To understand the problem, a little must be explained about the intimate structure of the kidney. This organ is made up of many identical small units - in the human about one million in each kidney - called "renal tubules." Each tubule commences in a peculiar structure formed by an invagination of a tuft of very small blood vessels into its expanded end. This structure is called the renal corpuscle or glomerulus. The remainder of the renal unit is composed of a long tube formed of epithelial cells, which, in the mammal, are found by microscopic examination to be of at least three distinct types. This tube is designated the tubule proper, and has a markedly different structure from the glomerulus. Ever since Bowman, nearly a century ago, discovered the true structure of the kidney unit, physiologists have interested themselves in two related problems concerning the production of urine; namely, the assigning of the proper functions to each distinct part of the renal unit and the "explaining" how each part carried out its function.

All theories which have been proposed to explain the secretion of urine by the kidneys have assigned different functions to the glomerulus and to the tubule. It has been generally agreed that the glomerulus eliminates most of the water or fluid part of the urine, but there has been, until recently, quite a controversy as to the nature of the process by which fluid is elaborated by this structure. One school has held that the process occurring at the glomerulus is a physical filtration or ultra-filtration, such as can readily be carried out in a dead collodion or parchment membrane - in other words, a process explicable by known laws of physics and chemistry. On the other hand, some have maintained that glomerular function cannot be explained as a filtration, cannot be expressed at present in terms of physical or chemical laws, or duplicated outside the body. Since the living cells around the glomerulus are, on this view, supposed to furnish the energy for fluid elaboration, the glomerulus is said to "secrete" fluid. Now, how has evidence been obtained favoring one or the other of these theories of glomerular function? We can only review here briefly some of the more important results of recent research, which seem at present to "prove" the filtration theory. The structure of the glomerulus makes it ideally adapted to function as a filter, and this argument from structure has been frequently used. As the rate at which filtration will take place across a membrane will depend, other things being equal, on the pressure-head available, the relation of urine formation to blood pressure has been studied. Two careful series of recent experiments have shown that the rate of urine formation by the kidney - the rate of fluid excretion - varies directly with the blood pressure. Again, if glomerular function is filtration, the fluid formed at the glomerulus should contain all the constituents of the blood plasma in the same concentrations as they exist in blood, except the plasma proteins to which the filter is impermeable. Quite recently a group of workers in Philadelphia have succeeded in collecting fluid from the frog's kidney immediately after it has passed through the glomerulus - a structure in this animal less than one hundredth of an inch in size. The fraction of a drop of fluid obtained has been subjected to analysis, and found to contain certain constituents in the same concentration as they are present in blood plasma, a finding strongly supporting the filtration theory of glomerular function.

In regard to the function or functions of the tubules, opinions have also been divided. Some have held that the cells of the tubules were reabsorptive; that is, substances passed from the lumen of the tube back through the cells to the lymph and blood stream; others have postulated the reverse process, a passage of substances from the blood and lymph into the lumen of the tubule - a process which is designated as secretion; and still others have assumed the tubules to be both reabsorptive and secretory in function. It has now been generally accepted by workers in this field that tubular reabsorption takes place; in fact, tubular reabsorption of some substances at least follows as a necessary corollary of glomerular filtration.

In regard to secretion by the renal tubule, however, there has been a long and active controversy for many years as to whether or not it occurs at all. Recently, experiments on the kidneys of two marine fish - lowly animals and apparently evolved for no other purpose than for physiological experimentation - in which only tubules and no glomeruli occur, have proven definitely that the renal tubule can secrete. How much tubular secretion occurs in the mammal and man, as well as in the different classes of vertebrates, must be determined by further work. Such research is in active progress and leads inevitably into the field of comparative physiology. It is possible that this line of work will also tell us something about the functions of the microscopically distinct parts of a single renal tubule.

It may well be asked if the above controversy about the mechanism by which the kidney eliminates urine is not purely academic, having no practical importance for mankind. Such is most certainly not the case. The kidneys are the organs of the body upon which falls the greatest r61e in keeping the composition of the blood plasma - the internal environment of the organism - at that degree of constancy which is essential to health. Stoppage of kidney function inevitably causes death. For a complete understanding of the problem of the kidneys, such questions as have been outlined must be solved, and it is only when the true solution is reached that we shall be able to interpret kidney disease in terms of physiology, and enable physicians better to cope with it.