34 



THE VITAL PROCESSES 



this function is mainly due to the white corpuscles, it is 

 due in part to the plasma. 1 Through its coagulation, the 

 blood also closes leaks in the small blood vessels. The 

 blood is thus seen to be a liquid of several functions. 



Changes in the Blood. In performing its functions in 

 the body the blood must of necessity undergo rapid and 

 continuous change. The red corpuscles, whose changes 

 have already been noted, appear to be the most enduring 

 constituents of the blood. The plasma is the portion that 

 changes most rapidly. Yet in spite of these changes the 



quantity and character of 

 the blood remain prac- 

 tically constant. 2 This is 

 because there is a balan- 

 cing of the forces that 

 FIG. 12. A balanced change in water. ... 



The level remains constant although the bring about the changes, 

 water is continually changing; suggestive The addition of Various 

 of the changes in the blood. materials to the blood 



just equals the withdrawal of the same materials from the 

 blood. Somewhat as a vessel of water (Fig. 12) having 

 an inflow and an outflow which are equal in amount may 

 keep always at the same level, the balancing of the intake 

 and outgo of the blood keeps its composition about the 

 same from time to time. 



Hygiene of the Blood. The blood, being a changeable 

 liquid, is easily affected through our habits of living. 

 Since it may be affected for ill as well as for good, one 



1 Certain substances, called opsonins, have recently been shown to exist in the 

 plasma, that aid the white corpuscles in their work of destroying germs. The 

 opsonins appear to act in such a manner as to weaken the germs and make them 

 more susceptible to the attacks of the white corpuscles. 



2 Some of the changes in the blood are very closely related to our everyday 

 habits and inclinations'. For example, a lack of nourishment in the blood causes 

 hunger and this leads to the taking of food. If the fluids of the body become 

 too dense, a feeling of thirst is aroused which prompts one to drink water. 



