25 October 2012

Digestion, or what happens when we eat?

Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed by the body through the blood stream, for instance. Digestion is a breakdown of large food particles to smaller ones.

When food enters the mouth, its digestion starts by the action of chewing, a form of mechanical digestion, and the contact of saliva (spit). Saliva, which is secreted by the salivary glads, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing chewing and starch digestion, the food will now be in the form of a small, round mass, called a bolus (It will then travel down the esophagus, a tube which leads from the mouth and into the stomach). Gastric juice, which is very acidic, in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and another enzyme called pepsin. As these two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats), the resulting thick liquid is called chyme. When the a valve in the bottom of the stomach opens, chyme enters the duodenum (the upper reaches of the small intestine) where it mixes with digestive enzymes from the pancreas, and then passes through the rest of the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine). Some vitamins, such as some B vitamins and vitamin K produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated during defecation.

When a proper diet and enough water is consumed, this process proceeds very naturally. If not enough water or fiber is consumed, problems may occur. That, however, is a topic for another day.

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