Methods of Raising a, Low Arterial Pressure. 387" 



blood-vessels exceeds the difference between the osmotic pressure of their 

 contents and that of the tissue fluids. If, on the other hand, the blood is 

 diluted, so that the osmotic pressure of its colloids is lowered, an internal 

 pressure of the same height as before will cause greater nitration, and, at the 

 same time, the difference between the osmotic pressure of the blood and that 

 of the tissue fluid being less, there is a decreased osmotic attraction of water 

 by the blood from the tissues. The two causes combine to produce oedema. 

 The colloid added to increase the viscosity of an intravenous injection must, 

 therefore possess an osmotic pressure equal to that of the colloids of the 

 blood. Table II, above, includes some determinations of the osmotic 

 pressures of certain solutions of interest in the question. These were all 

 made under the same conditions with Moore and Eoaf's osmometer, arranged 

 for changing the outer fluid as required, practically as described by me in a 

 previous paper (8). It will be noticed that the value found for serum against 

 Einger's solution is the same as that found by Starling, but that against 

 distilled water the reading is considerably higher. The water and the saline 

 solution used were made just faintly alkaline to neutral red in order to 

 approximate to the reaction of the blood, and also to avoid the loss of cations 

 from the colloidal salt which occurs when the outer solution becomes acid 

 from any cause, such as absorption of carbon dioxide from the air (see my 

 paper [10, p. 251]). The fact that the osmotic pressure of gelatin is lowered 

 by salts was noticed by Moore and Eoaf, and interpreted as due to the 

 aggregation of the colloid. I accepted this explanation at first, but subse- 

 quently found that a sufficient explanation is to be found in the unequal 

 distribution of the salts between the two sides of the membrane, when the- 

 colloid is itself a salt of a colloidal acid with a diffusible cation. The 

 explanation is discussed in my 'General Physiology' (pp. 120, 160,161). 

 Owing to the small molecular weight of the salts in Einger's solution, a very 

 small difference of concentration in favour of the outer fluid suffices to- 

 produce a considerable fall in the apparent osmotic pressure of the colloid. 

 In the case of the sodium salt of gum, for example, there is a difference of' 

 180 mm. of mercury between the osmotic pressures against Einger's solution 

 and against water. Supposing the sodium chloride in the former case were- 

 90 per cent, dissociated, a difference in concentration between the inner and 

 outer liquids of 0-034 per cent, in sodium chloride, or 3 - 8 per cent, of the 

 total concentration, would account for the difference in osmotic pressure- 

 Thus, the osmotic pressure of the contents of the osmometer is the sum of 

 those of the colloid and the salts ; the osmotic pressure of the outer fluid is 

 that of a solution of the salts of a slightly higher concentration ; the pressure 

 shown by the manometer is the difference between the two. It is interesting 



