420 THE ELIMINATION OF WASTE PRODUCTS. 



that in birds, the urine of which contains little water, urates maybe detected 

 in the epithelium of the tubules, but not in the capsules. 



Though much remains to be cleared up, we may, for the present, con- 

 clude that the secretion of urine does consist of two separate and distinct 

 acts; secretion by the glomeruli, which we may for brevity's sake speak of 

 as glomerular secretion, and secretion by the epithelium of the tubuli, which 

 we may speak of similarly as tubular secretion. But these forms of secre- 

 tion, especially the former, but to a certain extent the latter also, differ from 

 the secretion of such a gland as the salivary, and both deserve some special 

 consideration. 



353. T/ie nature of glomerular secretion. We have seen that the ex- 

 pansion of the kidney which has for its accompaniment an increased flow 

 of urine is one brought about by the renal artery and its various branches 

 becoming dilated, under such circumstances that the difference between the 

 blood-pressure in the aorta at the mouth of the renal artery and the blood- 

 pressure at the vena cava at the mouth of the renal vein is at the same time 

 increased, or at all events is not diminished. 



In dealing with the vascular system we saw that relaxation of a small 

 artery, taking place without any marked change in the general blood-pres- 

 sure and in neighboring arteries, leads to a fuller and more rapid stream of 

 blood through the capillaries supplied by the artery, and that at the same 

 time the pressure in the capillaries themselves is increased ; owing to the 

 decrease of peripheral resistance through the widening of the artery, the 

 great fall of pressure (see 105) so characteristic of the peripheral region 

 is shifted from the arterial side of the capillaries toward the venous side and 

 to the capillaries themselves. 



Hence, as we have already said, when the renal artery dilates two things 

 happen in the loops of the glomeruli : a fuller, more rapid stream of blood 

 passes through them, and that blood as it flows through them is exerting a 

 greater pressure than before on their walls. How does each of the events 

 stand toward the secretion of urine? 



We have not at present the means of inducing a fuller and more rapid 

 flow without increasing the pressure ; but we may easily obtain increase of 

 pressure without the fuller and more rapid flow. If we hinder or obstruct 

 the outflow through the renal vein we at once increase the pressure in the 

 glomerular loops as in the other capillaries of the kidney. Now, when the 

 blood-pressure in the glomeruli is thus raised by partial obstruction to the 

 venous outflow, the flow of urine so far from being increased is diminished. 

 Obviously, then, the passage of water and material through the walls of the 

 glomerular loops, to go to form the urine, is not the result of mere pressure, 

 and cannot, therefore, be spoken of properly as a process of filtration. 

 (Cf. 255.) And we may here draw a comparison between the passage of 

 water and material through the wall of a capillary in an ordinary situation 

 to form lymph and the passage through the wall of the glomerular loop to 

 form urine or part of urine. The former, as we have seen ( 255), appears 

 to be directly dependent on pressure, though influenced as we have also 

 seen in a very material way by the condition of the vascular wall ; and 

 hindrance to venous outflow, so inefficient in promoting a flow of urine, is 

 as we have seen especially favorable to the transudation of lymph. In the 

 former case the substances which pass through the capillary wall may be 

 described as the constituents of the blood generally, proteids as well as salts 

 and other soluble and diffusible matters. Through the wall of the glomer- 

 ular loop there pass, so long as that wall is sound and intact, neither 

 albumin nor globulin nor fibrin factor, but only water accompanied by 

 some, and apparently a selection of some, of the soluble diffusible constitu- 



