In many of these cases of persistent albuminuria the condition is not one of progressive disease, but rather the result of a former attack of nephritis, and such patients often live for many years without suffering from any marked impairment of general health, although doubtless they run considerable risks of contracting fresh attacks of nephritis. This class of case illustrates very well that treatment should not be determined solely by considerations of the state of the urine, or even by the amount of albumin present.
In most renal diseases the excretory activity of the kidney is impaired to a greater or less extent, and this is revealed by the small amount of nitrogenous extractives present in the urine, and by the dilute character of the urine with its well-known low specific gravity, so characteristic of chronic renal disease. The imperfect elimination leads to the retention not only of the nitrogenous extractives, but also of the salts of the urine, and it is possible that this retention may be a factor of great importance in determining the occurrence of some of the complications of renal disease, as, for example, dropsy. Too much attention is sometimes paid to the retention of nitrogenous extractives, and but little to that of the retention of the salts. In the dietetic treatment of the disease it may be of greater importance to regulate the ingestion of the saline constituents of the food than to concentrate attention on the cutting down of proteins. In many renal diseases the quantity of water is also considerably diminished, and in some, suppression may occur; but even when the quantity of urinary water is normal or above the normal in amount, the excretory activity of the organ may be greatly impaired, as is shown by the dilute urine that is excreted. This is more especially true of chronic degenerative diseases of the kidney, especially granular kidney and cystic degeneration, and it is also the case in other chronic inflammatory affections where there is a large overgrowth of fibrous tissue.
Another point of considerable importance is the fact that the excretion is also often extremely slow, so that substances introduced into the blood stream that normally appear in the urine within a few minutes are not excreted until after the lapse of half an hour, or even longer. This delay in the excretion is often an index of the degree of inefficiency of the excretory mechanism. The deficiency of the urine in solid constituents in chronic renal disease affords clear evidence of imperfect excretory activity, but the problem in the case of the excretion of water is a little more complex, owing to the relationship existing between the production of renal dropsy and the flow of urine. This is also a point of considerable importance in the dietetic treatment of renal disease. Renal dropsy is sometimes looked upon as directly dependent on the deficient excretion of urinary water, but on the other hand there are many who consider that the deficient excretion is the result rather than the cause of the dropsy. It is obvious that the advisability of regulating the amount of fluid in the diet must depend largely upon which of these two views is accepted. The incidence of dropsy in renal disease is always accompanied by a diminution in the quantity of urinary water excreted, and conversely a subsidence of the dropsy is accompanied by an increased urinary flow. Dropsy is however only present in some renal diseases, more especially in some forms of acute and chronic nephritis, but it is not an invariable accompaniment of either of these diseases. Further complete suppression of urine may occur in a considerable number of renal diseases without the development of any dropsy. This is not only true of the suppression accompanying calculous obstruction, but also with the suppression, partial or complete, that is so often seen in the more severe forms of acute nephritis. These facts are strong arguments in favour of the view that the diminution in the flow of urine, so constantly associated with the development of dropsy, is rather a result than the cause of the dropsy. At the same time it must be admitted that, especially in acute nephritis, the state of the circulation in the kidney, and especially in the glomeruli, is not favourable to the elimination of water by the organ, and that it must be undesirable to produce a condition of plethora by the ingestion of large quantities of fluid.
In most renal diseases, but especially in acute and chronic nephritis, a toxaemic state exists, and this toxaemia is doubtless in part dependent on the retention of normal products of metabolism which ought to be excreted, but it is possible that, owing to disordered metabolism, other abnormal substances are also present in the blood stream. Just as dropsy does not necessarily occur as the result of complete urinary suppression, so uraemia is not necessarily seen in the same conditions. Complete suppression, occurring as a result of calculous obstruction, or in the rare cases where a single kidney has been excised, or where the circulation has suddenly been arrested as the result of thrombosis in the renal arteries of both kidneys, is not followed by the development of the ordinary type of uraemia that is such a familiar complication of acute and chronic nephritis and of granular and cystic degeneration of the kidneys. It would thus seem that uraemia can scarcely be looked upon as due to the toxic action of some normal constituent of the urine that is retained as the result of deficient renal elimination. It may well be, however, that the cause of uraemia is some toxic substance formed as a result of abnormal metabolism. Inefficient excretion may play a part in the development of uraemia, and a possible hypothesis is that, as the result of such inefficient excretion, an abnormal toxic substance is retained.