THE FUNCTIONS OF THE The business of the kidney is to separate certain substances from the blood which have gained access to it in its course through the body, and are the result of the decompositions effected in the tissues by their activity. The chief of these substances removed by the kidney is urea, a solid crystalline body. But the ex cretion of the kidney is a fluid—the urine— in which urea and various other organic and inorganic substances are in solution. What is known of the structure of the kidney indicates that in the formation of the urine there are two processes at work. One process is per formed in the Malpighian bodies. An artery enters the capsule of the body and immediately breaks up into a tuft of capillaries. Now the blood is always exerting considerable pressure on the walls of the vessels, and the capillary vessels are very thin walled. One would at once conclude that fluid would ooze through the thin capillary walls and be received in the capsule. The capsule is the expanded end of a uriniferous tubule, so that the escaped liquid would find its way down the tubule, and so into the pelvis of the kidney, into which the tubule opens. As it collected in the pelvis it would flow into the ureter and so reach the bladder. In short, the structure of the Mal pighian body suggests that it is a sort of filter, the filter being formed by the thin walls of the blood-vessels of the tuft with the fine layer of cells which covers them, having on one side of it blood under pressure and on the other side the cavity of the capsule (see Fig. 161). As the blood streams through the capillary tuft, fluid filters from it into the capsule, and the greater the pressure of the blood the more fluid will be passed through. Force and con firmation are given to this view by the fact that the vessel, which the capillaries of the Malpighian body form, and which leaves the capsule, is narrower than the entering artery. The supply pipe is larger than the escape pipe; blood passes into the Malpighian body more readily than it escapes from it. Consequently the blood in the capillaries will be at greater than usual pressure, and filtration of liquid from it will be encouraged. The structure of the apparatus, therefore, suggests that one part of the process of urine formation consists in the separation from the blood by filtration of certain of its fluid constituents. This does not, however, appear to be all. As soon as the
blood has passed out of the Malpighian bodies in the efferent vessel it is distributed over the convoluted tubules by fine capillary blood vessels into which the outgoing vessel breaks up (see Fig. 159). Now the convoluted tubules consist of a very delicate wall lined within by large active cells (Fig. 160), and blood is brought in thin-walled vessels into intimate connection with the tubes. These are just the conditions of secretion. The active cells of the tubules are separated from nourishing blood only by the thin walls of the tubes and vessels. It is therefore probable that the cells separate cer tain substances from the blood, which they work up and then pass into the tubule among the fluid filtered into the capsule and finding its way down towards the ureter. It has been, therefore, concluded that the process of for mation of urine consists of two parts: (1) of a separation of fluid parts of the blood by filtra tion in the Malpighian tufts, and (2) of a sepa ration by active cells from the blood of probably more solid substances, which are added to the filtered fluid in its course down the tubules.
To this view of the action of the kidney there is one great objection. The blood is an albuminous fluid (pp. 294-295). Careful ex periments have shown that if an albuminous fluid be placed on a filter the liquid that passes through contains albumin, though in much less quantity than the original fluid, and that if the solution on the filter contains saline substances dissolved in it, these hinder the passage of the albumin, though they do not arrest it altogether. The experiments have also shown that as more pressure is exerted on the fluid on the filter more albumin will pass through with the fluid. If, therefore, the urine is largely a filtration from the blood under pressure, the fluid ought not only to contain some of the salts of the blood, but also some of the albumin, though less albumin ought to exist in the urine, be cause the blood is a saline solution, than would be if the blood were only albuminous and con tained no salts. In health, however, the urine contains not a trace of albumin. Indeed, if a physician finds albumin in the urine of a patient he regards it as a grave sign of disease. It is then not easy to understand why, if the urine is -mainly a filtration from the blood, it does not in health contain albumin.