We have just seen that the arterial twig, or afferent vessel, which enters the capsule of Malpighi, breaks up into a set of capillary loops, which are only covered by a single layer of extremely thin epithelial cells separating them from the lumen of the urinary tubule, and that the pressure in the vessels of the glomerulus is habitually higher than that in most capillaries, and constantly greater than that of the second capillary network around the convoluted tubules.
The general arrangement of these vessels, and the high pressure in the glomerulus, give the impression that it is simply a filtering apparatus by means of which the fluid parts of the blood pass into the urinary tubules. This view seems supported by the fact that the quantity of urine secreted bears a direct proportion to the blood pressure in the minute renal vessels, whether the change in pressure depends on local vascular mechanisms or on changes in the general blood pressure.
Such a theory, however, cannot adequately explain the formation of urine, because the urine differs so materially from the fluid one could obtain as a filtrate from the blood. In health it contains no albumin, a substance in which the blood is very rich; and it is much richer in urea and salts than the blood. There is, therefore, both a quantitative and qualitative difference, which implies a distinct process of selection, and although filtration may not be altogether excluded from the process, it must be completely modified by other forces.
In the general description of the structure of the organ it was seen that in a great part of the tubules, both the epithelial and vascular supply give the idea of actively secreting gland tubes. From the mere construction of the different portions of the gland, it has been concluded that there are two distinct departments, each of which plays a different part in the production of the urine. One is said to be a simple filtering mechanism, and the other a definitely secreting glandular tubule.
It is not surprising that, with such a complex arrangement as the tubules above mentioned, there should exist different views as to the exact mode in which the urine is secreted. As these are more or less at variance in their explanation of the method of secretion, and as it is difficult to put any of them aside as quite erroneous, it becomes necessary to enumerate each somewhat in detail.
Feeling convinced of the filter-like function of the glomerulus, and recognizing the fact that some other agency was also at work in the formation of urine, Bowman explained the process thus: From the glomerulus the watery parts of the fluid are filtered, while the glandular epithelium selects the important solid constituents which it is necessary to remove from the blood.
Ludwig takes a different view. He believes that the watery part of the plasma, bearing with it the salts, etc., is filtered from the glomerulus. As this fluid passes through the tortuous urinary tubules, a large portion of the water is reabsorbed into the capillary networks surrounding them. This reabsorption is assisted by the high specific gravity of the blood and the low pressure in these capillaries as compared with those of the glomeruli, where the filtration of the liquid occurs. The role of the epithelium is not then selection from the blood of specific materials, but possibly the prevention of the return of the solids with the water back to the blood vessels.
Heidenhain attempted to settle the question as to the function of the renal epithelium, by introducing into the blood a blue coloring matter - pure sodium sulphindigotate - which he found to be eliminated by the kidneys, giving rise to blue urine. On examining the organ with the microscope at a suitable time after the injection of the color into the blood, the tubulessare found to be filled with the pigment, and in some cases the peculiar epithelium of the convoluted tubules is stained with the blue substance, while the glomerulus and capsule are entirely free from the color. If the stream of fluid from the glomerulus be stopped in any way - tying the ureter, section of the spinal cord, or local destruction of the glomeruli - the blue color is only to be found in the convoluted tubes and their epithelium, and hence it has been concluded that its presence in the looped and collecting tubes of the kidneys and urinary bladder depends upon its being washed out of the convoluted tubes by the stream of fluid filtered from the blood at the glomerulus.
The following facts may also be adduced in further support of the view that the glandular epithelium has a considerable share in the removal of the more important solid constituents of the urine.
The epithelium in the tubules of the kidney of birds is found impregnated with acid urates, which form the chief solid constituents of the urine of birds.
The amount of liquid passing out at the kidneys is in direct proportion to the blood pressure, whereas the excretion of the specific constituents of urine is independent of the pressure, but is related to the amount existing in the blood, and the condition of the epithelium. This is shown by the increased elimination of urea when that substance is artificially introduced into the circulation, even after the flow of the fluid has been checked by section of the spinal cord.
Another view has been put forward, which, with some modification, appears plausible, or at least worthy of mention. Paying attention to the fact that where vascular filtration - i. e., the passage of liquid under pressure through the capillary wall - occurs elsewhere in the body it is not. only water and salts, but plasma that passes out of the vessels into the interstices of the tissues, we may then assume that the fluid part of the blood, as such, and not merely its watery part, escapes at the glomerulus. That is to say, the solid ingredients of the urine in a diluted form, plus serum-albumin, pass into the tubules. But on its way down the long and circuitous route through the tubules the albumin with much water is reabsorbed by the capillaries of the convoluted tubes. The first step in this case is a mechanical filtration; the second is a vital process of reabsorption of a solution of serum-albumin carried on by the gland cells in the tubules, aided by the low pressure in the peri-tubular capillary plexus. This view seems supported by pathological experience, which teaches that the removal of the epithelium of the tubes (the glomeruli remaining perfect), is followed by the appearance of albumin in the urine, and cysts formed by the destruction of the epithelium and occlusion of the tubules commonly contain a fluid somewhat like plasma.
Doubtless much remains to be found out as to the exact method of secretion of the urine, and possibly future research may show us that all the views here enumerated have some truth in them. That a filtration, not mere osmosis, takes place, seems probable from the special vascular mechanism of the glomerules. Why simply water and salts without albumin should pass through the capillaries of the glomerulus and not through any other capillaries, is not sufficiently explained to make it sure that such a filtration really occurs. That the glandular epithelium does take an active part in the elimination of the urea is rendered almost indisputable from the researches of Heidenhain. And yet there remain other parts, e.g., the loops of Henle, which are invariably found in the kidney, and have a special vascular mechanism, to which none of the foregoing theories assign any special or peculiar function.
From the foregoing evidence we may fairly suppose that most of the urea, and possibly some other-solid constituents of the urine, are selected from the blood by the epithelial cells of the convoluted tubules, that the fluid part of the blood escapes at the glomerulus, and flows along the varied and circuitous route of the tubules, carrying with it the matters poured into the tubes by the cells, and that in some part of the tubules the dilute filtrate loses much of its water and all its albumin.