THE CIRCULATION OF THE BLOOD. 269 



de-arterialized by a second but opposite exchange of gases the 

 giving up of a portion of its oxygen to the tissues and the absorption 

 of carbon dioxid from the tissues and changed in color from scarlet 

 to bluish-red. The venous blood is again returned by the systemic 

 veins to the venae cavae. Though the blood is thus described .as 

 flowing first through the right side and then through the left sid^yrit 

 must be kept in mind that the two sides fill synchronously; that 

 while the blood is flowing into the right side from the venae cavae, 

 it is also flowing from the pulmonary veins into the left side in equal 

 quantities and velocities.^ 



Though there is but one set of capillaries, as a rule, between 

 arteries and veins, there is an exception in the case of the arteries and 

 veins of the abdominal viscera. Thus the veins emerging from the 

 capillaries of the stomach, intestines, pancreas, and spleen, instead 

 of passing directly to the inferior vena cava, unite to form a large 

 vein the portal vein which enters the liver. In this organ the 

 portal vein divides to form a second capillary system which is in 

 close relation to the liver cells and from which arise the veins which 

 unite to form the hepatic veins. These latter vessels empty into the 

 inferior vena cava just below the diaphragm. 



From the foregoing facts physiologists frequently divide the 

 general circulation into: 



1. The pulmonary circulation, which includes the course of the blood 



from the right side of the heart through the lungs, to the left 

 side of the heart. 



2. The systemic circulation, which includes the course 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 of the heart. 



3. The portal circulation, which includes the course of the blood from 



the capillaries of the stomach, intestines, pancreas, and spleen 

 through the portal vein to the liver. 



Orifices and Valves. The movement of the blood along the 

 path of the circle above outlined is accomplished by the alternate 

 contraction and relaxation of the muscle walls of the heart. That 

 the movement may be a progressive one, that there shall be no 

 regurgitation during the relaxation, it is essential that some of the 

 orifices of the heart be closed. This is accomplished by the heart 

 valves. 



The right auriculo-ventricular opening is surrounded and strength- 

 ened by a ring of fibrous tissue to which is attached a membrane par- 

 tially subdivided into three portions or cusps, which during the period 

 of relaxation are directed into the ventricle (Fig. 116); during the 

 period of contraction they are raised and placed in complete apposi- 

 tion, when they act as a valve preventing a backward flow into the 

 auricle (Fig. 117). In the former position the valve is open; in the 



