In studying the functions of the kidneys we have to remember in the first place the course of each uriniferous tubule, which may be followed In the annexed diagram (Fig. 418). It begins in the Malpighian body (i). Issuing thence the tubule becomes convoluted (ii), and then it dips down in a long loop (Henle's loop) whose bend (It) is usually in the pyramidal portion. Turning upwards (iv) the loop comes back to the cortex, again becomes convoluted (v), and then opens, sometimes at an acute angle, into a straight tubule (vi) which passes directly downwards, joined by other tubules (vii), till it opens at the apex of the pyramid (ix) into one of the calices.
So far as the water of the urine is concerned, it is generally agreed that it passes from the blood at the glomeruli; it filters, in fact, from the capillaries into the ends of the tubules. According to Bowman's view, it is mainly the water which passes through at the Malpighian bodies, the urea, urates, etc., being secreted from the blood by the large granular epithelium which lines the convoluted tubules.
It seems probable that Ludwig's view is correct, that the water passing through at the glomeruli is partly re-absorbed, but that the function of the epithelium is not confined to this. In experiments, in which indigo-sulphate of sodium was injected into the blood, it was found that this substance is excreted by the epithelium of the tubules, chiefly that of the convoluted tubules. The colour of the substance rendered it possible to see the seat of its excretion. As an inference from this it may be supposed that the epithelium is actively engaged in separating urea and other urinary constituents, perhaps changing some of them in transit.
The actual amount of the urine secreted will depend on the blood-pressure in the vessels of the glomeruli and on the speed with which the blood passes through these vessels. The amount secreted will be increased by increase of pressure in the vessels, as, for instance, by relaxation of the renal arteries, and it will be diminished by any cause which diminishes the pressure in these vessels. Considering the close relationship of the renal vessels to the systemic arteries and veins, it is clear that the blood in the former will be liable to considerable variations in pressure and in the speed of the current from circumstances affecting the general circulation, such as disease of the heart and lungs.