest portion of the small intestine, and beyond that is the ileum, the last and narrowest section of the small intestine. Along this whole length, carbohydrates, proteins, and fats are broken down into sugars, amino acids, fatty acids, and glycerin. The lining of the small intestine absorbs these nutrient compounds as rapidly as they are produced. The bulky and unusable parts of the diet pass into the large intestine.





Sound e31a43d441998f862b764d17930467a5a23de45c433219259bed5eb42e3a7615