CIRCULATION I 1 1 



merits are slightly heavier (sp. gr., 1.090) than plasma (sp. gr., 1.030), and 

 they slowly sink by gravity. This difference in specific gravity is insuf- 

 ficient to cause settling of the- formed elements from plasma in the circula- 

 tion where blood is kept in continual agitation. 



The formed elements make up about 40-50 per cent, of the volume of 

 whole blood; and plasma, 50-60 per cent. These figures vary somewhat, 

 even in health, with temporary physiological changes in the water con- 

 tent of the blood. If one sweats profusely, the cells are temporarily more 

 concentrated, and the volume percentage of the plasma is reduced. If con- 

 siderable water is drunk, for a short time there will be a larger proportion 

 of plasma. In any case, the fluctuations are slight and temporary, and the 

 relative proportions of cells and fluid remain fairly constant. 



Approximately 90 per cent, of the plasma consists of water, in which 

 many substances are dissolved or suspended. Obviously, at one time the 

 plasma contains every product which tissue cells use and obtain from the 

 outside and also all substances produced by cells which are transported 

 to other organs to be used, in turn, by them, or excreted from the body. 

 In addition, other materials are found, all of which contribute in some 

 way or other to the maintenance of the relative constancy of the internal 

 environment of cells. 



In view of the importance of the circulating fluid, it is not surprising 

 to find that elaborate mechanisms have been evolved which guard against 

 its loss should a blood vessel chance to be ruptured. One of nature's de- 

 vices in invertebrates has been to produce spasms or strong contractions of 

 ruptured vessels, which serve to pinch off the opening. In vertebrates the 

 same end is served by the coagulation or clotting of blood, which everyone 

 has observed in his own blood escaping from a ruptured vessel. When 

 blood is drawn from a vein into a beaker, it retains its fluidity only a short 

 time. It is converted into a semisolid gelatinous mass, or clot, in some four 

 to eight minutes. If clotted blood is examined under the microscope, 

 threadlike or needle-like processes are seen to appear. As they increase in 

 length and number, they form an entangled interlacing netw'ork called 

 fibrin. Between the meshes the formed elements (red and white blood 

 cells) and some of the fluid become entrapped in the solidifying mass. 

 As it solidifies, the clot also shrinks, squeezing out from its interstices a 

 straw-colored fluid known as serum, which collects above the clot and 

 retains its fluid consistency indefinitely. 



Two distinct factors seem capable of initiating the clotting process: 

 contact of the blood with injured tissues or damaged cells or contact of 

 blood with "foreign" surfaces possessing certain physical properties dif- 

 ferent from those of the smooth lining of the blood vessels, with which the 

 blood is normally in contact. 



Contact with injured cells — If blood is drawn into a vessel whose inner 

 surfaces are properly prepared, taking care that fluid from tissues neces- 



