Freezing Point Depression of Plasma ‡

In an earlier paper2 we have reported results demonstrating that changing the concentrations of the carbon dioxide and oxygen with which the bloods of sheep and goats are equilibrated has a measurable effect on the total osmotic pressure of the true plasma. At constant oxygenation the osmotic pressure of the true plasma is increased 0.9 milliosmoles/Kg H20 for each millimole of C02/Kg H20 added to the whole blood. When the carbon dioxide content of the whole blood is held constant, reduction of one millimole of hemoglobin/Kg H20, (M.W. 17,000), lowers the osmotic pressure of the true plasma by circa 0.3 milliosmoles/Kg H20. From these data the effect of the exchange of the respiratory gases on the osmotic pressure of the plasma as the blood passes through the pulmonary capillaries can be calculated: