142 THE CIRCULATION OF THE BLOOD 



taining a sufficient amount of gum acacia or gelatin to make the viscos- 

 ity about equal to that of blood, are injected, the original level of blood 

 pressure is recovered as well as it would have been had blood itself been in- 

 jected. A 7 per cent solution of gum acacia almost fulfills these require- 

 ments, but unfortunately this solution contains a slightly greater amount 

 of calcium than it is safe to inject into an animal. The excess of calcium 

 may, however, be removed by exactly neutralizing the gum solution with 

 sodium hydroxide, neutral red being used as an indicator. Most of the 

 calcium becomes precipitated as phosphate. The mucilage of the British 

 Pharmacopeia, diluted five times with water, makes a 7 per cent solu- 

 tion of gum acacia. A 6 per cent solution of gelatin, after being heated 

 to 100 C., gives a viscosity similar to that of blood, but on account of 

 the possible presence of tetanus spores such solutions must be very care- 

 fully sterilized before injection, and the process of sterilization causes 

 a decrease in viscosity. The injection of a quantity of one of the above 

 solutions equal to that of blood lost by a hemorrhage will usually bring 

 the blood pressure back to its original height and hold it there for an 

 hour or so. 



Viscosity is, however, not the only property of such solutions upon 

 which their desirable effect depends. The osmotic pressure of the colloids 

 also comes into play. By injecting saline solution containing a sufficient 

 amount of a colloid such as soluble starch, which gives it the correct 

 viscosity but has no osmotic pressure, the blood pressure, although it 

 temporarily recovers after transfusion, does not maintain its recovery in 

 the same way as with solutions containing gum or gelatin. The difference 

 between a starch solution and one of gum or gelatin is that the former 

 has no osmotic pressure. This property influences the rate at which water 

 passes through the walls of the capillaries as can be shown by observing 

 the outflow of urine from the ureters during the injection into animals 

 of saline alone or of saline containing either starch or gelatin (Knowl- 

 ton 6 ). Diuresis is produced with either of the first two, but not with 

 the gelatinous solution. The reason that the osmotic pressure of certain 

 colloids prevents passage of water from the blood into the uriniferous 

 tubules is that the development of this pressure on the blood side of 

 the renal epithelium tends to counteract the filtration pressure by which 

 the urine is formed (see page 547). 



Although the urinary factor will not in itself explain the efficiency of 

 the colloids in recovering the blood pressure, the conditions controlling 

 it reveal the mechanism by which the passage of fluid from the blood 

 vessels into the tissues is prevented when solutions of correct composi- 

 tion are injected. Normally the protein content of the blood plasma is 

 higher than that of the tissue lymph, so that there is a continual attrac- 



