Digestive system
The
food we eat is usually very complex (see Food:
The chemicals of life) and our bodies cannot use it in this form.
Most animals, including humans, have evolved a system that breaks down
the complex molecules in our food into simple units that can be easily
absorbed and transported throughout the body by the bloodstream.
The human digestive system is about 8 metres long. It has several main
organs with specific roles contributing to the overall process of digestion.
Mouth
The teeth start digestion by physically reducing
the food into smaller pieces (mechanical digestion). The canine teeth
rip through flesh, the incisors bite off chunks and the molars grind
and mash. This is helped by the rolling action of the tongue and the
moistening effect of saliva. Mechanical digestion increases the surface
area of the food so that the digestive juices can act more efficiently
to break down the complex food chemicals (chemical digestion).
Saliva lubricates the food for easy swallowing, as well as containing
the enzymes needed to commence starch breakdown. Enzymes are protein
molecules produced by specialised glands. These "digestive juices"
increase the rate of chemical reactions, with a separate enzyme required
for each specific reaction.
The
epiglottis is a flap which prevents swallowed food entering the trachea
and lungs. Instead it
continues down the oesophagus (gullet) to the stomach.
Stomach
The strong muscle walls of the stomach churn the food to make a thick
"soup" (chyme). Acid and protein-digesting enzymes are added
to the chyme by specialised gland cells in the stomach wall. The acid
maximises the action of the protein-digesting enzymes. Mucus prevents
the acid and enzymes digesting the muscles of the stomach wall (which
are made of protein), although stress can reduce the effectiveness of
the mucus and cause ulcers. (The "burning" sensation in the
throat after vomiting is due to this acid.)
Small intestine
Food passes along the digestive tract by peristalsis, wave-like contractions
of the smooth muscle in the walls.
Digestion of carbohydrates and protein is completed in the first
section of the small intestine (duodenum), involving enzymes from glands
in the intestine walls and the pancreas. Bile from the liver (stored
in the gall bladder) aids fat digestion.
By the time food reaches the second section of the small intestine
(ileum), all food has been reduced to simple units (except for plant
cellulose "fibre" which passes through largely undigested).These
simple units are now absorbed into the bloodstream through the villi
of the intestine wall (see Systems,
transport and exchange). These capillary-rich folds increase the
area for absorption of nutrients to about 300 square metres.
Excess sugars are stored in the liver as glycogen. The hormones insulin
and glucagon regulate the uptake or release of sugar to maintain a constant
sugar level in the blood regardless of the body's activities.
The circulatory system then carries the absorbed nutrients from the
liver to all parts of the body.
Large intestine
Water loss is a major problem for organisms which live on land.
Therefore it is essential that water is reabsorbed from the chyme in
the large intestine to form the solid faeces containing fibre and other
undigested food. These are stored in the rectum and released through
the anus by the anal sphincter out of the body.
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