Absorption of Digested Food

 Glucose , amino acids, fatty or carboxylic acids and glycerol, as well as , vitamins and mineral salts are absorbed in the small intestine. The walls of small intestine are specially adapted to carry out this function.

     For efficient absorption, a large surface area is necessary. The walls of the small intestine have folds and furrows which serve to increase it's surface area. In addition, there are small finger like projections called villi (singular : villus) which increase the surface area even more.

     The inner surface e layer or epithelium of each villus is thin; this allows the absorption of the end products by either diffusion or active transport through it. There is a rich supply of blood vessels and lymphatic vessels containing blood and lymph respectively to carry away the absorbed food substances. In each villus, there is a blind lymphatic tube called a lacteal surrounded by a network of blood capillaries. The lymph in the lacteals transports carboxylic acids and glycerol which usually recombine to form fats again in the lacteals. The lymphatic vessels eventually empty their contents into the blood vessels near the heart. The blood then carries the fats to where they are needed. Excess fats are stored in fats cells which group together to form adipose tissue, found usually under the skin and around organs. The capillaries transport sugars and amino acids away from the intestine. Mineral salts and vitamins are also absorbed by these capillaries. All these food substance are brought by the blood stream to the hepatic portal vein which runs from the small intestine to the liver. The liver processes the food substances. Some are broken down, some converted to other substances, some stored and some left unchanged. The food is then carried in the bloodstream to the cells in the rest of the body where they are assimilated.

     Undigested food passes into the Colon (large intestine). Here, water is absorbed. This concentrates the waste products and makes them semi solid. In this state, the waste products are called faeces. Faeces pass into the rectum and out of the body through the anus. Movement of faeces is facilitated by dietary fibre or roughage which gives the guy muscles something to squeeze against. If there is insufficient roughage in the diet, constipation will result. Constipation is difficulty in removing the bowels.

      The food we eat during a meal takes about one and a half days to pass through the gut, before the undigested remains are egested out as faeces