BLOOD AND BLOOD DERIVATIVES — COHN 419 



pressure is, as a first approximation, a function of the number of dis- 

 solved molecules to which the membrane is impermeable. Since the 

 blood vessels are impermeable to the normal plasma proteins they exert 

 sustained osmotic pressure. Since these proteins cannot pass through 

 the membranes their presence in the blood stream holds water in it in 

 an amount roughly proportional to their concentration, more exactly 

 to their effects on the osmotic pressure. 



Should an increase in permeability of the blood vessels occur which 

 is sufficiently great to allow plasma proteins to escape into the tissue 

 spaces, fluid will also pass into the tissue spaces. Commonly such a 

 change in permeability occurs where there has been a severe burn or 

 injury and usually is not sufficiently great to permit many red cells to 

 leave the blood stream. As a result, there is an increase in red cell 

 concentration (often measured in terms of a hematocrit reading), an 

 increase in viscosity, and a decrease in the volume of circulating plasma. 

 This diminution of blood volume ultimately results in poor peripheral 

 circulation and hence inadequate oxygen transport, while the in- 

 creased viscosity makes it more difficult for the heart to move the re- 

 maining blood through the blood vessels. This condition may be over- 

 come by injecting into the blood stream molecules which will not 

 readily leave it and which, by exerting osmotic pressure, will pull 

 water back into the blood stream, decrease viscosity, and permit a 

 return to normal circulation of the blood with the transport of oxygen 

 to the tissues and the fulfillment of the other physiological functions 

 of the blood. 



The diameters of all the plasma proteins are so nearly the same that 

 a change in permeability that will permit one type of protein to pass a 

 membrane will generally permit all to pass. The amounts of each 

 protein type that will pass will, however, now depend upon other 

 factors controlling permeability. Thus the longer molecules will on 

 the average leave the blood stream less readily than the more sym- 

 metrical albumins. The electrically charged conditions of the mole- 

 cules will also play a role. The albumins bear the largest negative 

 charges, and a species of globulins, known as the y-globulins, precisely 

 because of their low negative electrical charge in neutral solutions, 

 the lowest. 



The plasma proteins that leave the blood stream and appear in the 

 urine or in the tissue spaces, under a variety of conditions, might be 

 expected to reflect all these influences. It has been repeatedly noted 

 by various investigators under various conditions that the distribution 

 of proteins lost from the blood is far more nearly constant than the 

 composition of the plasma under pathological conditions. It is more 

 nearly characteristic of normal than of pathological plasma. Gen- 



