CmCULATION OF THE NUTRIENTS 



119 



The right ventricle by its contraction drives the blood 

 through an artery to each of the lungs, until it finally reaches 

 the countless capillaries in the interior of these organs. Veins 

 now receive this blood and convey it to the left auricle, 

 whence it • again enters the left ventricle. About one half 

 minute is required to complete the circulation. 



168. Changes in the composition of the blood. — The 



composition of the blood is continually changing in its pas- 

 sage through the various tissues of the body. We may, 

 perhaps, make clearer these various changes by expressing 

 them in tabular form as follows : — 



VI. Hygiene of the Circulation 



169. Effect of exercise on the heart. — The pulse rate 

 is slowest when we are asleep. As the activities of the day 

 begin, the heart beat is quickened, and after violent exercise 



