Physiology: The Science of the Body

by Ernest G. Martin

THE metabolism that goes on in all our body cells results in the formation of waste products. The chief of these, carbon dioxide, is taken care of by means which were described in the last chapter. Besides this gaseous substance there are produced a number of compounds which are soluble in water and which are poured out from the cells into the tissue spaces surrounding them and which pass thence into the blood stream. The special places in the body wherein these substances leave the blood stream make up the apparatus for the removal of waste. This apparatus must be able to take out of the blood stream all the waste products of cell metabolism except carbon dioxide, also any materials which may have been absorbed into the blood stream from the digestive tract, but not used by the tissues, and finally all the accumulations of water. The discharge of water makes up a topic by itself which will be considered after a word has been said about the apparatus by which the removal of wastes is carried on. Sharing in this function we have first the kidneys, second the liver, and third the sweat glands.

The kidneys are the most important of the organs for removing waste. They are a pair of bean-shaped bodies lying in the small of the back. Each

one is made up of thousands of tiny tubes; each of these tubes starts as a little sac in which is a knot of blood vessels which is really a tiny filter, filtering out from the blood water and the inorganic salts that are dissolved in it.

All the other materials that are in the blood are held back by the filter, so that nothing escapes from it in these places except water and salts. Further along each of the tiny tubes is a section which is surrounded by a network of fine blood vessels where two things take place; the first of these is the taking out from the blood of the various kinds of waste products; the second is the absorbing back into the blood of part of the water which filtered out through the knot at the tip of the tube. Beyond the point where these things happen the tube does nothing except to convey the finished kidney secretion, or urine, down to the outlet where opens a very large tube, the ureter, by which the kidney secretion is carried down to the bladder. The formation of urine is made up then of three stages; first the filtration of water and salts out of the blood, second the escape from the blood into the kidney tube of the various waste products of cell activity, and third, the reabsorption into the blood of part of the water. It is estimated that about four times as much water filters out from the thousands of tiny filters at the tips of the kidney tubes as comes out at their ends to be carried down to the bladder. We do not understand exactly why this should happen; one result of it is to make the urine contain about four times as much salt as the blood; the water that filters out at the beginning of the kidney tube carries with it just the amount of salt that is dissolved; after the water is reabsorbed the salt is left behind in the kidney tube, so that all the salt that filters out stays in the urine, but only about a quarter of the water. The discharge of waste products is easier to understand than the way in which the water is handled; all organic substances which happen to be in the blood and which do not belong there pass out from the blood during its passage through the fine network surrounding the kidney tubes. Of course we do not know just how this is carried on; there must be some special features about the protoplasm in this part of the kidney to cause these substances to pass out from the blood here and nowhere else. Not only waste products of cellular activity, but also all or nearly all the organic accessories of the diet and all drugs taken as medicine leave the blood in this region.

An interesting thing about the removal of water from the blood by the kidneys is the relation it shows to the activity of the sweat glands. The total amount of water gotten rid of in any day is the amount taken in in drinking during the same time. The amount we drink is controlled chiefly by the sense of thirst, assuming that we are in a place where water can be had. The amount of water that passes out in the form of sweat is not under our control; in a later chapter we shall see how the formation of sweat is regulated. There is also a constant and fairly steady loss of water from the lungs, since every breath that we exhale carries with it all the water vapor which it can hold. What the kidneys have to do is to get rid of the water that is not passed out either from the sweat glands or the lungs. The amount of urine that is formed in a day is, therefore, extremely variable; on days that are warm and when a great deal of exercise is taken there will be so very much sweat formed that in spite of copious drinking comparatively little water will be discharged from the kidneys. On the other hand on cold days in which not much exercise is taken there will be a large amount of urine formed. It is interesting to note that some substances like asparagus, and to a less extent coffee, cause a large increase in the amount of urine. We do not know certainly whether these act by increasing filtration from the tips of the kidney tubes or by diminishing the reabsorption in the part farther along, but so far as our information goes either or both may occur.

The kidney is not the only organ by which wastes are removed; the liver has a part in this as well. The action of the liver is not, however, simply to assist the kidney; there are certain special substances which are taken care of by the liver and which are never gotten rid of through the kidneys. The first of these are the broken-down blood pigments. We have already seen that after the red corpuscles become worn out they are decomposed in the spleen and the hemoglobin carried to the liver where the iron and other usable parts are saved and the useless remainder is passed on into the bile to be conveyed out of the body. Bile is green in vegetable-eating animals and golden brown in flesh-eating; the difference depends on the so-called bile pigments which are the waste remains of hemoglobin. The difference in color is the result of a slight chemical difference in the pigments which in turn depends on the diet, so that in man the bile is either green or golden brown according to whether the diet is vegetarian or chiefly of flesh. The liver also takes out from the blood a waste substance which has the formidable name of cholesterin. The interesting thing about this substance is that unless the bile is altogether normal chemically it will not be dissolved and so will be thrown down in the form of little grains or lumps. So long as these grains are small they can pass down the bile duct to the small intestine and be carried out of the body, but after they become bunched together into lumps it is difficult or even impossible for them to be carried down the narrow bile duct. These lumps make up what are commonly called gallstones. The acute pain known as gallstone colic is due to the stretching of the bile duct by the passage of one or more of these stones down it. At the present time no way is known to prevent the formation of gallstones in persons in whom it starts, nor of softening them after they have once been formed; the only relief is through surgery, which, fortunately, is entirely adequate, so that thousands of persons are living in great comfort to-day who would be dead or living in acute suffering if the surgeon had not been able to remove the accumulation of gallstones from the passages in their livers.