CHAPTER VI 

 CIRCULATION 



The Heart. The blood is forced through the vessels that 

 contain it mainly by the rhythmical contrations of the heart. 

 Its path, in general, is as follows: Beginning with its exit 

 from the left ventricle, it passes into the aorta, and through 

 its various branches is rapidly taken into the systemic capil- 

 laries of all portions of the body. From the capillaries it is 

 passed into the veins, which lead it back to the right auricle, 

 through which it passes to the right ventricle. The latter, 

 in turn, forces it through the arteries, capillaries, and veins 

 of the pulmonary system to the left auricle, and then to the 

 left ventricle, thus reaching its starting point. That portion 

 of the blood which happens to pass into the capillaries of the 

 stomach, intestines, spleen, or pancreas necessarily circulates 

 through a second set of capillaries which are found in the liver, 

 giving rise to. what is known as the portal circulation. The 

 kidney exhibits a somewhat similar arrangement. 



Every particle of blood follows a path which, no matter how 

 deviating it may be, finally returns into itself. The blood, more- 

 over, is flowing always in a certain definite direction. This 

 is the meaning of the expression "circulation of the blood." 

 The left side of the heart forces the blood through the systemic 

 vessels, while the right side forces it through the vessels of 

 the lungs. It is thus possible for the blood to carry the food- 

 stuffs absorbed from the intestines and those brought to it 

 by the thoracic duct, as well as the oxygen which is absorbed 

 in the lungs, to all the cells of the body. In addition it carries 

 the waste products of the cells, in order that they may be 

 removed by the proper excretory organs. 



The energy of the contractions of the heart is derived from 

 the potential chemical energy of the foodstuffs, and is entirely 

 converted into heat before it leaves the body. Each contrac- 



