716 ADEQUACY OF BLOOD PRESSURE 



that the ureter pressure rose with a rise in the arterial blood 

 pressure, and tended to become stationary at a pressure of 

 about 40 mm. Hg below that in the carotid artery. Gottlieb 

 and Magnus were unable to confirm Starling's observations. 

 They found that urine was still secreted when the difference 

 between the arterial and ureter pressures was only a few mm. Hg, 

 and further that urine was still formed when the carotid blood 

 pressure had fallen as low as 16 mm. Hg. These observations 

 would invalidate Ludwig's view only if the osmotic pressure of 

 the plasma proteids were anything approaching 40 mm. Hg. 

 Starling's method of estimating their osmotic pressure was as 

 follows. He freed serum of proteid by filtering it under pressure 

 through a gelatinised Chamberland filter. This filtrate was placed 

 in an osmometer and separated from the original serum by a 

 gelatine membrane supported on peritoneum, and values were 

 obtained in mm. Hg. Waymouth Reid has thrown doubt on 

 the correctness of Starling's conclusions. For he has shown 

 that, when serum is filtered through gelatine, the filtrate is not 

 the original serum minus proteid only ; for, the A of the filtrate, 

 instead of being within '005 C. of that of the original serum, 

 differs from it by '035 C. Although Reid has demonstrated 

 that pure crystalline proteids have no osmotic pressure, he 

 admits that blood serum does give a readable but variable 

 osmotic pressure on a gelatine membrane due to substances 

 other than proteids. The question, therefore, resolves itself into 

 how far the cells of Bowman's capsule have properties anything 

 like those of the gelatine membrane, and this, as we have already 

 seen, it is impossible to answer. 



It is also necessary on the filtration theory that the osmotic 

 pressure of the glomerular filtrate should never be less than that 

 of the blood plasma by more than the osmotic pressure of the 

 plasma proteids. As we have seen, unless the proteid osmotic 

 pressure is extremely low it is impossible to explain filtration 

 with very low arterial blood pressures. Assuming, then, that the 

 osmotic pressure of these proteids is something much less than 

 40 mm. Hg, it is clear that the osmotic pressure of the glomer- 

 ular filtrate must be but little below that of the plasma. 

 Unfortunately we cannot obtain the glomerular filtrate until it 

 has already passed down the tubules and been converted into 

 urine, and our ideas as to its osmotic pressure are derived en- 



