The excretion of sugars by the kidney

In studying the excretion of any substance by the kidney there are at first glance three separate things to be considered.

First, the passage from the blood into the kidney cell; second, the transit through the cell substance; and third, the passage from the cell into the lumen of the glomerulus or tubule. It is obvious that if the transfer through either cell boundary is impossible that none of the substance can be excreted, and it is also obvious that if anything prevents the passage through the cell substance itself that the kidney will appear impermeable.

In considering the passage of glucose through the body of the cell there are two well-known facts to be borne in mind that so far as we know have not received sufficient attention in this connection. The first is that the kidney uses a relatively large amount of oxygen and the second is that glucose is one of the main sources of energy for the body in general. It is therefore very probable that the kidney normally oxidizes a certain amount of glucose. Hence, even if both cell boundaries are permeable for glucose not all the sugar that gets into the cell will reach the lumen of the tubule. Normally only the minutest trace of sugar is present in the urine, whereas if the blood sugar rises beyond a certain level some will appear. Although it is true that in certain cases of diabetes there may be only a very slight hyperglycemia yet it is in the main true that glycosuria is dependent on a more or less pronounced rise in the percentage of sugar in the blood. Many hypotheses have been advanced to account for this so-called threshold phenomenon and I hope you will pardon me for dismissing them with the statement that none of them are generally accepted.