BLOOD AND BLOOD DERIVATIVES — COHN 417 



thing more than half the volume. The proportion of the blood oc- 

 cupied by the red cells varies somewhat from individual to individual. 

 If the red cells are too few in number, the condition of anemia obtains ; 

 if they are too concentrated, the viscosity of the blood is increased, and 

 an extra burden is imposed on the heart in forcing the blood through 

 the circulatory system. 



Shock as observed in military medicine generally results from a 

 rapid decrease in the volume of circulating blood due to the loss of 

 blood and plasma proteins externally and into damaged tissues. It 

 is most frequently treated, or prevented, by injection of plasma protein. 

 The injected proteins, insofar as they cannot readily pass through the 

 kidney, increase the body's reservoir of plasma proteins and, insofar as 

 they do not readily transverse the capillary walls, increase the plasma 

 volume by drawing water from the tissues into the blood stream, and 

 by holding it there. 



The maintenance of the fluid balance between the blood and the 

 tissues is one of the functions of the plasma proteins. While they are 

 like hemoglobin in their general organic structure, their special proper- 

 ties and their functions differ widely from hemoglobin or globin. 

 Moreover, there are many different kinds of plasma proteins, each serv- 

 ing a different function. The function of controlling the equilibrium 

 between the water and the electrolytes in the blood and in the tissues is 

 largely performed by the smaller proteins of the blood stream, known 

 as the albumins, although all dissolved colloids will, of course, exert 

 some osmotic effect. 



Osmotic pressure is the force exerted by the dissolved molecules so 

 large that they cannot pass through the pores of the membranes that 

 contain them. The walls of the blood vessels are the retaining walls for 

 the plasma proteins. Oxygen, water, electrolytes, sugars, and other 

 small molecules readily pass these walls. But normally the plasma 

 proteins do not, nor do they pass the kidney and appear in the urine. 

 A variety of conditions influence the permeability of membranes to 

 dissolved molecules. Besides the size and shape of the molecules, these 

 include the charged state of the membranes as well as of the molecules 

 and the nature of their surface groups. The limiting condition for 

 passage through a membrane is, however, the size of the pores and the 

 size of the dissolved molecule. 



The plasma proteins vary in mass and in length. All that have been 

 adequately studied, however, have closely the same diameter, ranging 

 from 33 to 38 angstrom units (an angstrom unit being 10 -8 cm.). The 

 albumin molecules are the most symmetrical of the plasma proteins, 

 being approximately four times as long as their diameters. (See 

 table 1.) 



