Digestive System
We require food for health, survival, movement and work. Materials required by our body are present in the foods that we eat. But most of them are not absorbed into our body in their original form; on the contrary they have to be digested. Digestion is the process of breaking up the food that we eat into a form that can be absorbed into our body to produce nourishment. And this important function is performed through the digestive system. Digestive system consists of a tube which begins from the mouth and ends at the last outlet (anus). Passing through this small workshop, food undergoes hundred of thousands processes and fine and amazing reactions and becomes digested and absorbable. So many elaborate processes take place, which can hardly be described over here. Some of them are mentioned by way of examples.
We shall divide the digestive tract into parts and explain the function of each part in brief.
Mouth: Some organs are present in the mouth which are effective in digestion: lips, tongue, teeth (in particular way), jaws and three pairs of salivary glands.
When the morsel enters the mouth, the lips close the entrance so that the food may not fall out. The tongue rotates in the mouth and places the morsel under the teeth. The lower jaw moves up and down so that the morsel may be chewed and become soft. All this time, the salivary glands secrete liquids and enzymes on the surface of the morsel so that it may help in digestion. Secretion of salivary glands continues at other times also and keeps the tongue and lips moist to facilitate speaking and breathing and also to help in the sense of taste.
Throat and the process of swallowing: Three passages are present in the gullet, the nasal tube, respiratory tube and the food tract, which is connected to the stomach. Food must not enter the air tube or it would disrupt breathing and we would choke. When the morsel in the mouth is ready to be swallowed, the tongue gathers it and sends it to the throat. At that same time the uvula is raised and closes the nasal tube. The air tube is also closed by the epiglottis. The only thing open at that moment is the food tube and the morsel enters it and by the worm like movements is propelled to the stomach.
Circular muscles in the last part Esophagus (cardia) is normally contracted and it prevents the stomach juices from entering the esophagus, but on receiving worm-like motions to this end, the contraction of muscles goes away and the entry of food to the stomach is facilitated.
Stomach: The food matter remains in the stomach for a period of time. Different types of foods stay in the stomach for different lengths of time. After the food enters the stomach, weak contractions appear in its muscles. They become stronger and more frequent gradually. These contractions in the form of worm-like waves begin from below (cardia) and throughout the length of the stomach travel to the pyloric sphincter, and as they reach the pyloric sphincter they become stronger and causing the softening of food matter and its mixing with the gastric juices. There are numerous glands in the inner wall of the stomach that secrete enzymes and special liquids on the surface of the food; with this action starch is converted into glucose and protein is changed to amino acids. Also the food matter is converted into smaller particles fit to be absorbed by the different cells of the body. The food matter through these actions and reactions assumes the form of a paste, which is called as chime and after that it gradually enters the duodenum.
Duodenum: The initial part of the small intestine is called duodenum. When the food enters the small intestine, it first passes through the duodenum and at this place through pancreas and gall bladder liquids are sprayed on its surface, which affect digestion of food in a systematic manner.
Small intestine: The food does not stay in the small duodenum; on the contrary it passes through it into the small intestine. Matter resultant from digestion of food through the small intestine is absorbed and enters the blood stream, so that with the circulation of blood it may come at the disposal of all the cells of the body. The inner surface of the small intestine has numerous folds through which the digested food is absorbed into the blood stream and transferred to the liver.
Large intestine: The large intestine is the last part of the digestive tract. When food enters the large intestine no useful or absorbable matter remains in it. Things that remain behind in it are a quantity of water, salts and a little undigested food matter. The walls of the large intestine absorb the water and salts and in the end, half solid excrement including dead and alive bacteria and indigestible matter remains in the rectum till it is excreted through the anus.