The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

fifteen of salt, and ten of other soluble substances. The remainder—more than ninety-six per cent. of the total weightis simply water. It is curious to note this continual loss of water, because water is one of the most necessary constituents in our blood. The reasons for its expulsion are two. Firstly, as we have seen, waterexcretion may under certain circumstances make up for lack of salt in the blood; and secondly, the kidneys can only deal with substances in dilute solution. Urea, salt, or creatinine do not form gases and cannot be got rid of in the lungs ; they must be expelled dissolved in water.

A kidney consists of a multitude of coiling tubes. It is built up of tubes of two kinds. Firstly, the arteries, the veins, and the capillaries which connect them together ;

across and examined through the microscope, its porous nature is revealed. The area shown is about 1/16th of an inch long.

and secondly, the kidney-tubules, end-branches of the ureters, in the walls of which the separating processes take place. There are, roughly, a million of these tubules in each kidney of a full-grown man.! The tubules end blindly at one end ; at the other they lead into the ureter. The details of their structure and working are complicated and it would occupy too much space to go into them at all fully ;_ the essential fact is that each kidney-tubule is surrounded by a dense network of blood-capillaries, and that a fluid oozes from the blood vessels through the walls of the tubule into its canal and so to the ureter. But the ooze is not a simple filtering like the lymph-ooze that we have already described. It has filtration as its

! The number of stars that a good eye can distinguish on a winter night is between two and three thousand.

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THE SCIENCE OF LIFE

CHAPTER 2

basis, but the cells of the tubule wall interfere actively and modify it in various ways, hurrying up the departure of unwanted

. Substances and delaying or preventing alto-

gether the escape of those that are desirable. Now this needs energy. A busily excreting kidney is working against diffusion just as a busily contracting muscle is working against gravity or inertia or friction, and in both cases the energy required is produced in exactly the same way—i.e., by burning chemical fuels. As a matter of fact, one gram of kidney tissue consumes, on the average, more oxygen per minute than the same weight of the conspicuously labouring heart, and, like the heart, its oxygen consumption rises three or four times when it is given heavy work to do.

This co-operation of liver and kidneys is the most important excretory mechanism with which we are provided. But among the other organs there are one or two which assist in the process to a small extent, although their primary duties are different. The sweat-glands, for example, are primarily organs which regulate the temperature of the body. Butsince sweat contains a little urea, the sweat-glands assist the kidneys to a certain extent in eliminating this substance. They also eliminate water. Again, the first duty of the salivary glands is to produce a digestive secretion, but they have a subsidiary excretory function. During the chemical operations of our cells minute traces of that deadly poison, prussic acid, are produced ; this is converted by the liver into a relatively innocuous substance, potassium sulphocyanide, which is excreted partly by the kidneys and partly by the salivary glands. In the former case it leaves the body at once, while in the latter it has first to run the whole length of the digestive tube. Similarly bile, the digestive juice made by the liver, has an incidental excretory function, for it carries the products of the continual breakdown of red blood cells out of the body via the bowels. But the most important accessory excretory organ is the large intestine. Among the end-products of vital activity is