ADEQUACY OF BLOOD PRESSURE 717 



tirely by inference. Dreser found that a urine whose A = 2'3 C. 

 was formed from blood with a A = -56 C. This difference re- 

 presents an enormous amount of work done by the kidney cells, 

 and can be explained equally well as brought about by the 

 absorption of water on Lud wig's view or the excretion of solids 

 by the tubules on Heidenhain's view. Starling found that during 

 the diuresis following intravenous injections of normal saline and 

 of strong salt or dextrose solutions the A of the urine was 

 rapidly reduced until it approximated but always remained 

 larger than that of the blood. In this he sees a strong argument 

 in favour of Lud wig's view, for, the less time the tubule has to 

 concentrate the glomerular nitrate, the more nearly should we 

 expect the osmotic pressure of the urine to be like that of the 

 blood plasma. Dreser, however, has found that the A of the 

 urine in diabetes insipidus, during caffein diuresis or after drink- 

 ing 1-5 litres of beer, may be as small as '16 C., and even 

 smaller values have been found in man by other observers. 

 Assuming that in these cases the A of the blood was as low as 

 46 C., there would be a difference between the A of blood and 

 urine of *3 C., which would correspond to an osmotic pressure 

 of many hundred mm. Hg. It is manifestly impossible for the 

 blood pressure to have filtered against such an osmotic resistance 

 as this. Dreser considered that these results could be explained 

 only by imagining that the glomerular epithelium had done work 

 and acted as a secreting membrane in Heidenhain's sense. To 

 explain these results on the filtration hypothesis we should have 

 to imagine that in these cases the glomerular filtrate in passing 

 down the tubules became actually more dilute instead of under- 

 going the more usual concentration. Starling has suggested that 

 this might take place in one of two ways : either by the tubule 

 cells secreting extra water practically without solids, or by their 

 absorbing solids out of all proportion to water. The first sugges- 

 tion supposes a mechanism which is contemplated in neither 

 Heidenhain's nor Ludwig's view, and is opposed to Starling's 

 explanation of diuresis in general. The second suggestion is that 

 natural to Ludwig's view. As has already been pointed out, it 

 is necessary with Ludwig's view to believe that absorption by 

 the tubules can be regulated to the needs of the organism. In 

 diabetes insipidus the daily excretion of urinary solids is normal, 

 and the only abnormality is the enormous quantity of water 



