Physiology of the Circulation 



42. GENEEAL VIEW. The blood within the vessels is in a state of continual 

 motion", being carried from the ventricles by the large arteries (aorta and pulmonary) 



and their branches to the system of capillary 

 vessels, from which again it passes into the veins 

 that end in the atria of the auricles (W. Harvey, 

 1628). 



The cause of the circulation is the difference of 

 pressure which exists between the blood in the 

 aorta and pulmonary artery on the one hand, and 

 the two venae cavae and the four pulmonary veins 

 on the other. The blood, of course, moves con- 

 tinually in its closed tubular system in the direction 

 of least resistance. The greater the difference of 

 pressure, the more rapid the movement will be. 

 The cessation of the difference of pressure (as after 

 death) naturally brings the movement to a stand- 

 still ( 81). The circulation is usually divided 

 into 



(1) The greater, or systemic circulation, 

 which includes the course of the blood from the 

 left auricle and left ventricle, through the aorta and 

 all its branches, the capillaries of the body and the 

 veins, until the two venae cavae terminate in the 

 right auricle. 



(2) The lesser, or pulmonic circulation, which 

 includes the course from the right auricle and right 

 ventricle, the pulmonary artery, the pulmonary 

 capillaries, and the four pulmonary veins springing 

 from them, until these open into the left auricle. 



i right, ( 3 ) ^he portal circulation is sometimes spoken 



B, left of as a special circulatory system, although it repre- 



ventricle ; 1, pulmonary artery ; sents only a second set of capillaries (within the 



2, aorta ; I, area of pulmonary, K, li ve r) introduced into the course of a venous trunk. 



ar ^l7 8 v ^ i( ;l r r r^'^n ^ consists of the vena portarum-formed by the 

 superior vena cava: (j, area sup- . . * * 



plying the inferior vena cava, u ; union of the intestinal or mesenteric and splenic 

 d, d, intestine ; m, mesenteric veins, and it passes into the liver, where it divides 

 artery ; q, portal vein ; L, liver ; j n t capillaries, from which the hepatic veins arise. 

 h, hepatic vein. The hepatic vein joins the inferior vena cava. 



Strictly speaking, however, there is no special portal circulation. Similar arrangements 

 occur in other animals in different organs, e.g., snakes have such a system in their supra-renal 



Fig. 25. 

 Scheme of the circulation. 

 6, left auricle ; A, right 



