THE CIRCULATION OF THE BLOOD 267 



the left side, by way of the right side of the heart, the cardio-pulmonic 

 vessels, and the left side of the heart. 



The venous blood flowing toward the heart is emptied by the superior 

 and inferior vena cava into the right auricle, from which it passes through 

 the auriculo-ventricular opening into the right ventricle (Fig. no); thence 

 into and through the pulmonic artery and its branches to the pulmonic 

 capillaries, where it is arterialized by the exchange of gases the giving up of 

 a portion of carbon dioxid to the lungs and the absorption of oxygen and 

 changed in color from bluish-red to scarlet-red. The arterialized blood, 

 flowing toward the heart, is emptied by the pulmonic veins into the left 

 auricle, from which it passes through the auriculo-ventricular opening into 

 the left ventricle; thence into the aorta and its branches to the systemic 

 capillaries, where it is de-arterialized by a second but opposite exchange of 

 gases the giving up of a portion of its oxygen to the tissues and the absorp- 



Fio. no. RIGHT CAVITIES OF THE FIG. in. RIGHT CAVITIES OF THE HEART. 

 HEART. Auricino-venlricular valve open, Auriculo-ventricular valve closed, semilunar 

 semilunar valves closed. (Dalton.) valves open. (Dallon.) 



tion 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 side, it 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 into the left side from the pulmonic 

 veins 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 some 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 

 and discharge the blood into the inferior vena cava just below the diaphragm. 



