THE DIGESTIVE SYSTEM
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The Digestive system is the series of organs that process and convert food into simpler substances that the body uses for nourishment. Starch and complex sugars are digested to simple sugars, fats to fatty acids and glycerine, and proteins to amino acids. These simpler substances consist of small molecules that can pass through the intestinal wall into the bloodstream for distribution to all parts of the body.
The digestive system consists of the alimentary canal - mouth, pharynx, esophagus, stomach and small and large intestines – aided by secretions from the liver and pancreas.
The teeth break up food by chopping and grinding it into fine particles. Glands in the mouth lubricate and moisten food with saliva, which also contains a digestive enzyme. The tongue conveys food to the throat, and the pharynx muscles push it down the esophagus (gullet) that leads to the stomach.
The stomach both stores and helps to digest food. It turns food around and mixes it with gastric juice, which includes hydrochloric acid to provide the acid medium needed for enzyme pepsin to break down protein. The partly digested food (chyme) passes from the stomach to the small intestine, usually after two to five hours.
The digestive process is completed in the small intestine, a narrow muscular tube about 6 meters long. Enzymes from the pancreas mix with enzymes from the duodenum. Bile, made by the liver and stored in the gall-bladder, also enters the small intestine. Bile helps in the digestion of fats.
The digested food particles are then absorbed by lymph or blood vessels in the intestinal wall. Tiny finger-like projections (villi) on the walls of the small intestine increase the surface area that can absorb the food. The digested particles are then carried by the bloodstream to the liver, which converts them into substances needed by the body.
Eaten material that cannot be digested as food, such as plant fiber, passes into the large intestine, which is about 1.5m long. There water is removed from the liquid waste, and bacteria convert it to its final form, feces. The waste material is excreted from the body through the end of the large intestine (rectum).
Food is propelled along the wavelike contraction of muscles in the stomach and intestines. This is called peristalsis. The food moves in one direction only. Sphincters, circular muscles that close tightly, prevent the food from moving backward. There are sphincters at the lower end of the esophagus, at the exit from the stomach, at the lower end of the small intestine, and at the exit from the rectum.
One fairly common disorder of the digestive system is ulcers of the stomach or duodenum. If bile stagnates in the gall-bladder because of a blocked bile duct, gallstones can form and must be removed surgically.
Disorders of the intestinal tract include colitis, diverticulitis, diverticulosis, and enteritis.