302 THE BLOOD 



saturated with COg at its tension in alveolar air and at body 

 temperature (37-5° C). For dog's plasma he finds the depression 

 of the freezing-point = 0-61° C, for vapour pressure 48-1 mm. Hg, 

 and calculates from these an osmotic pressure of 8-2 atmospheres. 

 This value for the osmotic pressure is due very largely to the crys- 

 talloids present, as shown by separating colloids from crystalloids 

 by the process of ultrafiltration [q.v.). It is then found that over 

 8 atmospheres pressure is given by the diffusible salts, leaving only 

 about 0-06 of an atmosphere, i.e. 46 mm. Hg., due to the colloids. 



That is, the osmotic pressure of separated plasma as taken 

 by an ordinary osmometer with a semi-permeable membrane or 

 by the depression of the freezing point is almost the same as that 

 exhibited by a 0-9 per cent, solution of sodium chloride. It 

 varies with the diet and with the amount of fluid ingested. If 

 the kidneys are not functioning properly, so that the products of 

 metabolism are not eliminated with sufficient rapidity, the osmotic 

 pressure will rise. 



The student cannot guard too carefully against the errors of 

 considering that {a) the osmotic pressure of plasma is due to the 

 presence of 0-9 per cent. NaCl in it, and [b) that the figure given 

 even approximates to the proper value of the osmotic pressure 

 in the blood vessels. These vessels are permeable to salts in 

 solution, and, therefore, the true osmotic pressure of plasma must 

 be due not to crystalloids, but to colloids. Further, plasma 

 divorced from its formed elements, especially the red corpuscles, 

 is very different from " true plasma," which is plasma removed 

 with such precautions that for any given tension of COo it is in 

 equilibrium with the cells of the blood. 



(vii.) Refractive Index (see p. 273). The refractive index of 

 plasma depends primarily on the amount and nature of the 

 proteins present. Its variations are governed by practically the 

 same factors as are responsible for the variations in specific gravity. 



(6) Components, (i.) Colloids. 



The major colloidal constituents of plasma are protein in 

 chemical nature. These proteins are : 



(a) Albumin . . 2-5 per cent, circa. 



(/3) Globulin . . 3-8 per cent, circa. 



(y) Fibrinogen . . O15-0-6 per cent. 



(a) Albumin, probably a mixture of three albumins. At least 

 it is possible by careful heating to discover three separate coagula- 

 tion temperatures. 



(j8) Globulin is similarly a mixture of two or more globulins. 

 Globulins are insoluble in distilled water, but soluble in dilute salt 



