THE EXCRETION OF URINE 389 



the wide part of Henle's loop secrete the specific constituents of the urine 

 and water. 



As an indirect support of this theory, the following consequence of the 

 filtration hypothesis, emphasized by Heidenhain, is to be considered. If the 

 outflow of fluid from the glomeruli takes place by filtration, the filtrate cannot 

 be richer in urea than the blood ; it would contain therefore about 0.05 per cent 

 urea. Since however the urine as voided contains two per cent of urea, the 

 filtrate must be concentrated forty times. With a daily excretion of 1,500 g. 

 urine containing 30 g. urea the total quantity of filtrate would thus amount to 

 60,000 g. of which 58,500 g. would have to be absorbed again into the urinary 

 tubules. 



Various other circumstances favor the idea of a secretory process in the kid- 

 neys. (1) The excretion of urine occasions a measurable rise of temperature (the 

 temperature of the urine may be 0.4 C. higher than that of the blood) (Grijns). 

 (2) Atropin which is poisonous for all glands reduces the excretion of urine to a 

 considerable extent (Thompson), although pilocarpiiie which is stimulating for 

 glands in general has no effect on the activity of the kidneys (Loewi). (3) When 

 by feeding benzoic acid and glycocoll the kidneys are called upon to synthesize 

 hippuric acid (cf. page 378), the output of NaCl in the urine is considerably 

 increased, notwithstanding that the flow of blood and the percentage composi- 

 tion of NaCl in it remain constant (Asher). 



The amount of work done by the kidneys depends essentially on two 

 factors, namely, the volume of the Hood flowing through them, and the 

 percentage of diuretic substances in the blood. 



The influence of the blood flow was first established by Ludwig and his 

 pupils on the basis of a great many experimental observations. Everything 

 which increases the blood flow, such as great but not excessive distention of 

 the vascular system, extensive vasoconstriction with the renal nerves cut, etc., 

 intensifies the secretion of urine. Conversely it falls pari passu with the 

 blood flow, whether that fall be occasioned by diminution of the general blood 

 pressure due to stimulation of the vagus, to bleeding, to section of the spinal 

 cord, or be caused by a local constriction or compression of the renal vessels. 



Since every change of the arterial blood supply alters the pressure in the 

 capillaries in the same direction, the above-mentioned facts were adduced as 

 the most important support for the filtration hypothesis; for it is evident 

 that filtration through the glomeruli should be more abundant, the higher the 

 pressure brought to bear on them. Likewise if the excretion of urine be the 

 result of a secretory process, the variations of the Hood flow are of vast im- 

 portance, for by this means the activity of the kidney cells can be influenced 

 in one way or the other. 



The action of diuretic substances is shown most clearly by experiments 

 on the secretion of urine with the renal veins compressed. It is well known 

 also that the kidney is surrounded by a tolerably firm capsule, and that its 

 mass is incompressible. Here, as in the brain (cf. page 241), venous stasis 

 must, therefore, cause an arterial anaemia. Consequently when the renal vein 

 or inferior vena cava is constricted, the secretion of urine declines or stops 

 altogether. If, however, a solution of sodium nitrate, for example, be then 

 injected into the blood, the urine gushes out in a strong stream, even if the 



