THE DIGESTIVE SYSTEM

Digestion is the process by which biological molecules in food are broken down so that they may be absorbed into the bloodstream and incorporated into our bodies.

Mechanical Digestion is the physical breakdown of large chunks of food into smaller, more easily managed chunks. Chewing, churning of the stomach and the emulsifying action of bile increases the surface area of food so that Chemical Digestion (chemical breakdown of food molecules) can work more efficiently.

Chemical digestion is carried out by digestive enzymes - proteins which speed up the rate at which large biological molecules break down. Examples of digestive enzymes include :

    Pepsin - produced in stomach - breaks down protein into smaller peptides
    Trypsin - produced in pancreas - breaks down small peptides into amino acids
    Lipase - produced in pancreas - breaks down lipids into fatty acids and glycerol
    Amylase - produced in saliva and pancreas - breaks down starch into glucose

Food travels down the oesophagus through muscular contractions (peristalsis) to the stomach. The stomach stores food and churns it to aid in breakdown. The broken down food (chyme) is released into the first part of the small intestine (duodenum).

In the duodenum, the chyme is combined with digestive enzymes from the pancreas and bile from the liver. Bile acts to emulsify fats so that lipase can break down lipids.

Nutrients are absorbed along the remainder of the small intestine, as the chyme is pushed along through peristalsis. In the large intestine (colon), the majority of the water is absorbed from the chyme to form faeces. Faeces is stored in the last part of the colon (rectum) until it is expelled from the body.

Diseases which irritate the lining of the intestine increase the rate of peristalsis. This means that the chyme is moved through the colon too quickly for water to be absorbed. The result is watery stool or diarrhoea. The greatest danger in diarrhoea is from dehydration.

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