Metabolism

Metabolism means tissue change and includes all the physical and chemical processes by which the living body is maintained, and also those by which the energy is made available for various forms of work. The constructive, chemical and physical processes by which food materials are adapted for the use of the body are collectively know as anabolism. The destructive processes by which energy is produced with the breaking down of tissues into waste products is know as catabolism. Basal metabolism is the term applied to the energy changes necessary for essential processes such as the beating of the heart, respiration, and maintenance of body warmth. This can be estimated, when a person is placed in a state of complete rest, by measuring the amounts of oxygen and carbon dioxide exchanged during breathing under certain standard conditions.

Heat production

Heat production results from all the metabolic processes in the body; under conditions of rest it occurs mainly in the skeletal muscles and to a less extent in the glands, particularly the liver and kidney.

It is said that in the tropics heat production at rest is only about 30 per square meter per hour compared with 40 in temperate zones. Conversely, the metabolism is stimulated by cold. Many factors increase heat production, e.g., the ingestion of food and, far more important, muscular activity (unconscious as well as conscious).

Heat loss

Heat loss occurs by 1) vaporization of water from the skin and lungs (25%); 2) radiation of heat from the surface of the body to bodies at a distance (60%); 3) by convection currents (15%). The figures in brackets apply to the resting person in an equable environment. Heat loss by conduction in the well clothed person or by warming the ingested food is of negligible importance. The amount of heat lost by radiation depends on the size of the body surface and on the average temperature difference between the surface of the skin and surrounding objects. Heat loss diminished if the outside air is still, because the body is then surrounded by a layer of warmed air. Moist air is a better conductor of heat than dry air and increases the loss by radiation; a cold dump day is thus less tolerable than a cold dry day. A warm moist day is also unpleasant, as evaporation is interfered with.

About 20-30 litres of air are entangled between the clothes. This is helpful in cold weather by diminishing heat loss by radiation and conduction. In hot weather the clothes retain moisture and the cooling effect of sweating is diminished. Opening the jacket, waistcoat, and shirt gives, as is well know, a feeling of greater comfort, and more moisture can evaporate from the skin surface.

Digestion

The alimentary canal begins above with the cavity of the mouth and terminates below at the anus, traversing in its course the entire length of the thoracic and abdominal cavities.

After the entrance into the mouth the food undergoes a twofold reduction: a mechanical one and a chemical one. The former process is called mastication. The chemical action taking places in the mouth is accomplished by secretion, the saliva. The end product of mastication is the bolus, a rounded mass of food thoroughly moistened with saliva. In this form the food is progected into the stomach where it is again subjected to a mechanical and chemical reduction. The chemical process is accomplished by means of the gastric juice.

The ultimate purpose of gastric digestion is the formation of the chime; in this form the food is transferred into the duodenum.

In the duodenum the food is influenced by the pancreatic juice and the bile, an important external secretion of the liver. The liver is the largest glandular organ in the body which exerts a most powerful influence upon all the metabolic functions of the body.

The principal organ of absorption is the small intestine. The function of the small intestine is to separate the useful from the useless constituents of the absorbing channels of the body, while the latter are allowed to escape into the large intestine to be eventually included in the faeces.

Water

Water makes up two-thirds of the body weight and in active tissues, such as muscle, it is three-fourths the total weight. The body, moreover, has a liquid carrier system. The blood and lymph, which are mainly water, carry food to the tissues and remove from them the waste products of combustion. The chief way of eliminating waste nitrogen from the body is by means of the water passing through the kidneys. In making thirst more insistent than hunger nature makes a constant and insistent demand for water in the diet.

So important is the activity of water than we might almost speak of a distinct water circulation consisting of the passage of water from the diet into the body and its elimination from the lungs in the form of moisture, from the skin in the form of swear, and through the kidneys in the form of urine. The amount of water excreted in these ways by an activity man in the course of a day amounts to over two quarts. Nevertheless, we find that a deficiency of water in the diet, especially among person engaged in a sedentary occupation, is a most common and serious fault. Water may be obtained in the body in a variety of ways. The greatest quantity, of course, is obtained through drinking water and other beverages, such as milk, tea an coffee, but much also is obtained from the solid food. This is notably true in the case of fruits and vegetables, many of which contain over 90 per cent of water. Watermelon is 98 per cent water. About a half pint of water is actually produced in the daily oxidation process. The amount of water necessary for proper waste elimination and for feeding the tissues in person of average weight is four or five pints per day. Of this, at least four to six glasses should be taken in the form of water it.

Hunger

Hunger may be defined as an unpleasant feeling of emptiness, pressure, discomfort or dull pain in the epigastric region. Sometimes other symptoms such as lassitude, weakness, faintness, headache or nausea are a part of the hunger complex, and occasionally one or more of these symptoms may dominate while the gastric sensations are unrecognized. The average individual partaking of three or more meals per day rarely experiences hunger sensations; his intake of food is ordinarily determined by routine and appetite. Real hunger develops when food is withheld for longer intervals of time, after excessive muscular exercise or exposure to extreme cold.