SECRETION 



49 



plasma filtrate is derived by studying the physical conditions 

 under which urine is produced. If the capsular membrane 

 is impermeable to the colloidal constituents of the blood, work 

 must be done (i) against the osmotic pressure of the proteins, 

 etc., in removing a filtrate of identical crystalloidal composition ; 

 (2) in driving the filtrate along the narrow lumen of the tubules 

 at the observed rate of flow. From the first consideration it 

 follows that no secretion of urine can take place when the blood- 

 pressure falls to a value below the osmotic pressure of the 

 serum proteins. As a matter of fact, secretion of urine has long 

 been known to cease in the mammal when the arterial blood- 

 pressure falls below about 35 mm. of mercury. Starling 

 (1899) was the first to recognise the theoretical significance of 

 this fact, and on comparing the osmotic pressure of a 

 crystalloidal filtrate separated from blood by a gelatine filter 

 with the osmotic pressure of the original serum, showed that 

 the osmotic pressure of the blood colloids is actually about 

 30 mm. of mercury. Conversely, urinary secretion can be 

 prevented by increasing the pressure in the ureter so that the 

 diflFerence between the pressure of the arterial blood and that 

 of the fluid in the capsule is of the same order as the osmotic 

 pressure of the serum proteins. Later it was shown by 

 Barcroft and Straub (191 1) that a great increase of urinary 

 secretion follows replacement of normal blood by a suspension 

 of red corpuscles in Ringers' solution without any rise in 

 arterial pressure or increase in oxygen consumption by the 

 kidney. 



However, the possibility that the first stage of renal activity 

 is a process of simple filtration deriving its energy from the 

 heart beat, does not throw any light on the essentially secretory 

 function of the kidney, namely, that of modifying the composi- 

 tion of the filtrate in such a way that the concentration of each 

 of the crystalloidal constituents is finally diff"erent from its 

 concentration in the plasma. To effect this, work must be 

 done by the epithelial portion of the tubule. Barcroft and 

 Brodie found in experiments on the gaseous metabolism of the 

 active kidney that the respiratory quotient (ratio of CO2 given 

 off to oxygen absorbed) is practically unity ; i.e. that the activity 



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