Metabolism is a word used to describe all processes that take place within the body from the time food is absorbed from the digestive organs until it is passed out of the body through some of the excretory channels. To be more accurate, it means the sum of both the anabolic, or constructive, and the catabolic, or destructive, processes that continually go on in the animal body.

The process of metabolism is chiefly one of tearing apart, or of breaking down, complex chemical substances into simpler forms of matter. Formerly, all processes in animal life were considered to be those of tearing down, or of simplifying, chemical compounds; while plant life was considered to be chiefly the process of building up complex substances from simpler forms of matter. This distinction, however, is rather general with many exceptions. The two terms, "anabolisni" and "catabolism" are sometimes used to distinguish between the processes of building up complex chemical compounds, and the oxidizing or tearing down of such compounds by effort or activity. Thus, the formation of muscular tissue from the digested proteid materials would be a process of anabolism, or construction, while the conversion of glucose in the muscle-cells, into carbon dioxid and water would be an example of catabolism, or destruction.

Meaning of metabolism.

Distinction between ana-bolism and catabolism.

The process of catabolism is, in general, one of oxidation; that is, oxygen is added to the chemical compounds taken from the food we eat, forming simpler substances which are excreted from the body as waste-products. Oxidized carbon in the body forms carbon dioxid; hydrogen is oxidized into the form of water, while nitrogen leaves the body in the more complex and incompletely oxidized substance known as urea, the chemical formula of which is COH4N2. A small portion of nitrogen leaves the body in the form of uric acid, C5H4N403.

Catabolism a process of oxidation.

The process of anabolism usually absorbs energy or heat from the surrounding material, while catabolism produces heat as a result of oxidation, as do ordinary fuels. This explains why muscular work warms the body.

We may study metabolism best by considering the two purposes food serves in the animal body, as follows:

First - The Building Of Actual Body-Tissue

Every atom composing the human body is constructed from food. The number and the proportion of the various chemical elements composing the body are well known, and were it not for the fact that the body is constantly casting out old cells and waste-products, the problem of nutrition would resolve itself into the simple process of supplying the body with the materials needed for growth.

Why muscular work produces warmth.

We could analyze an adult man and a new-born infant, and know that the infant, in order to reach maturity, would need to add to its body so many pounds of oxygen, carbon, sulfur, iron, etc. The problem of nutrition, however, is more complex. Not only must we consider the formation of new tissue, but we must also allow for the rebuilding of the old, and for all those processes of vital activity that involve the consumption of food material and the destruction of body-tissue. Nor can this allowance be accurately proportioned from the analysis of the body, because the various elements composing it do not change with equal rapidity. Thus, a man in a harvest field might pass through his blood in one day ten or fifteen pounds of oxygen (in the form of water and carbon dioxid), which would amount to ten per cent of the oxygen contained in his body, but if he should take calcium or fluorin to the extent of ten per cent of that contained in the body, death from poisoning would speedily ensue.

Formation of new tissue and destruction of old.

We can better understand the use of foods and the process they undergo in building the body by considering separately each class of food material from the time it is absorbed from the alimentary tract until it is excreted from the bowels, or from the lungs and the kidneys, or deposited in the body as bone, fat, or tissue.

Second - The Generation Of Heat And Energy

The second function, or rather group of functions to be considered in the study of metabolism is the generation of heat and energy. If the reader will recall what was said in Lesson II (Simple Principles Of General Chemistry), regarding the production of heat by the process of oxidation, he can more clearly comprehend the method by which heat is produced in the animal body. However, as heat is only one form or expression of energy, these two subjects - heat and energy - should be considered together.

The production of heat and energy in the body occurs almost entirely through the oxidation of food. All three classes of foods, namely proteids, carbohydrates, and fats can be oxidized to produce heat.

Energy may be mechanical, chemical, electrical, or thermal. The conservation of energy, which is one of the fundamental laws of science, teaches that no energy can be lost, but can only be changed into other forms. This being true, and because all energy can be changed into heat, we use heat as a measure of energy.

Heat and energy produced by oxidation.

Heat, a measure of energy.

The unit of heat, and consequently of energy, that is used by scientists is the "calory," which is the amount of heat required to raise the temperature of one thousand grams of water one degree on the centigrade thermometer scale. The energy in food is measured in calories, as will be learned from the explanation given in the lesson entitled "Vieno System of Food Measurement."

The Vieno is merely a unit especially convenient in measuring the energy in food. In order that this energy may be drawn upon or liberated in the body, it is necessary for the food to pass through the process of metabolism, as heretofore described.