The process whereby food is broken down by digestive juices to enable nutrients to be absorbed into the blood and lymphatic system and used for energy, growth and repair.
The digestive process begins in the mouth, with chewing of the food and mixing with saliva, which contains the enzyme salivary amylase that begins the breakdown of starch. Once the food is swallowed and reaches the stomach, it is mixed with acid and pepsin, which begins the breakdown of protein in the food.
When food leaves the stomach it is in the form of a thick creamy liquid called chyme, and is very acidic. As chyme leaves the stomach and enters the duodenum (the first part of the small intestine) its acidity stimulates the liver and pancreas to release bicarbonate, which changes the pH of the chyme, making it alkaline. This alkalinity is necessary for the next stage of digestion, using enzymes secreted mainly by the pancreas but also by the small intestine itself. The pancreas contributes lipase for fat (lipid) digestion, amylase for starch digestion, and protease for protein digestion. Bile released by the gall bladder emulsifies fat particles, enabling pancreatic enzymes to work on them more easily.
Once digested, the nutrients from the food, including vitamins and minerals, are absorbed into the cells which line the small intestine. Finger-like projections called ‘villi’ increase the surface area of the small intestine approximately to the size of a tennis court, thus optimizing absorption. Fatty substances pass into the lymphatic system, only entering the bloodstream once they have travelled as far as the large veins in the neck. Other substances are absorbed into the blood and carried straight to the liver for processing, after which nutrients are distributed and taken up (assimilated) by the cells of the body’s various tissues.
Substances which aid digestion
|The saliva glands||Salivary amylase, also known as ptyalin||Begins to break down starch|
|The stomach(gastric juice)||Gastrin||Hormone stimulating the production of gastric juice|
|Hydrochloric acid||Kills microbes, solubilizes protein. Stimulates secretin production, which starts pancreatic stage of digestion|
|Pepsin||Enzyme which prepares proteins for pancreatic digestion|
|Liver and gall-bladder||Bicarbonate||Neutralizes acid coming from the stomach|
|Bile salts||Emulsify fats|
|Bicarbonate||Neutralizes acid coming from the stomach|
|Proteases (trypsin and chymotrypsin)||Protein-digesting enzymes|
|Small intestine||Carboxypeptidase||Enzyme required for final stages of protein digestion|
|Cholecystokinin||Hormone stimulating the gall bladder to release bile|
|Disaccharidases (sucrase, lactase, maltase)||Enzymes needed to split disaccharides to simple sugars|
|Secretin||Hormone stimulating the production of pancreatic juice|