Undoubtedly, the most important function of the skin is protection and it achieves this for us in many ways. First, by any rating, is the prevention of the drying up by evaporation of the fluid which protects all the live cells of the body, since cells have to be wet to live.
Life began in the sea many millions of years ago. That sea was salt and ever since, all animal life has existed in salt water. Water performs its important duties because there is nothing else that will hold so many different substances in solution. Dr. James L. Gamble, a great authority on the fluids of the body, says: “Before our extremely remote ancestors could come ashore to enjoy their Eocene Eden or Paleozoic Palm Beach’, it was necessary for them to establish an enclosed aqueous medium which could carry on the role of sea water.” That is why the skin was formed so early in the development of animals. Fluid will not soak through skin any more than it will soak through leather, which is tanned skin. So we are living, all wet, in a water-tight container.
The body is composed of an enormous number of units called cells and every cell is saturated with fluid. This fluid within the cells comprises about 50 per cent of the body weight. The fluid surrounding the cells might be considered for each of us as our share of the Cambrian Sea, from which our progenitors emerged. Part of it is in the blood and part is in the lymph. If you have ever raised a blister, you have seen lymph. All this fluid outside the cells comes to 20 per cent of the body weight.
The function of the body fluid is to assist in what we call metabolism. This is the infinite number of chemical and physical changes which occur in the body as long as life persists. Substances are taken into the body, changed as necessary, and transported by the fluid to where they build up the protoplasm, the material of which the cells are composed. Then as protoplasm is continuously replaced by new, the old waste products are carried away.
To keep the chemical composition and the physical properties of the body fluids delicately balanced is an intricate piece of work. A lot of apparatus had to be assembled for this as the body grew. The heart stirs and moves the fluid about. The lungs handle the relations of the gases in the atmosphere and within our bodies. The kidneys excrete substances that we do not need and help maintain the chemical and physical balance.
The chemistry of the body fluids is extremely complex. The sea water is referred to as salt, and to most of you salt is the common table salt, sodium chloride. Had you visited the salt works of Cape Cod in my youth and tasted the brine produced by evaporation, you would have realized that there were other salts in it: Glauber’s salt and Epsom salts, for example. Still, sodium chloride is easily the chief one as far as bulk is concerned, both in the ocean and the extra-cellular fluid of the body. But in the intra-cellular fluid potassium chloride has replaced it, although only a gossamer membrane separates the two fluids.
Those of you who have recently been hospital patients realize that treatment by introducing fluids into the veins is much employed. This is undoubtedly a great advance but an enormous amount of research has been required to bring it about and extreme caution in its use is necessary. The tissues do not take kindly to pure water, and the most common fluid used is weakly salted water – just the strength of the body fluids inside you. (The fluid in the body has 0.9 per cent salt. This is the strength which we inject into the veins. We call it normal salt solution.) The variations from this starter are many, but they are all guided by a desire to keep the tissues of the healthy body bathed as though it were still floating in the primitive ocean.
Any cells, which dry out, immediately die. All we see of the skin is the epidermis. This consists of the cells formerly living beneath the surfaces which have now died and dried. They are as inanimate and insensitive as the shell of a turtle. They are used ingeniously by the skin as a protection for the live layer of skin underneath, the dermis.
The fact that the skin retains the fluid surrounding all our cells so it cannot get out and the fact that no matter how long we soak in water, little gets in, may not seem to agree with your experience in sweating. Sweat is, however, a definite secretion of the sweat glands, used chiefly to regulate the heat of the body. This will be described later. When our sweat pours out, it does not mean that any fluid surrounding our body cells is evaporating or escaping.