The Determination of the Colloidal Osmotic Pressure in Blood Serum and Similar Fluids

The importance of the colloidal osmotic pressure in biological fluids is becoming increasingly recognized, but difficulties in the older methods as yet prevent their general use in inquiries for which needed information can only be obtained from colloidal osmotic pressure data. The membrane bag arrangement used by Starling (1896) is capable of yielding admirable results (see, e.g., Adair, 1925; Adair and Robinson, 1930), but it requires so much time and material that its use has been limited. The methods in greatest use are those of Schade and Claussen (1924), Govaerts (1923), also Verney (1926), and Krogh and Nakazawa (1927) (see also Wells, 1933; Hill, 193233).l These all require special apparatus which is not easy to construct and requires considerable skill to use. In addition to technical difficulties, there is a further point of criticism of the methods in current use in physiological and medical laboratories in that they are based on the assumption that it is desirable to obtain a colloid osmotic pressure reading of the colloid fluid equilibrated against its own ultrafiltrate. The justification of this is based on the assumption that the effective colloid osmotic pressure in the organism is most closely -estimated in this way. However, there is no valid proof that this is the case; in this laboratory we have found evidence to indicate that, in man, neither are the capillaries wholly impermeable to blood colloids nor are the colloids alone effective as osmotic agents determining the fluid balance across the capillary membrane. In other words,