| From the stomach food enters the duodenum and small intestine, squeezes into the caecum through a small valve, then passes into the ascending, transverse, and descending parts of the colon or large intestine, and finally to the rectum and anus. The total distance between the stomach and the anus is about 8 m (26 ft), the whole journey taking 18-30 hours from the time food leaves the stomach. Liver and pancreas empty their digestive secretions into the duodenum through a common duct, the bile duct. Bile produced by the liver emulsifies fats, and enzymes produced by the pancreas get to work on proteins, starch, and fat.
The liver does many things besides aiding digestion. It stores glycogen (a compact form of glucose) and also vitamins (A, B, D, E, and K); it removes toxins, for example alcohol, from circulation; it manufactures blood proteins and blood clotting agents; it synthesizes cholesterol, a vital component of all cell membranes and the precursor of many hormones; it converts saturated fats to unsaturated fats when the body is obliged to use fat rather than glucose for energy; it breaks down and recycles the iron-rich, oxygen-carrying pigment haemoglobin; it dismantles excess amino acids and excretes their nitrogen-containing components into the blood as urea (urea is one of the constituents of urine); it also produces a great amount of heat because of all the chemical reactions going on inside it.
The pancreas, a gland about 15 cm (6 in) long lying beneath and behind the stomach, produces large amounts of sodium bicarbonate as well as digestive enzymes, sodium bicarbonate is alkaline, and therefore neutralizes the hydrochloric acid produced by the stomach. It also produces two important hormones, insulin and glucagon, which control sugar uptake by every cell in the body.
The gall bladder has no secretory function. It merely stores and concentrates bile sent to it by the liver and empties it into the duodenum in response to a hormone called cholecystokinin, secreted whenever fatty food leaves the stomach. The small intestine has two functions, to complete the food breakdown process, and to absorb nutrients into the bloodstream. Although it is only 6.4 m (21 ft) long, it has a total absorptive area of about 18.5 m2 (200 ft2). This is because its inner walls are lined with millions of finger-like projections called villi, each containing blood capillaries and tiny tributaries of the lymphatic system called lacteals. Amino acids, glucose, small fat droplets, vitamins, and minerals pass into the capillaries, and larger fat droplets into the lacteals.
Whatever remains unabsorbed by the small intestine - about 5 per cent of fats, 10 per cent of amino acids, fibre, some vitamins, bacteria, intestinal secretions, salts, water - passes into the colon where it is gradually compacted into semi-solid faeces. The bacterial populations of the colon manufacture Vitamin K, B12 B1 and B5, and certain amino acids; these are absorbed by the walls of the colon, together with salts, water, and other valuable substances. Transit time through the colon depends mainly on the amount of roughage in the diet. Too little roughage causes constipation, and related ailments such as haemorrhoids and diverticulitis. |
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