When the food enters the stomach the muscular walls contract and relax to churn and mix up the food. The stomach produces hydrochloric acid for two reasons, firstly to kill any bacteria that might be in the food and secondly to provide the correct pH for the protease enzymes to work. The enzyme protease works best at pH 2. The protease enzyme is produced in the stomach. Protease begins the digestion of proteins in the stomach. The large protein molecules are broken down to the smaller amino acids.
Bile is produced in the liver. It is poured in to the small intestine. Bile neutralizes the hydrochloric acid from the stomach so that the pH is correct for digestive enzymes to work in the small intestine. Bile also emulsifies fats so that they are mixed up with the churned up liquid food. Bile is stored in the gall bladder before it is poured in to the small intestine.
The digestive enzymes, protease, carbohydrase, and lipase are also produced in the pancreas.
The food passes from the stomach into the small intestine. The inside of the small intestine is covered with thousands of villi. They greatly increase the surface area of the small intestine for maximum absorption to take place. Villi are tiny finger shaped projections about 0.5mm long. They have microvilli, which further increases the surface area. The walls of the villi are very thin so that fluids can pass quickly through them. Each villus has a very rich blood supply by means of tiny capillaries, which are in contact with all parts of the wall, so that abortion is increased. In the intestine digestive juices from the pancreas and the liver are mixed with the food. Enzymes are contained in the juices. Protease enzymes continue with break down of the proteins into amino acids. Carbohydrates continue with the break down of starch to glucose. Lipase breaks down fats to fatty acids glycerol. All the products of digestion are very small soluble molecules, which can pass through the lining of small intestine. Glucose, amino acids, fatty acids and glycerol are small enough to defuse into the blood stream.
The small molecules of the products of digestion defuse into the capillaries and into the blood stream, which transports them around the body. The hepatic portal vein caries blood from the small intestine to the liver where some of the products of the digestion are stored or changed. These products are released from the liver when they are needed into the blood circulatory system. The products defuse out of the blood cells where they are needed.
The material that passes through the large intestine is mainly water and digested food and this is mainly fibre. No enzymes are secreted here. The bile salts are absorbed and returned to the liver in the blood. The colon absorbs the water from the undigested food so that the body does not to become dehydrated.
Elimination occurs when the semi solid waste passes to the rectum by peristalsis and is passed out of the body at intervals-this is egestion.
