CHAPTER XII 



THE VASCULAR SYSTEM CONTINUED: THE GENERAL CIRCULA- 

 TION; BLOOD PRESSURE; THE PULSE; LYMPH; F(ETAL 

 CIRCULATION 



THE GENERAL CIRCULATION OF THE BLOOD 



To trace the general circulation, we will begin with the venous 

 blood, which is returned to the right auricle by the superior and 

 inferior venae cavae. It enters and fills the right auricle, and 

 beyond into the right ventricle, then the auricle contracts and 

 forces the blood over the open tricuspid valve into the ventricle, 

 which has already been passively filled, and now becomes well 

 distended by the extra supply of blood. Almost instantly the 

 ventricle contracts, the blood gets behind the cusps of the tri- 

 cuspid valve and closes them, and thus is forced over the open 

 semilunar valves into the pulmonary artery. The pulmonary 

 artery divides into two branches and carries the blood to the lungs, 

 where it passes through the innumerable capillaries that surround 

 the alveoli or air sacs of the lungs. These capillaries unite to 

 form veins, and these unite to form larger veins, until finally two 

 pulmonary veins return the blood from each lung to the left auricle. 

 The left auricle now contracts and forces the blood over the open 

 bicuspid valve into the left ventricle, just as described for the 

 right side of the heart. Upon contraction of the left ventricle the 

 bicuspid valve is closed (in the same way as the tricuspid), and 

 the blood is forced over the open semilunar valve into the aorta 

 to be carried through the body. As soon as the ventricles relax 

 the semilunar valves are closed by the excessive pressure within the 

 pulmonary artery and the aorta. From the aorta and its branches 

 the blood travels in the capillaries to every part of the body. 

 The capillaries unite to form veins, and finally the blood is returned 

 by means of the vense cavse to the right auricle, which brings it 

 back to where we started from. 



The puhnonary circulation. — The lesser circulation from the 

 right ventricle to the left auricle is called the pulmonary circula- 



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