Try not to become a man of success, but rather try to become a man of value - Albert Einstein
THERE is a striking contrast between the durability of our body and the transitory character of its elements. Man is composed of a soft, alterable matter, susceptible of disintegrating in a few hours. However, he lasts longer than if made of steel. Not only does he last, but he ceaselessly overcomes the difficulties and dangers of the outside world. He accommodates himself, much better than the other animals do, to the changing conditions of his environment.
He persists in living, despite physical, economic, and social upheavals. Such endurance is due to a very particular mode of activity of his tissues and humors. The body seems to mould itself on events. Instead of wearing out, it changes.
Our organs always improvise means of meeting every new situation. And these means are such that they tend to give us a maximum duration. The physiological processes, which are the substratum of inner time, always incline in the direction leading to the longest survival of the individual. This strange function, this function, this watchful automatism, makes possible human existence with its specific characters. It is called adaptation.
All physiological activities are endowed with the property of being adaptive. Adaptation, therefore, assumes innumerable forms. However, its aspects may be grouped into two categories, intraorganic and extraorganic. Intraorganic adaptation is responsible for the constancy of the organic medium and of the relations of tissues and humors. It determines the correlation of the organs. It brings about the automatic repair of tissues and the cure of diseases.
Extraorganic adaptation adjusts the individual to the physical, psychological, and economic world. It allows him to survive in spite of the unfavorable conditions of his environment. Under these two aspects, the adaptive functions are at work during each instant of our whole life. They are the indispensable basis of our duration.
Whatever our sufferings, our joys, and the agitation of the world may be, our organs do not modify their inward rhythm to any great extent. The chemical exchanges of the cells and the humors continue imperturbably. The blood pulsates in the arteries and flows at an almost constant speed in the innumerable capillaries of the tissues. There is an impressive difference between the regularity of the phenomena taking place within our body and the extreme variability of our environment.
Our organic states are very steady. But this stability is not equivalent to a condition of rest, or equilibrium. It is due, on the contrary, to the unceasing activity of the entire organism. To maintain the constancy of the blood's composition and the regularity of its circulation, an immense number of physiological processes are required. The tranquillity of the tissues is assured by the converging efforts of all the functional s ystems. And the more irregular and violent our life, the greater are these efforts.
For the brutality of our relations with the cosmic world must never trouble the peace of the cells and humors of our inner world. The blood is not subjected to large variations of pressure and volume. However, it receives and loses a great deal of water in an irregular manner. After each meal, it takes in the fluids absorbed by the intestinal mucosa from the food and the digestive juices.
At other moments its volume tends to decrease. In the course of digestion, it loses several liters of water, which are used by the stomach, intestines, liver, and pancreas for manufacturing their secretions. An analogous phenomenon occurs during violent muscular exercise, a boxing-match for example, if the perspiration glands work actively. Blood also diminishes in volume in the course of certain diseases, such as dysentery or cholera, when a great deal of liquid passes from the capillary vessels into the lumen of the intestine. The administration of a purgative is followed by a similar waste of water.
The gains and losses are exactly counterbalanced by mechanisms regulating the blood volume. These mechanisms extend over the whole body. They maintain constant both the pressure and the volume of the blood. The pressure does not depend on the absolute amount of the blood, but on the relation of this amount to the capacity of the circulatory apparatus. This apparatus, however, is not comparable to a system of pipes fed by a pump. It has no analogy with the machines constructed by man. Arteries and veins It has no analogy with the machines constructed by man.
Arteries and veins automatically modify their caliber. They contract or dilate under the influence of the nerves of their muscular envelope. In addition, the walls of the capillaries are permeable.
The water of the blood is thus free to enter or to leave the circulatory apparatus. It also escapes from the body through the kidneys, the pores of the skin, the intestinal mucosa, and evaporates in the lungs. The heart realizes the miracle of maintaining constant the pressure of the blood in a system of vessels whose capacity and permeability ceaselessly vary. When blood tends to accumulate in too large a quantity in the right heart, a reflex, starting from the right auricle, increases the rate of cardiac pulsations, and blood escapes more rapidly from the heart into the vessels.
Moreover, serum traverses the wall of the capillaries and inundates connective tissue and muscles.
In this manner, the circulatory system automatically ejects all excess of fluid. If, on the contrary, the volume and the pressure of the blood diminish, the change is recorded by nerve endings hidden in the wall of the sinus of the carotid artery.
This reflex determines a contraction of the vessels and a reduction in the capacity of the circulatory apparatus. At the same time, the fluids of the tissues and those contained in the stomach pass into the vascular system by filtering through the wall of the capillaries. Such are the mechanisms responsible for the nearly perfect constancy of the amount and the tension of the blood.
The composition of the blood is also very stable. Under normal conditions, the quantity of red cells, plasma, salts, proteins, fats, and sugars varies only in a small measure. It always remains higher than is really necessary for the usual requirements of the tissues. Consequently, unforeseen events, such as privation of food, hemorrhages, or intense and prolonged muscular efforts do not modify in a dangerous manner the state of the organic fluids.
The tissues contain abundant reserves of water, salts, fats, proteins, and sugar. Oxygen, however, is not stored anywhere. It must be unceasingly supplied to the blood by the lungs. The organism needs variable quantities of this gas, according to the activity of its chemical exchanges.