THE EXCRETION OF SUGAR 645 



at which it undergoes tluingc into something else within the cell, the 

 same holding in the case of the cells forming the kidney membrane. 

 Thus sugar passes from the kidney membrane into the urine irhen. the 

 rate at which sugar molecules enter the kidney membrane exceeds the 

 rate at which they are denatured or utilized within it. In order to under- 

 stand the various ways in which melituria may be produced it is 

 necessary to analyze further these factors of supply and utilization. 



The Sugar Supply to the Kidneys is cliiefly via tho hlooci, althouRh some may 

 be liberated within the kidney cells themselves from glycogen deposits or from the 

 transformation of protein during metabolism. The factors which infliienre the 

 rate at which sugar molecules enter the kidney membrane from the blood, are 

 immediate and remote. 



The more remote factors are those which determine the quantity of sugar in 

 general circulation, and the distribution of blood to the kidneys. The relative 

 quantity of sugar circulating in the blood will depend on the combined rates at 

 which sugar enters the blood from the outside and from all the organs, and on the 

 rate at which sugar passes out of the blood into the cells. The absolute value of 

 these factors, supply and depletion, and their ratio, will determine fluctuations of 

 the total blood sugar. The supply to the kidneys is accordingly influenced by the 

 balance of supply and depletion elsewhere. 



The immediate factors which determine the rate at which sugar is brought to the 

 kidney membrane will include all of those which may influence the state of the 

 blood as to viscosity etc., and the rate at which sugar actually enters the kidney 

 membrane will be influenced by changes in the state of that membrane, but apart 

 from those things there are two factors of major importance in determining 

 the supply of sugar to the kidney membrane at any instant, namely, the concentrn- 

 tion of sugar in the blood plasma of the kidney capillaries and the extent of the surface 

 of contact between the blood plasma and the kidney membrane. In accordance with a 

 method followed by M. H. Fischer it is convenient to think of the kidney membrane 

 simply as including all that lies between the blood plasma on the one hand and the 

 urine on the other. So considered, the surface of contact between the blood plasma 

 and the kidney membrane is the internal surface of all the capillaries of the cortex, 

 in so far as these are filled with circulating blood. The kidney membrane might be 

 conceived otherwise and be made to include only the layer of epithelial cells, in 

 which case the surface of contact might be considered as the surface of those cells 

 in so far as they are bathed in plasma; or the matter might be carried within the 

 cells, in which case the problem of surface would become one of surfaces between 

 phases of an heterogeneous system in the chemical sense, and so on. 



For present purposes it is convenient to mass all such intermediate factors and 

 deal with the sum of their effects, i. e., to think of this surface as the internal 

 surface of the active capillaries. The surface of contact between the blood 

 plasma and kidney membrane, so considered, is clearly subject to variations, for 

 as more or less blood is forced into the capillaries of the cortex there must be changes 

 in the diameter, length or number of capillaries containing circulatory blood, or in 

 all three. Any or all of these changes implj' changes of capillary surface. It 

 therefore becomes apparent that other factors remaining the same, changing vol- 

 umes of blood in the cortical capillaries would imply changing rates of sugar 

 diffusion from the blood into the kidney cells, even though the concentration of 

 sugar in the blood should remain unchanged. And it is also apparent that if the 

 blood should be diluted with water in such a way as to double its volume and halve 

 the blood sugar percentage this would not of necessity change the rate at which 

 sugar was passing from the blood into the kidney membrane, provided the extra 

 volume of blood developed for itself a proportional amount of extra capillary 

 surface.'"' Thus it has been shown by Epstein* that the rate at which sugar is 



*» This raises the question of the geometrical form of capillaries and the mechan- 

 ism by which an organ or a tissue accommodates var>-ing volumes of blood. It is 

 interesting to note that in capillary systems, notably that into which the efferent 

 .artery from the glomerulus breaks up, the capillary strands may vary in calibre, 

 * which would lead one to expect that a pressure just sufficient to force blood through 



