Chapter 7
Digestive system
Anatomy and Physiology of the Digestive System
The digestive system has three main functions: digesting food, absorbing nutrients, and eliminating wastes. Digestion is the mechanical and chemical breakdown of food into forms that cell membranes can absorb. Mechanical digestion breaks large pieces into smaller ones without altering their chemical composition. Chemical digestion breaks food into simpler nutrient molecules like gluco, triglycerides and amino acid. The simpler nutrient molecules are absorbed from the intestines and circulated throughout the body by the cardiovascular system. They are ud for growth and repair of organs and tissues. Any food that cannot be digested or absorbed becomes a waste product and is expelled.
The organs of digestive system are traditionally parated into two major groups: the alimentary canal, or gastrointestinal(GI) tract，and the accessory digestive organs. The alimentary canal is approximately 9 meters long in a cadaver but is considerably shorter in a living person. It consists of the mouth, 爱情傻瓜歌词pharynx, esophagus, stomachangie, and small and large intestines(or colon), rectum, and anus. The accessory structures include the 英语投诉信范文salivary glands, gallbladder, liver, and pancreas, each of which is connect
ed to the alimentary canal by a duct, and crete its products into the alimentary canal.
Oral cavity
The digestive system begins when food enters the mouth and is mechanically broken up by the chewing movement of the 幼儿园学英语teeth. The tongue, with its muscular action, moves the f
ood within the mouth and mixes it with saliva. Saliva contains digestive enzymes that break down carbohydrates and lubricants that make it easier to swallow the food. Taste buds are found on the surface of the tongue and can distinguish the bitter, sweet, sour and salty flavors in our food. The roof of the oral cavity is known as the palate. The roof of the mouth consists of the hard palate, the bony anterior portion, and the soft palate, the flexible posterior portion. Hanging down from the posterior edge of the soft palate is the uvula. The uvula rves two important functions. It helps in the production of speech and is the location of gag reflex. The gag reflex helps prevent us from accidentally inhaling food or liquids without first swallowing. The cheeks form the lateral walls of this cavity and the lips are the anterior opening. The entire oral cavity is lined with mucous membrane.
Teeth
Teeth are important for the first stage of digestion. The teeth in the front of the mouth bite, tear, or cut food into small pieces. The cutting teeth include the incisors, and the cusp
ids, or canines. The remaining teeth grind and crush food into even finer pieces. The grinding teeth include the bicuspids or premolars and the molars. A tooth can be subdivided into the crown and root. The crown is the part of the tooth above the gum line. The root is below the gum line and anchors the tooth in the jaw bone. The crown of the tooth is covered by a layer of enamel, the hardest substance in the body. Under the enamel is dentin, which makes the main bulk of the tooth. The hollow interior of a tooth is the pulp cavity关于环境的英语作文 in the crown and root canal in the root. The cavities contain soft tissue made up of blood vesls, nerves and lymph vesls.
Humans have two ts of teeth. The first ts, often called baby teeth, or milk teeth, are the cute是什么意思英文deciduous teeth. There are 20 teeth in this t erupt through the gums between ages of 6 to 28 months. At approximately 6 years of age, the teeth begin to fall out and are replaced by the 32 permanent teeth. This replacement process will continue until about 18 to 20 years of age.
Pharynxsince
After food has left the mouth, it enters the oropharynx and then the laryngopharynx. The epiglottis covers the larynx and trachea so that food is shunted away from the lungs and into esophagus.
克龙Esophagus
The esophagus is a muscular tube that is about 10 inches long in adults. Food entering the esophagus from the pharynx is delivered to the stomach. The food is propelled along the esophagus by wave-like muscular movement called 有理数混合运算计算题peristalsis. In fact, peristalsis will work to push food through the entire gut tube.
Stomach
The stomach is on the left side of the abdominal cavity and is hidden by the liver and diaphragm. This J-shaped muscular organ acts as a bag or sac to collect, churn, digest and store food. Different regions of the stomach are the cardiac region(the area surrounding the cardiac orifice through which food enters the stomach from the esophagus), the fundus (the expanded portion of the stomach, superolateral to the cardiac region), the body or main portion (midportion of the stomach, inferior to the fundus), and the funnel-shaped pyloric region(consisting of the superior-most pyloric antrum, the more narrow pyloric canal, the terminal pyloris, which is continuous with the small intestine through the pyloric sphincter).
The folds in the lining of the stomach are called keyboardrugae. When the stomach is filled with food, the rugae are stretched out and disappear. Hydrochloric acid is creted by glands in the mucous membrane lining of the stomach. Food mixes with HC1 and other gastric juices to form a liquid mixture called chyme, which then pass through the remaining portion of the digestive system.
