PHYSIOLOGY OF THE CIRCULATION. 



CAUSE, PURPOSE, DIVISION. 



The blood maintains itself within the vascular system in an unin- 

 terrupted circulating movement that, proceeding from the cardiac ven- 

 tricles through the largest arterial trunks 

 k arising therefrom (the aorta and the pul- 



monary artery) to the furthermost branches 

 of these vessels, then through a system of 

 capillary vessels, from which it is collected 

 into the venous channels, which progressively 

 increase in size by coalescence, terminates 

 finally in the auricles. 



The cause of this circulatory movement 

 resides in the difference in pressure to which 

 the blood is exposed in the aorta and the 

 pulmonary artery, on the one hand, and the 

 two venae cavae and the four pulmonary 

 veins on the other. The blood naturally 

 flows continuously toward that portion of 

 the closed system of tubes where the pres- 

 sure is lowest. The greater this difference in 

 pressure the more active will be the move- 

 ment of the stream. Abolition of this differ- 

 ence in pressure, as after death, will natur- 

 ally cause a cessation of the flow. 



The purpose of the circulation is, on the 

 one hand, to carry nourishment through the 

 blood to all the tissues of the body, while on 

 the other, the blood carries away from the 

 tissues to the organs of excretion the waste 

 products of their metabolism. 



The circulation of the blood is divided 

 into: 



1. The greater circulation, comprising the 

 pathway from the left auricle and the left 

 ventricle through the aorta and its branches, 

 the capillaries and the veins of the body, 

 to the termination of the two venae cavse in 

 the right auricle. 



2. The lesser circulation, comprising the 

 pathway of the right auricle and the right ventricle, the pulmonary 

 artery, the pulmonary capillaries and the four pulmonary veins arising 

 therefrom up to their point of entrance into the left auricle. 



3. The portal circulation is occasionally considered as a separate 

 circulatory system, although it is only a second capillary ramification 



88 



FIG. 21. Diagrammatic Representa- 

 tion of the Circulation: a, right 

 auricle; A, right ventricle; b, 

 left auricle; B, left ventricle; i, 

 pulmonary artery; 2, aorta, with 

 semilunar valves; 1, lesser cir- 

 culation; k, greater circulation, 

 including superior vena cava, o; 

 G, greater circulation, including 

 inferior vena cava, u; d d, intesti- 

 nal tract; m, mesenteric arteries; 

 q, portal vein; L, liver; h, hepa- 

 tic veins. 



