Digestion

From Free net encyclopedia

Digestion is the process whereby a biological entity processes a substance, in order to chemically convert the substance into nutrients. Digestion occurs at the multicellular, cellular, and sub-cellular levels, usually in animals.

Digestion usually involves mechanical manipulation and chemical action. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical manipulation.

1 Human digestion
2 Digestive organs
3 Digestive hormones
4 External links

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Human digestion

See: Gastrointestinal tract

In humans, digestion begins in the mouth where food is chewed with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of food, particularly carbohydrates. Chewing also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus. The esophagus is about 20 centimeters long. The food is pushed down by a movement called peristalsis, which is the wave-like contraction of smooth muscle tissue characteristic of the digestive system.

The food enters the stomach upon passage through the cardiac sphincter. In the stomach, food is churned and thoroughly mixed with a digestive fluid, composed chiefly of hydrochloric acid, and other digestive enzymes to further decompose it chemically for a few hours. As the acidic level changes in the stomach and later parts of the digestive tract, more enzymes are activated or deactivated to extract and process various nutrients.

After being processed in the stomach, food is passed to the small intestine via peristalsis. It passes through the pyloric sphincter and enters the first 10 inches of the small intestine, the duodenum, where it is further mixed with 3 different liquids, which are bile (which helps aid in fat digestion), pancreatic juice (made by the pancreas), and intestinal juice. They also add several enzymes: maltase, lactase and sucrase, to process sugars. (Bile also contains pigments that are by-products of red blood cell destruction in the liver; these bile pigments are eliminated from the body with the feces.) Most nutrient absorption takes place in the small intestine. The nutrients pass through the small intestine's wall, containing small, finger-like structures called villi. The blood, which has absorbed nutrients, is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.

After going through the small intestine, the food then goes to the large intestine. The large intestine has 3 parts: the cecum (or pouch that forms the T-junction with the small intestine), the colon, and the rectum. In the large intestine, water is reabsorbed, and the foods that cannot go thorugh the villi such as fiber, can be stored in large intestine. Fiber helps to keep the food moving through the G.I. tract. The food that cannot be broken down is called feces. Feces is stored in the rectum until it is expelled through the anus.

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Digestive organs

Organisms develop specialized organs to aid in the digestion of their food, for example different types of tongues or teeth. Insects may have a crop (or the enlargement of oesophagus) while birds and cockroaches may develop a gizzard (or a stomach that acts as teeth and mechanically digests food). A herbivore may have a cecum that contains bacteria which can produce cellulase that helps break down the cellulose in plants. Ruminants, for example cows and sheep, have a specialised fore-stomach called a rumen where microbes help to break down cellulose before the food passes onto the "true" stomach or abomasum.

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Digestive hormones

There are at least four hormones that aid and regulate the digestive system:

Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.
Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
Gastric inhibitory peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach.