The Human Digestive System Biology Revision

1155 words - 5 pages

The Human Digestive System
The Digestive System:
· Eating food involves several different processes:
· Ingestion - taking food into the body and the alimentary canal (through the mouth in humans).
· Digestion - the breakdown of food; mechanical digestion breaks down  food into smaller pieces (such as by chewing and the muscular action by the stomach) ready for chemical digestion by enzymes.
· Absorption - the movement of digested food molecules from the gut into the bloodstream.
· Assimilation - the process by which simple food molecules are made into complex molecules in the body so that the body can use them.
· Egestion - the removal of undigested materials (faeces) from the body.
· All these different processes take place in different parts of the alimentary canal.
· The alimentary canal is a long tube through the body, from the mouth where the food is ingested through to the oesophagus, stomach, small intestine and large intestine, to the anus where faces are egested.
· You can say that the materials in the alimentary canal are not truly in the body.
· Not until food molecules are absorbed do they cross cell membranes into body tissue. Then they can be assimilated and waste products can be excreted.
· The digestive system includes the alimentary canal and the other organs that contribute to digestion, such as the liver, pancreas and gall bladder.

Part of digestive system
What happens there

Mouth
Teeth and tongue break down food into smaller pieces. Saliva from salivary glands moistens food so it is easily swallowed and contains the enzyme amylase to begin breakdown of starch.

Oesophagus (gullet)
Each lump of swallowed food, called a bolus, is moved from the mouth to the stomach by waves of muscle contraction called peristalsis.

Stomach
Food enters through a ring of muscle known as a sphincter. Acid and protease enzymes are secreted to start protein digestion. Movements of the muscular wall churn up food into a liquid known as a chyme (pronounced ‘kime’). The partly digested food passes a little at a time through another sphincter into the small intestine.

Liver

Cells in the liver make bile. Amino acids not used for making proteins are broken down to form urea which passes to the kidneys for excretion. Excess glucose is removed from the blood and is stored as glycogen in liver cells.

Gall bladder
Stores bile from liver. The bile is passed along the bile duct into the small intestine where it neutralises the stomach acid in the chyme.

Pancreas
Secretes amylase, lipase and protease enzymes, as well as sodium hydrogencarbonate, into small intestine.

Small intestine (made up of duodenum and ileum)
Secretions from the gallbladder and pancreas as well as carbohydrase, protease  and lipase enzymes from the wall of the small intestine complete digestion. Digested food is absorbed into the blood through the villi.

Large intestine (or colon)
Water and some...

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