THE TRANSPORTATION SYSTEM 



509 



very small doses of two of the B-vitamins, 

 folic acid and vitamin Bn; (p. 472). These 

 vitamins are extracted from liver, which 

 was a logical place to find them, since large 

 doses of liver extract were used for years to 

 lessen the symptoms of this particular dis- 

 ease. 



The oxygen and COo carrying capacity 

 of erythrocytes has been discussed earlier 

 (p. 487). 



White blood cells 



White blood cells are called leucocytes 

 because they are colorless. They are not so 

 numerous as the red corpuscles, and there 

 are several different kinds, all with nuclei 

 (Fig. 19-4). They can be distinguished by 

 the way they take certain stains, and when 

 counted they serve as an excellent criterion 

 for determining certain kinds of diseases. 

 They usually increase rapidly in number 

 ( normal count is about 7000-9000 per cubic 

 millimeter) during an infection, because 

 some of them (neutrophils and monocytes) 

 act as scavengers and attack the bacteria 

 by engulfing and digesting them (Fig. 

 19-14). Such a process is termed phagocy- 

 tosis. 



White blood cells have the ability to 

 move like an amoeba, and in a localized in- 

 fection they migrate between the cells of 

 the capillary walls out into the infected 

 region where they do their work. Pus is 

 primarily leucocytes. They probably also 

 aid in healing by transforming into other 

 types of cells in order to repair damaged 

 tissue. Other functions have been assigned 

 to them, but considerable information is 

 still needed to understand their complete 

 role in the body. 



The plasma 



The fluid portion of the blood, the 

 plasma, has many and diverse functions 

 besides that of carrying prothrombin and 

 fibrinogen, already referred to in blood 

 clotting. It may be recalled that it carries 

 the food essentials — amino acids and sugars 



— as well as the wastes, urea and carbon 

 dioxide. Such ions as Na, K, Ca, CI, and 

 several others are also present in plasma, 

 and all perform specific functions in main- 

 taining a stable internal environment for 

 the organism. 



The plasma also transports a very inter- 

 esting and important group of substances 

 called antibodies. Their presence was dis- 

 covered many years ago, in Pasteur's day, 

 and their significance has become more and 

 more important, although as far as their 

 exact nature is concerned we know little 

 more today than was known when they 

 were first discovered. It is best to describe 

 an actual experiment in order to understand 

 their production and what they do, even 

 though how they are produced and how 

 they work is unknown. 



Antibody production 



It was learned in the chapter on digestion 

 that proteins must be degraded to amino 

 acids before they can pass into the blood 

 stream. Any more complex compound of 

 this sort initiates deleterious reactions. For 

 example, if rattlesnake venom is injected 

 into the blood, serious complications fol- 

 low; hence the value of venom as a defen- 

 sive and offensive mechanism. If, on the 

 other hand, a very tiny amount of the 

 venom is injected into a large animal, as a 

 horse, the reaction is not severe, and if tlie 

 injections are gradually increased at short 

 intervals the horse will eventually tolerate 

 a tremendous dose; enough that, had it 

 been injected in one initial dose, it would 

 easily have killed the animal. Just what has 

 taken place to protect the animal from this 

 poison? 



The snake venom is a foreign protein 

 and, like any foreign protein, initiates a 

 reaction in the body of the horse. The reac- 

 tion produces a substance that can combine 

 with the snake venom and neutralize its 

 effect. This substance is called an antibody 

 and the snake venom is the antigen. Any 

 foreign protein can act as an antigen. In 



