EXCRETION 469 



intra-glomenilar pressure on the contrary, it must increase it 

 but the secretion of urine stops. If the venous outflow from 

 the kidney is only partially interfered with, the flow of urine 

 is immediately diminished, but the administration of a diuretic 

 like potassium nitrate causes an increase. It is more than likely 

 that in these experiments the secretion stops or slackens not 

 because a high blood-pressure, but because an active circulation 

 is its necessary condition. When the blood stagnates in the 

 kidney the natural stimulus to the renal apparatus speedily 

 disappears owing to the elimination of the urinary constituents 

 to the neutral or indifferent point (p. 464). The experiment, 

 however, is not perfectly conclusive. For few glands can go 

 on performing their function after the circulation has ceased. 

 The kidney must be able to feed itself in order to continue its work. 

 Above all it needs oxygen ; and it might be urged that if the blood 

 in the glomeruli could be kept at the normal standard of arterial 

 blood, secretion might still go on after ligation of the renal vein. 



According to Ludwig, indeed, the flow of urine stops, in spite 

 of continued filtration through the glomeruli, because the 

 swelling of the veins in the boundary layer compresses the 

 tubules, and may even obliterate their lumen. There is no 

 conclusive experimental evidence, however, and no a priori 

 probability, that the obstruction so produced is sufficiently 

 sudden or sufficiently complete to cause instant and total cessa- 

 tion of the flow. It is even less justifiable to conclude from the 

 experiment that the liquid part of the urine is, at any rate, not 

 separated by the epithelium of the tubules, since the blood- 

 pressure in the capillaries around the tubules must rise very 

 greatly after ligature of the vein, and yet secretion is stopped. 

 It might equally well be argued that the renal epithelium nor- 

 mally secretes water under a low blood-pressure, but is dis- 

 organized under the excessive and entirely unaccustomed 

 pressure which follows the closure of the vein. 



It is not only through nerves directly governing the calibre 

 of the vessels of the kidney that the rate of urinary secretion 

 can be affected. Any change in the general blood-pressure, if 

 not counteracted by, still more if conspiring with, simultaneous 

 local changes in the renal vessels, may be followed by an in- 

 creased or diminished flow of urine ; and the law which explains 

 all such variations, or at least serves to sum them up, is that 

 in general an increase in the rate of the blood-flow through the 

 kidney is followed by an increase in the rate of secretion. It wilt 

 be remarked that this is the converse of the great law, of which 

 we have already seen so many illustrations, that functional 

 activity increases blood-flow. It is probable that this law holds 

 for the kidney as well as for other organs, but that the influ- 



