974 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



protein osmotic pressure of 24 to 26 mm Hg, cor- 

 responding to a total protein concentration of about 

 7 per cent. It is impossible to give a significant mean 

 value because the protein concentration depends 

 upon procedures used for drawing blood samples 

 and in any given sample the value obtained depends 

 upon the method of measurement. Electrophoretic 

 measurements yield slightly lower values for total 

 protein than estimates based upon salt precipitation 

 or protein nitrogen. In the following discussion a 

 nominal value of 7.0 g per 100 ml will be considered 

 normal. 



The osmotic pressure-concentration curves for 

 albumin and for normal plasma are described by the 

 following empirical equations which fit the experi- 

 mental data closely over the range o to 25 per cent 

 protein. 



II albumin = 2.8 c + 0.18c* + 0.012c 3 (3.4) 



n plasma * 2.1c + 0.16 c z + 0.009c 3 (3.5) 



In each equation the first term represents the ideal 

 limiting law of van't Hoff. Thus the molecular weight 

 of albumin, calculated from the first term of equation 

 3.4, is 10 RT/2.8 = 69,000. The second and third 

 terms in each equation represent deviations from 

 van't HofPs law caused by Donnan effects and 

 protein-protein interaction. 



The chief osmotically active protein in normal 

 mammalian plasma is albumin, which can be sepa- 

 rated and identified as a homogeneous component 

 representing about 50 per cent of the total protein in 

 plasma and contributing about 65 per cent of the 

 protein pressure. The globulins, on the other hand, 

 comprise a spectrum of components with molecular 

 weights ranging from 45,000 to 1,000,000 as shown in 

 table 3.1. The widely different osmotic activities 

 of jSj-globulin and 7-globulin shown in figure 3.1 

 serve to emphasize that no simple physicochemical 

 meaning can be attached to the osmotic pressures 

 developed by crude, heterogeneous globulin frac- 

 tions. Precipitation methods fail to separate albumin 

 from low molecular weight globulins which contribute 

 substantially to total protein osmotic pressure; for 

 this reason many early studies attempting to relate 

 total protein pressure to albumin: globulin ratios 

 (380) need to be reevaluated. Current estimates of 

 A : G ratio in normal plasma are close to 1 . 1 , in 

 comparison with values in the range 1.8 to 2.6 ob- 

 tained by classical fractionation procedures. 



The osmotic pressure contributed by globulins 

 can be calculated from the difference between 

 albumin and whole plasma (equations 3.4, and 3.5), 



it being assumed that the A:G ratio is 1.1 and that 

 osmotic interactions between globulins and albumin 

 are not significantly different from interaction be- 

 tween albumin and albumin (270, 314). The "aver- 

 age" globulin curve so calculated is given by 



//globulins ' 1.6c + O.I5c z + 0.006c 3 (3. 6) 



In normal plasma about 15 per cent of the total 

 protein pressure is contributed by known globulin 

 components and about 20 per cent by unidentified 

 components (table 3.1). Bennhold et al. (14) have 

 studied two extremely interesting cases of complete 

 analbuminemia; the osmotic pressure-concentration 

 curve of the albumin-free plasma from these unique 

 patients (brother and sister) conforms closely to 

 equation 3.6 (271). These patients have been in good 

 health for many years despite the fact that the protein 

 osmotic pressure of their plasma is less than 50 per 

 cent of normal. Presumably they have compensated 

 by permanent reduction of mean capillary pressure 

 to balance the low protein pressure. 



C. Species Differences, Fetal Plasma 



Comparative studies of colloid osmotic pressure 

 have been reviewed by Meyer (251) and by Keys & 

 Hill (175). A summary of data pertaining to plasma 

 of Elasmobranchs, Pisces, Amphibia, Reptilia, 



Prepared with J. L. Oncley; cf also ref. 267. 



