462 A MANUAL~OF PHYSIOLOGY 



even by those who uphold a modified ' mechanical ' theory, that 

 even if the urine is originally separated from the blood by filtra- 

 tion at the expense of the energy of the heart-beat represented 

 by the pressure of the blood in the glomeruli, the reabsorption 

 in the tubules cannot be attributed to simple diffusion, but 

 must be a selective process analogous to absorption in the intes- 

 tine and entailing the expenditure of a large amount of work 

 at the expense of the food materials or the protoplasm of the 

 epithelial cells. Every attempt at a strictly mechanical explana- 

 tion breaks down for the kidney, as for other glands. 



The practical absence from urine of the proteins and sugar 

 of the blood under normal circumstances, and the elimination 

 by the kidney of egg-albumin, peptone, and other bodies when 

 injected into the veins, show a selective power inexplicable 

 except by reference to the vital activity of cells. Urea and 

 dextrose, both highly diffusible substances, circulate side by side 

 in the bloodvessels of the kidney. The one is taken and the other 

 left. The urea is a waste-product of no further use in the 

 economy. The sugar is a valuable food-substance. The kidney 

 selects with unerring certainty the urea, of which only 4 parts 

 in 10,000 are present in the blood, but rejects the sugar, of 

 which there is three times as much. 



Egg-albumin injected into the blood passes through the renal 

 circulation side by side with the serum-albumin of the plasma. 

 Both are indiffusible through membranes, and to the physical 

 chemist the differences between them may appear superficial and 

 minute. But the kidney does not hesitate for an instant. A 

 large part of the egg-albumin is promptly excreted as a foreign 

 substance ; the serum-albumin passes on untouched. 



Not only does the kidney exercise a power of qualitative selec- 

 tion ; it also takes cognizance of the quantitative composition of 

 the blood. So long as there is less sugar in the plasma than about 

 2 to 3 parts in 1,000, it is refused passage into the renal tubules. 

 But when this limit is passed, and the proportion of sugar in the 

 blood becomes excessive, the kidney begins to excrete sugar, and 

 continues to do so till the balance is redressed. 



The advocates of the theory of filtration through the glomeruli 

 have made their firmest stand on the excretion of the inorganic 

 constituents of the urine, and have laid stress particularly on the 

 fact that the hydraemic plethora caused by intravenous injection 

 of salts is accompanied by diuresis. It is true that the direct intro- 

 duction of water into the blood, or its attraction from the lymph- 

 spaces when the osmotic pressure of the blood is increased by the 

 injection of substances like urea, sugar, and sodium chloride, may 

 cause a condition of hydramia plethora, and that this plethora 

 may sometimes be associated with an increase of pressure in the 

 capillaries in general, and therefore in the vessels of the Mai- 



