The Digestive System
In The Mouth
Digestion begins in the mouth as foods are chewed and mixed with saliva. Enzymes in saliva begin to break down some complex carbohydrate (also called “Starches”). Chewing food thoroughly is essential for good digestion. As we swallow, muscular contraction moves food down the oesophagus to the stomach.
In The Stomach
The cells lining the stomach produce acidic gastric juices which contain some enzymes to begin breaking down protein into smaller pieces. The mass of food in the stomach is churned by the stomach acid to a soupy consistency, ready to pass to the small intestine. Alcohol and some drugs can be absorbed directly from the stomach but most of the mass of food passes through to the small intestine. Carbohydrates eaten on their own tend to leave the stomach quickly. Proteins and fats delay the emptying of the stomach and so make you feel more satisfied after eating. If you consume large quantities of fat, the mass of food will take many hours to leave the stomach and may produce unpleasant feeling of “fullness”.
In The Small Intestine
Most of the digestion of foods and the absorption of nutrients occurs in the small intestine. Here juices from the pancreas neutralise the acid from the stomach. Enzymes from pancreatic juice and intestinal secretions break down carbohydrates, proteins and fats. The liver also produces bile which is stored in the gall bladder until it is needed in the small intestine to help digest fats. Once the various food components have been digested in the small intestine, they are absorbed into the bloodstream and go to the liver. From the liver, the nutrients are distributed to the body and provide kilojoules of energy to all body cells. Dietary fibre and some of the complex carbohydrates are not digested in the small intestine and pass to the large intestine.
In The Large Intestine
Bacteria in the large intestine break down many types of dietary fibre and any carbohydrates which have escaped digestion in the small intestine. During this bacterial digestion, more kilojoules of energy are made available. Most dietary fibre is broken by these helpful bacteria but some types are totally undigested and are eventually excreted. The bacteria themselves multiply by the million as they digest fibre and carbohydrates and they contribute to the faecal material which is excreted. A large amount of water is reabsorbed from the large intestine. if this does not occur, diarrhoea results.