CIRCULATION 



75 



blood vessels do not all enlarge at the same time. When 

 certain blood vessels dilate, certain others contract, so 

 that the volume of blood is 

 always slightly in excess 

 of the capacity of the blood 

 vessels. Elasticity is then 

 called upon to make room 

 for the excess. The sup- 

 ply of blood sent to a tis- 

 sue is thus adjusted to the 

 need of the tissue. The 

 more active the tissue the 

 more the vessels dilate and 

 the more blood passes 

 through. The power of 

 the vessels to change their 

 size also has an important 

 effect on the temperature 

 of the body, as we shall see 

 later. 



Composition of the 

 Blood.— T he blood is 

 largely composed of water. 

 In it sodium chloride and 

 other salts are dissolved. 

 This salt solution holds in 

 suspension several proteids 

 and the red and white corpuscles. In it also are 

 food for the tissues, secretions of various glands, and 

 waste products of oxidation. Through the give and take 

 of the tissues the food substances are present in the 



Fig. 49.— Diagram showing the relation 

 between general arterial tone and the 

 supply of blood to the brain. In A, 

 the arterioles in organs m, n, s, are 

 constricted, raising the general arte- 

 rial pressure and forcing a large 

 amount of blood through the brain. 

 In B, they are dilated, lowering the 

 general arterial pressure and dimin- 

 ishing the amount of blood sent to 

 the brain. (After Hough and Sedg- 

 wick.) 



