Some nutrients, such as minerals and salts, can be absorbed directly into the blood-stream. The digestive system breaks down others, such as carbohydrates, proteins and fats, into smaller nutrient molecules that can be readily absorbed into the bloodstream. The digestive system also disposes of indigestible and unwanted substances from food and body processes.
The gastrointestinal tract is like a food-processing plant with a series of conduits and processing chambers, each with its own special functions.
The digestive process begins in the mouth, where food is chewed and manipulated by the teeth and tongue and mixed with saliva, which contains the enzyme amylase. Amylase begins to break down starch into sugar.
Once chewed, food passes with little or no change through the oesophagus (helped by lubricating mucus from glands in the oesophagus wall) and into the stomach.
The stomach both churns the food and acts as a temporary storage area between the oesophagus and the small intestine. Hydrochloric acid kills bacteria and helps an enzyme called pepsin to begin the breakdown of protein into peptides and polypeptides. Food now takes the form of a semi-fluid mass called chyme.
In the small intestine, lipase, an enzyme produced by the pancreas, breaks down fats into glycerol and fatty acids. The pancreas also releases enzymes that further break down proteins, carbohydrates and fats. Fat digestion is aided by bile from the liver and gall bladder. Digestion is completed in the small intestine by maltase, sucrase and lactase – enzymes that are secreted from the intestinal lining. The final breakdown products of carbohydrates are mono-saccharide sugars – glucose, fructose and galactose; proteins split into individual amino acids; and fats break down into fatty acids and glycerol. These nutrient molecules are absorbed into the bloodstream via the wall of the small intestine. Food that cannot be digested passes into the large intestine.
The large intestine
The main function of the colon (the major part of the large intestine) is to transport undigested food to the rectum where it leaves the body in the form of faeces. The colon also absorbs water from waste matter.