148 STUDIES IN PHYSIOLOGY 



rich in oxygen ; while the darker blood may be described as 

 de-ox'y-gen-a'ted blood. The former is not necessarily pure 

 blood, nor is the latter always impure. The purest blood 

 of the body is undoubtedly the dark colored blood that 

 pours into the inferior vena cava from the kidneys, for 

 when it reaches the kidneys it has already lost in the lungs 

 its carbon dioxid; in the kidneys the water and urea are 

 removed. 



Regulation of the Blood Supply to the Various Organs. Every 

 one knows that the pulse beat is more rapid and the body 

 feels warmer during vigorous exercise. The feeling of warmth 

 is due to the more rapid metabolism that is going on in the 

 muscles, and because of this metabolism there is need of 

 a greater supply of blood to repair the wasting tissues. 

 Since there is seldom more than twelve pounds of blood 

 in the body of an adult, other organs must do without 

 their usual supply if the working muscles are to receive 

 the amount of blood that they need. It will be remembered 

 that in the description of the arteries, the presence of muscle 

 tissue was noted ; it is these muscles in each artery that de- 

 termine its size. When a considerable amount of blood is 

 needed, for instance, in the brain, the muscles of the arteries 

 in this part of the body relax, and the vessels become larger. 

 In other parts of the body, however, there must be at the same 

 time a corresponding decrease in the size of arteries by the 

 contraction of the circular muscles. All these changes in 

 the diameter of arteries are regulated by the sympathetic 

 nervous system. 



4. THE LYMPHATIC SYSTEM 



The Lymph. From our discussion of the structure and 

 use of the capillaries, one might infer that the liquid nutri- 

 ents of the blood pass directly from these thin-walled tubes 

 into the tissue cells, and that waste substances are given off 

 from the cells directly to the blood. In reality this is not 

 true. In the tissues of the body there are countless small 



