EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



975 



Aves, and Mammalia will be found in reference (4). 

 It is a striking fact that all mammals have approxi- 

 mately the same protein osmotic pressure. Mean 

 values for the various species of mammals listed by 

 Meyer (251) range from 19 mm Hg (guinea pig) 

 to 26 mm Hg (human), but data from most species 

 lie in the range 21 to 25 mm Hg. These relatively 

 small variations among species may well be spurious. 

 Excitement, posture, anesthesia, and many other 

 factors can cause substantial hemodilution or hemo- 

 concentration prior to withdrawal of blood samples. 

 It would be unjustified to compare blood obtained 

 from a confident, unanesthetized human with blood 

 obtained from animals in various states of excitement 

 and anesthesia. 



Protein concentrations and osmotic pressures in 

 plasma of poikilotherms are in general far lower than 

 in mammals. Values in the range of 5 to 10 mm Hg 

 have been reported for frogs and turtles (35, 189, 

 200), the lower values being associated with the 

 spring season. The low protein osmotic pressure of 

 poikilotherms is to be expected in view of their 

 relatively low blood pressure. It is surprising, however, 

 to find similarly low values in birds in which blood 

 pressure is relatively high. 



The protein osmotic pressure of fetal plasma is of 

 special interest in relation to fluid exchanges be- 

 tween maternal and fetal blood. Meschia (249) 

 has carried out a careful series of osmotic measure- 

 ments in fetuses from sheep and goats. At midterm 

 the protein osmotic pressure in fetal blood is 7 to 10 

 mm Hg; the pressure gradually increases during 

 gestation but never exceeds that in maternal blood. 

 The average molecular weight of proteins in fetal 

 plasma is only 65,000 as compared with 96,000 in 

 the adult. This difference is largely due to the presence 

 of fetuin (283a), a low molecular weight globulin 

 which accounts for about 90 per cent of the globulin 

 fraction. The mean molecular weight increases 

 dramatically within 24 hours after birth, owing partly 

 to absorption and retention in the blood of 7-globulins 

 from colostrum. 



D. Physiological Significance of the 

 Deviations from van't Hoff's Law 



The disproportionate increase in protein osmotic 

 pressure as a function of concentration tends to 

 amplify osmotic forces contributing to homeostasis 

 of blood volume. The magnitude of this effect is 

 seldom appreciated and will therefore be discussed 

 in some detail. Figure 3.2 shows the rate of change of 



osmotic pressure per unit change in concentration, as 

 a function of concentration. At small protein con- 

 centrations, such as those existing in tissue fluids, 

 the osmotic coefficient, dU'dc, is relatively small and 

 the proteins behave as the osmotic equivalent of an 

 ideal solute of molecular weight 80,000 to 90,000. 

 Thus the osmotic forces in interstitial fluid tending to 

 withdraw fluid from blood are relatively small and 

 insensitive to quite large percentual alterations in 

 protein concentration. At concentrations which 

 exist in plasma, however, the osmotic coefficient is 

 almost threefold greater so that the plasma proteins 

 behave as the osmotic equivalent of an ideal solute 

 of molecular weight 37,000. If the proteins behaved 

 as ideal solutes they would have to be present in 

 plasma at a concentration of 1 2 per cent in order 

 to exert the same osmotic pressure. The viscosity of 

 such a concentrated solution would more than double 

 peripheral resistance to blood flow. On the other 

 hand, an ideal solute of molecular weight 37,000 

 (i.e., osmotically equivalent to the proteins in plasma) 

 would pass rapidly through the capillary walls and 

 therefore be ineffective in balancing capillary hydro- 

 static pressure. 



The functional significance of the nonlinear osmotic 

 properties of the plasma proteins is further illustrated 

 in figure 3.3. Fluid loss from plasma, amounting to 



CHANGE IN OSMOTIC 



PRESSURE OF PLASMA 



PROTEINS 



mm Hg 

 + 40 T 



~*--Actuol 



deviotion from 

 Van't Hoff's Law 



X-20 



fig. 3.3. Physiological significance of deviations from 

 van't Hoff's law. Fluid loss from plasma causes a dispro- 

 portionately large increment in the osmotic restoring force; 

 conversely, hemodilution causes a relatively small diminution 

 of protein osmotic pressure. 



