The working of the body
The functioning of the body is activated by the nervous system and performed by the muscular system and the main purpose of the other organs is to keep nerves and muscles in the fit and healthy condition necessary to do their work. The energy for this work is derived from food and from the oxygen which is taken into the blood via the lungs during breathing. The body uses food in three distinct stages namely:
Food is prepared for digestion and digested by the digestive organs in the following manner: food is broken up into smaller particles by the action of chewing, and becomes warm and moist. The chemical action of the digestive juices splits these particles into simple substances which are soluble in water, allowing them to be taken up more easily into the body. The action of the digestive juices is helped by the mixing and churning movements of the involuntary muscles of the stomach and intestines (also called plain or smooth muscles because they are not striped as are the skeletal or voluntary muscles).
In the mouth the food is broken up by chewing, and moistened by saliva, a digestive juice poured into the mouth from several glands (salivary glands)
situated near it. The act of swallowing passes the moistened food down the gullet into the stomach. The stomach acts as a reservoir in which the food is retained until it is thoroughly warmed and reduced to a fluid condition. The walls of the stomach pour out a second juice (gastric juice) which also acts upon the food. Upon leaving the stomach the partially digested food enters the small intestine in a liquid state, and is there acted upon by the digestive juice of the pancreas (pancreatic juice). The bile also acts to some extent upon the food. At the same time the walls of the intestine also secrete a digestive juice (the intestinal juice). The small intestine is the main area in which digestion takes place.
Thus, in its passage through the digestive canal, food is continuously undergoing change. Finally, after passing through the large intestine, the undigested and unabsorbed remainder of the food is expelled from the body. The passing of the food through the digestive canal takes about 24 hours.
A normal diet contains, in proportions suited to the needs of the body, the following three main classes of nutrients:
· Nitrogenous nutrients found mainly in animal foods (meat, fish, etc), and which also form the main part of these foods. Several vegetable foods, however, also contain a considerable amount of nitrogenous nutrients.
· Starches and sugars, found mainly in vegetables, bread, potatoes, sugar, fruits, etc. Meats contain virtually no starch or sugar.
· Fats, found for example in butter, oil, meat, fat and cream.
While all these nutrients serve as sources of energy, it is chiefly starches, sugars and fats which supply energy. In a normal diet starches, sugars and fats amount to more than three-quarters of the total food intake. The main use of nitrogenous nutrients is to repair the break-down in the living structures of the body.
In addition to these nutrients a normal diet contains mineral salts and vitamins. These are necessary to the body, but they are not sources of energy, and the same is true of the water which we drink.
Digestive juices act upon the nutrients in different ways. Saliva only acts upon starches in the food, and gastric juice mainly upon the nitrogenous nutrients. Gastric juice is the only acid digestive juice, and for this reason it has an important action in checking the development of any infective germs which may have been swallowed.
Partially digested foodstuffs are acted upon in the small intestine by pancreatic juice, bile, and intestinal juice. All these are alkaline juices, and together they aid digestion, but the strongest is pancreatic juice. By the joint action of these juices the digestion of all three classes of nutrients is continued and completed in the small intestine where they are reduced to the simple forms in which they can be absorbed into the body.
In the lower part of the small intestine and in the large intestine a certain amount of putrefactive change also takes place. This is due not to the digestive juices, but to the action of germs or bacteria.
The nutrients which have been prepared by digestion are absorbed (ie taken up through the walls of the stomach and intestines) and passed directly or indirectly into the blood. The small intestine is the chief area where absorption of the nutrients take place. Water, however, is largely absorbed in the large intestine.
There are two separate pathways by which the various nutrients reach the general bloodstream of the body:
· Nitrogenous nutrients together with sugars, salts and water are taken into the blood mainly in the walls of the small intestine. They are then carried by a large blood vessel, the portal vein (fig 2.13 PV), to the liver and are to some extent acted upon by that organ before they reach the general bloodstream of the body through the veins leaving the liver (fig 2.13 HV). This special system is known as the portal circulation. In addition to other important functions the liver has to store the sugars and prepare them for further use in the body. Any substances (including poisons) absorbed from the gut must thus pass via the bloodstream to the liver, where they could (as in the case of poisons) damage the liver, or be modified by it.
· Fats, on the other hand, are taken up by the lymphatic vessels of the small intestine and pass through these to veins and so to the general circulation (fig 2.15).
Both pathways of absorption are shown in fig 2.15. A further description of the portal circulation and of the lymphatic vessels will be found in the next section.
The body is a living ``machine'' and its living structures or tissues make use of the nutritional substances and oxygen in order to:
· perform mechanical and other forms of labour
· maintain bodily heat
· repair the constant decline of living structures
In a steam engine coal and oxygen from the air are used similarly to produce heat, which in turn result in useful activity. In the body there is less active ``burning'' or oxidation of nutritional substances which are absorbed in the lungs and blood. In both cases energy is produced in the form of heat, resulting in activity. The burning of coal in the engine converts the coal into smoke and ash. So too the activity of bodily tissues in consequence of oxidation and the conversion of food produce certain waste material of which the body must rid itself.
The most important of these waste materials are:
· carbon dioxide
The process of removing waste matter from the blood and from the body is called excretion.
Carbon dioxide is eliminated by the lungs into the air through breathing. The lungs also give off a considerable quantity of water as moisture in the breath. The lungs thus serve the dual purpose of taking up oxygen from the air into the blood, and of eliminating carbon dioxide from the blood to the air. This is the essential purpose of respiration or breathing.
Water, along with a small quantity of other waste substances, is eliminated by the skin in the form of sweat. The kidneys withdraw from one to one-and-a-half litres of water daily from the blood which contains dissolved mineral salts and several other important forms of waste matter (the most important being urea). These form the urine. The urine passes down from the kidneys to the bladder, from where it is expelled (fig 2.14).
Other waste matter is eliminated by the liver in the bile, and this together with the unabsorbed and undigested portions of the food is expelled from the body through evacuation of the bowels, as faeces or stools. The faeces also contain countless numbers of germs or bacteria, as well as a variety of viruses and yeast.
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