Digestion is the process of mechanical digestion and chemical cleavage on a soluble, digestible substances, which are then transported by the blood to cells in the body. It is a collection of bodies, carrying out this process is called digestive system. Its structural units are the alimentary canal and digestive glands. The alimentary canal consists of the following departments: oral cavity, pharynx, esophagus, stomach, colon and small intestine. Small the digestive gland contained in large amounts in the mucous membrane of the bodies involved in the processing of food. And the large glands, such as salivary, pancreas and liver are outside the digestive tract and ducts secrete enzymatic juices into its cavity. The juices of the digestive glands contain enzymes that catalyze certain reactions: some groups of enzymes break down proteins, the second is fat, third is carbs.The digestive system in the body of a human performs three functions: secretory, motor and absorptive.Secretory function is in the chemical processing, food juices, which produce the digestive gland. As a result of complex carbohydrates, fats and proteins are broken down into simple soluble monomers are able to penetrate cell membranes.Motor function occurs due to peristalsis (contraction of the muscle wall of the digestive tract. And it promotes thorough mixing of food in the course of moving from one Department system to another.After the process of digestion the nutrients are taken up in lymph and blood flow through certain parts of the mucous membrane of the digestive canal. And thus runs the absorptive function.Since the digestive system inaccessible to direct observation, so there have been developed various methods of investigation: x-ray, ultrasound diagnostic, biopsy, laboratory techniques and etc.