Physiology of the Circulation, 



42. General View of the Circulation. 



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). 



The cause of the circulation is the differ- 

 ence of pressure which exists between the 

 blood in the aorta and pulmonary artery on 

 the one hand, and the two venae cavse and 

 the four pulmonary veins on the other. 

 The blood, of course, moves continually 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 standstill. The circulation 

 is usually divided into 



(1.) The greater, or systemic circulation, 

 which includes the course of the blood from 



the left auricle arid left ventricle, through the 



aorta and all its branches, the capillaries of Scheme of the circulation- a, 

 the body and the veins, until the two vena? right auricle ; A, right ven- 

 cava3 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 right auricle. 



(3.) The portal circulation, which is some- 

 times spoken of as a special circulatory system, 

 although it represents only a second set of 

 capillaries (within the liver) introduced into' the course of a venous 



5 



Fig. 16. 



tricle; b, left auricle; B, 

 left ventricle ; 1, pulmonary 

 artery ; 2, aorta with semi- 

 lunar valves ; I, area of pul- 

 monary circulation ; K, area 

 of systemic circulation in 

 region supplying the supe- 

 rior vena cava, o ; G, area 

 supplying the inferior vena 

 cava, u; d, d, intestine; m, 

 mesenteric artery; q, portal 

 vein; L, liver; U, hepatic 

 vein. 



