字幕列表 影片播放 列印英文字幕 How our Digestive System Works? Our digestive system is a combination of mechanical and chemical actions. Imagine, putting your food in a petri dish, chopping it up, and exposing it to a bunch of chemicals and microbes. Can you imagine what it would look like in the end? This is what the digestive system does. MOUTH The journey down the alimentary canal begins in the mouth. Here, the food is broken down into smaller chewable pieces. Chewing, breaks the food into pieces, while the saliva mixes with food to begin the process of breaking it down into a form your body can absorb and use. Rolling action of the tongue and secretion of saliva rolls food into a bolus. The saliva contains water, electrolytes, antibacterial components, and enzymes such as amylase. Amylase converts carbohydrates into sugars. THROAT The throat is the region where the mouth cavity and the nasal passages join. Swallowing pushes the food through the throat or pharynx and into the esophagus. An important function of the throat is that it prevents the food from entering into the trachea, more commonly known as the windpipe. When the food enters our throat, the larynx or our voice box closes. This results in epiglottis covering the entrance of the trachea or windpipe. The epiglottis is a flap of tissue. ESOPHAGUS Now that the food has reached the esophagus, a wave of smooth muscle contractions occurs, pushing the food into the stomach. These smooth muscle movements are called peristalsis. The importance of sphincter muscles. Three types of sphincter muscles help in the digestive system. Here, at the junction of the esophagus and the stomach is a thick ring of circular-smooth-muscle, that prevents the movement of food pass from esophagus into the stomach. It is called the esophageal sphincter. Another sphincter muscle, the pyloric sphincter, directs the passage of food from the stomach into the intestine. The third sphincter muscle surrounds the anus. STOMACH Most of us eat our food in a matter of minutes, but digesting it can take hours. One of the important functions of our stomach is to store food until it is digested. Food can be stored here for 2-6 hours. It also kills the microorganisms we consume unconsciously along with our food, and begins the digestion of the proteins we took in our diet. The stomach secretes gastric juice, hydrochloric acid, water, mucus, pepsin, and renin, that continue the process of breaking down the food. Pepsin is secreted as pepsinogen by cells in the gastric glands that are present in the deep folds of the stomach lining. Other cells in the gastric glands produce hydrochloric acid, which has a pH balance between 1 and 3. The low pH helps convert pepsinogen to pepsin and is also the right pH for pepsin enzymatic action. Hydrochloric acid or HCl also helps break the bonds holding the ingested contents together. The breakdown of these food contents exposes more surface area to the action of pepsin, and later to the other digestive enzymes in the small intestine. Mucus secreted by the stomach lines the walls of the stomach and protects them from being digested by HCl and pepsin. If this coating is eroded at some place of the stomach, for instance by the attack of bacteria Helicobacter pylori, it can cause an ulcer. Contractions of the smooth muscles in the walls of the stomach, roll around its contents, mixing partly digested food with enzymes and acids. This acidic fluid mixture of gastric juice is called chyme. Peristaltic movements of the stomach walls push chyme towards the end of the stomach. These waves of peristalsis cause the pyloric sphincter to relax briefly, so that very little amount of chyme can enter the small intestine. In this way, our stomach empties itself gradually over a period of almost four hours. The small intestine works on a small amount of food at a time. We'll continue to explain the small intestine, pancreas, liver, gall bladder, down to colon and rectum in the next video. Actually, we were told to make this title into two parts, so we can publish more. It's a cheat, really. But a new project is a kind of refreshing to our animators and designers, giving them some false sense of novelty and freedom. So, at least we've got that going for us, which is nice.