132 HUMAN PHYSIOLOGY. 



aorta and its branches to the systemic capillaries where it is de- 

 arterialized by an opposite exchange of gases, *'. e., yields up a 

 portion of its oxygen to, and absorbs carbon dioxid from the tissues, 

 and changes in color from scarlet to dark blue. The venous blood 

 is again returned to the systemic veins to the venae cavae. 



While there is but one circulation, physiologists frequently divide 

 the circulatory apparatus into 



1. The systemic circulation, which includes the movement of the 

 blood from the left side of the heart through the aorta and its 

 branches, through the capillaries and veins, to the right side. 



2. The pulmonary circulation, which includes the course of the 

 blood from the right side through the pulmonary artery, through 

 the capillaries of the lungs and pulmonary veins, to the left side 

 of the heart. 



3. The portal circulation, which includes the portal vein. This vein 

 is formed by the union of the radicles of the gastric, mesenteric, 

 and splenic veins, and carries the blood directly into the liver, 

 where the vein divides into a fine capillary plexus, from which 

 the hepatic veins arise ; these empty into the ascending vena 

 cava. 



The Mechanism of the Heart. The immediate cause of the 

 movement of the blood through the blood-vessels is the alternate 

 contraction and relaxation of the muscular walls of the heart, and 

 more especially the walls of the ventricles, each of which plays 

 alternately the part of a force pump and to a slight extent of a 

 suction pump. The motive power is furnished by the heart itself. 

 The contraction of any part of the heart is termed the systole, the 

 relaxation, the diastole; as each side of the heart has two cavities, 

 the walls of which contract and relax in succession, it is customary 

 to speak of an auricular systole and diastole and a ventricular systole 

 and diastole ; as the two sides are in the same physiologic relation 

 they contract and relax in the same periods of time. 



Movements of the Heart. At each beat, during the systole, the 

 heart hardens and becomes shortened in its long diameter ; its apex 

 is raised up, rotated on its axis from left to right, and thrown for- 

 ward against the walls of the chest. The impulse of the heart, ob- 

 served about two inches below the nipple and one inch to the sternal 

 side, between the fifth and sixth ribs, is caused mainly by the apex 

 of the heart striking against the chest walls, assisted by the dis- 

 tention of the great vessels about the base of the heart. 



