COLLOIDS OF BLOOD 233 



determinations have shown that the pH. of plasma is 7-4, i.e. just 

 on the alkaline side of neutral. This alkalinity is due to the 

 presence of sodium bicarbonate (see below). 



( vi) Osmotic Pressure. The osmotic pressure of 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 electrolytes but to colloids (see below). 



(vii) Refractive Index (see p. 216). 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. 



(b) Components, (i) Colloids. 



The major colloidal Constituents of plasma are protein in 

 chemical nature. These proteins are : 



(a). Albumin 2-5% circa. 



(/3). Globulin 3-8% 



(y). Fibrinogen 0-15-O6%. 



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

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

 tion temperatures. 



0@) Globulin is similarly a mixture of two or more globulins. 

 Globulins are insoluble in distilled water but soluble in dilute salt 

 solutions. They therefore require to have a certain concentration 

 of electrolytes present if they are to remain in solution. They 

 may be partially separated from albumin by dialysis. When the 

 salt contents of plasma is forced below a concentration of about 

 0-2 per cent., the globulins are almost completely precipitated. 

 In the -blood stream they function to a great extent as regulators 

 of the amount of NaCl present. It is of importance that this fact 



