OSMOTIC PRESSURES OF SOME BIOLOGICAL FLUIDS 203 



and the serum globulins have a higher value, averaging 175,000. They 

 are formed by living matter and occur in the tissues and liquids of plants 

 and animals. They are composed of carbon (50-55 per cent), hydrogen 

 (6-7 per cent), nitrogen (15-18 per cent), oxygen (19-24 per cent), with 

 some sulphur, phosphorus, or iron. 



Since proteins are colloids they do not penetrate the semi-permeable 

 boundaries of living cells and tissue structures, yet despite their giant 

 molecular forms they can develop a small osmotic pressure. As early 

 as 1861 Graham suggested that plasma colloids might exert an osmotic 

 pressure in the vascular system, but it remained for Starling [1896] to 

 measure the osmotic pressure of colloidal solutions directly. He took 

 advantage of the fact that colloidal membranes, while permitting the 

 passage of water and salts, are impermeable to colloids in solution. 

 With this technique Starling [1936] found that blood serum containing 

 about 7 per cent proteins developed, owing to the presence of the pro- 

 teins, an excess osmotic pressure of about 35 mm mercury. The osmotic 

 pressure developed by blood plasma, which is about 6.4 atmospheres at 

 37° C, is due primarily to the presence of sodium chloride. Its colloid 

 osmotic pressure, which is of prime importance for the transfer of fluids 

 from tissue to blood to urine, is mainly due to the presence of plasma 

 proteins and is approximately 28 mm of mercury. Equally low values 

 were found by Pfeffer for egg albumen (1.25 weight per cent at 20° C), 

 22.4 mm of mercury. 



Since a A change of 0.01° corresponds to a pressure change of 91.5 mm 

 mercury, it can be concluded that the presence of proteins will change the 

 freezing point only 0.0034°. This quantity lies just within the limit of 

 accuracy of the micro-Beckmann thermometer. For all practical pur- 

 poses, in the osmotic-pressure determinations of blood, the presence of 

 proteins adds a negligible magnitude to the depression of the freezing- 

 point determination. 



Osmotic Pressures of Some Biological Fluids 



One interesting hypothesis that has been used in discussing the theory 

 that life originated in the sea is that in the blood of all vertebrates the 

 content of sodium, potassium, calcium, and magnesium is proportional 

 to that of Archean sea water. 



In certain marine invertebrates the body fluids contain the ions of the 

 above salts in almost the same concentrations in which they occur in sea 

 water. The tissue fluids of the j elly fish are very mu ch like sea water when 

 one compares their relative concentrations of sodium, potassium, calcium, 

 and magnesium ions. The serum cf the lobster, which is of a higher 

 order in the scale of evolution, contains a concentration of sodium and 



