SOURCES OF UREA. 411 



merely by dialysis or filtration; and others (the specific ele- 

 ments of the secretion), especially urea, which are selected 

 or made by a special activity of the renal gland-cells. The 

 total quantity of the twenty-four hours' urine thus depends 

 largely on the pressure in the renal arteries, since the 

 higher this is the greater will the amount of filtered liquid 

 be. Under ordinary pressures such substances as albumen 

 will not filter, but they do under higher; accordingly in 

 healthy conditions none of the albumen of the blood plasma 

 passes into the urine, but if the pressure in the capillaries of 

 the glomeruli is considerably raised it does; its presence in 

 the urine being the most prominent symptom of that danger- 

 ous class of maladies grouped together under the name of 

 Bright' s disease. Filtration in the glomeruli is largely pro- 

 moted by the fact that the calibre of the efferent vessel of each 

 is rather less than that of the afferent; and thus the pressure 

 in the thin-walled vessels of the vascular tuft is raised. 



The Role of the Renal Epithelium. Water and salines 

 being passed out mainly through the glomeruli, we have 

 now to consider what part the secreting cells of the kidney 

 play; and especially as regards urea, the most important 

 constituent of the urine. Urea represents the final state 

 in which the proteids taken into the Body from the alimen- 

 tary canal (or at least their nitrogen) leave, after having 

 yielded up, by chemical changes, a certain amount of energy. 

 In this process the proteids are oxidized and broken down 

 into carbon dioxide and water and urea; and the kidneys 

 get rid of the latter. 



Since the life and activity of every tissue is accompanied 

 by a breaking down of proteids (though not necessarily at 

 once into urea, as many intermediate stages may, and doubt- 

 less do, occur in different tissues), there is no doubt that the 

 main chemical degradation of albuminous compounds takes 

 place outside the kidneys. Whether the final steps by 

 which urea is formed occur in those organs or elsewhere is 

 not yet certainly known. According to one view the urea is 

 carried to the kidney in the blood of the renal artery, and 

 there merely picked up and passed on by the excreting cells; 

 while, according to another, not urea, but the penul- 



