Alimentary tract, food digestion

The food that we eat (or drink) enters the alimentary tract, which is the passage that starts at the mouth, passes through the food-pipe (esophagus), stomach, small and large intestine, rectum (lower end of bowel) and ends at the anus. The part of this passage from the stomach downward is also referred to as the gastrointestinal tract. Digestion of food starts in the mouth, firstly by the act of biting and chewing, and secondly through the action of the saliva. Saliva, which comes from small glands (salivary glands) situated below and on the side of the jaws, contains biochemical substances called enzymes. Enzymes that play a part in the digestion of food are referred to as digestive juices. These salivary enzymes start the presses of digestion. Further digestion takes place in the stomach (with the help of other enzymes and hydrochloric acid). Partially digested food passes from the stomach into the duodenum (the part of the tract immediately below the stomach). Pancreatic enzymes-enzymes produced in the pancreas, which is a small organ situated behind and below the stomach – are transported to the duodenum through a small tube (the pancreatic duct). These pancreatic enzymes, trypsin being one of the most important for digestion of proteins, play a most effective role in the main process of digestion of the fats, proteins and carbohydrates. From the duodenum the digested food passes to the small intestine, where intestinal juices (from the cells of the lining of the intestine) further help to complete the process of digestion.

In the small intestine, the digested foods are absorbed into the blood stream, which carries them into the liver. The undigested and unabsorbed food, along with most of the water and the intestinal juices, now passes on to the large intestine (colon). At the junction of the small and large intestine is a small outgrowth of the intestine, called the appendix. The terminal part of the large intestine is called the rectum. The opening of the rectum to the outside is called the anus. In the colon most of the water and minerals are absorbed. What is left behind is excreted as stools (feces).

The liver is situated in the upper part of the abdomen to the right of the stomach. In the liver, the nutrients are reassembled to suit the needs of the body, and distributed to the various parts of the body. Some nutrients, (e.g. vitamins A and D, folic acid, vitamin B 12, iron, etc.) are stored in the liver. In addition to nutrients, other substances are absorbed and transported to the liver. These may be drugs, alcohol, poisons etc. The liver has a limited capacity to safeguard the body against harmful substances, by neutralizing the harmful effects. For example alcohol is utilized to provide energy (calories) with the aid of Vitamin B 1. As a result of the various activities that go on in the liver; some waste products are formed within it. Most of these are expelled into the duodenum as a thick greenish-black liquid called bile. Bile contains substances such as bile pigments (resulting from the breakdown of red blood cells), bile salts, cholesterol, and some enzymes; Bile drains out of the liver through a small tube (bile duct) into the gall bladder, and from there into the duodenum.