A diuretic is a remedy which tends to promote the flow of urine. Diuresis is copious flow of urine.

The kidney is a highly vascular organ, with numerous vasomotor nerves and readily influenced arterioles. Its function is to preserve the normal composition of the body fluids by ridding the blood of certain substances which are present in excess or are not normal constituents, hence it reacts readily to changes in the blood composition.

The blood from the renal artery passes along the afferent arterioles into the capillaries of the glomeruli, and there loses a certain amount of water, containing substances in solution. This escapes through the endothelium of the capillaries and their covering membrane of Bowman's capsule into the uriniferous tubule; while the blood, thus concentrated, leaves the glomerulus by an afferent vein, which is smaller than the afferent arteriole (perhaps only two-thirds the size). "This vein divides into branches after the manner of an artery, and from these arises a dense network of capillaries which everywhere ramify over the wall of the uriniferous tubule" (Quain). The blood in the capillaries surrounding the tubule is, therefore, blood with a diminished total of dialyzable substances and concentrated by the loss of water; and it differs by so much from the blood in the capillaries of the glomeruli.

The average daily urine amounts to about 1500 c.c., is of acid reaction, and contains about 33 gm., i. e., 2.2 per cent. of urea; while the blood from which it is derived is alkaline and contains only 0.05 to 0.1 per cent. of urea. The liquid must, therefore, undergo striking changes in its passage from the glomerular capillaries to the ureter.

We might review very briefly the functions of the different parts of the kidneys:

The Glomerulus - -While there seems to be no doubt that this acts largely as a mechanical filter, there is some evidence that its cells may, in addition, select and secrete certain of the elements of the blood. Brodie believes it to be an expulsor organ, capable of expansion and contracton.

The Tubules

That the tubules have the power to reabsorb water and some of its dissolved substances is apparent from a number of experiments. Cushny showed not only that water was absorbed, but that there was a differential reabsorption of certain of its salts, apparently in proportion to their diffusibility, e. g., sodium chloride more readily than sodium sulphate. He found also that in marked diuresis the proportion of these salts in the urine was more nearly equal; and he figured that reabsorption failed to take place because of the rapidity of the passage of the liquid through the tubules. Moreover, destruction of the tubule cells experimentally or by disease is regularly followed by increase of urine excretion.

That the tubules have also a specific secretory power is suggested by the results of the injection of sodium sulphindigotate into the blood. Within a minute or two the urine secreted is blue, showing that the pigment passes out in the urine. If the kidney is at once removed and the coloring-matter fixed by perfusion with alcohol, microscopic examination shows the tubule cells deeply stained with blue, while the glomeruli are not stained at all. This suggests that the pigment has passed through the tubule cells (presumably was excreted) rather than through those of the glomerular capillaries. Again, if the blood-pressure is reduced below 40 mm. mercury (below which pressure all urine flow ceases), the cortex alone is blue, and the pigment is found deposited in granules in the striated epithelial cells and the lumen of the first and second convoluted tubules. After the injection of uric acid in a solution of piperazin Starling found uric acid in the cells and lumen of the convoluted tubules. Nussbaum's experiment on the reno-portal vein of the frog and some experiments on poisoned kidneys also point to a specific secretory power.

By injecting acid indicators into the blood it may be shown that the glomerular fluid is alkaline, and that the urine becomes acid in the convoluted tubules; if it is hurried through the tubules by active diuresis, it is less acid and may be alkaline.

Without entering further into the theories of kidney action, which are not yet soundly established, and can be read up in any recent book on physiology, we will assume that the function of the glomerulus is to pass from the blood to the uriniferous tubules large quantities of an alkaline fluid which contains urea, chlorides, phosphates, sulphates, and under some circumstances sugar and other substances, in the proportion in which they occur in the blood. And that the functions of the tubules are: (1) To change the reaction of the glomerular fluid to acid. (2) To add to it certain substances by excretion, such as urea, uric acid, creatinin, urinary pigment, phosphates, and, under certain circumstances, water. (3) To concentrate the urine, by the reabsorption of much of its water and of some of its dissolved substances. These are reabsorbed somewhat according to their absorption power, i. e., sodium chloride readily, sulphates less readily, and urea not at all. But it has been demonstrated that the excretion of various substances reaches its maximum at different times and not necessarily when diuresis is greatest.

As the function of the kidneys is in large measure to keep the blood of normal composition, even minute quantities of foreign substances, such as potassium iodide, or excessive quantities of normal constituents, such as sodium chloride and sodium bicarbonate, may be rapidly excreted without any apparent re-absorption.

The urine is, therefore, made up essentially of - (1) water, (2) such dissolved substances as have been removed from the blood in the glomeruli and have escaped reabsorption, and (3) the substances excreted by the tubule cells. Either its quantity or its quality may be changed by an alteration - (1) in the constituents of the blood; (2) in the filtration or secretory power of the glomeruli; (3) in the secretory power of the tubules; or (4) in the reabsorptive power of the tubules; but in the production of diuresis we are not always certain which of these are the factors involved.