22 THE HUMAN MECHANISM 



and the aorta from the left ventricle. The pulmonary artery 

 supplies the lungs with blood, while all other organs are 

 supplied by the aorta. 



The veins are tubes which conduct the blood from the 

 various organs to the heart. Beginning in the tissues as 

 microscopic tubes, they unite to form larger and larger tubes 

 as 'they approach the heart; those visible through the skin 

 of the hand may be regarded as of medium size ; as the 

 union goes on, the size of the vessels increases until finally 

 at the heart there are only two great veins on the right side 

 (superior vena cava and inferior vena cavd) and four on the 

 left (pulmonary veins). The venae cavae bring blood back 

 from those portions of the body which are supplied by the 

 aorta, that is to say, from all parts of the body except the 

 lungs ; the pulmonary veins bring blood back only from 

 the lungs, that is to say, from the organs supplied by the 

 pulmonary arteries. The venae cavae empty into the right 

 auricle, the pulmonary veins into the left auricle. The gen- 

 eral arrangement of heart, arteries, and veins is shown in 

 Fig. 15, and the figures in Chapter IX (especially 70 and 71) 

 should also be consulted. 



The blood flows in the following circuit: 



Right ventricle to 



Pulmonary artery to 

 Pulmonary circulations T 



.Lungs to 



Pulmonary veins to 



Left auricle to 



Left ventricle to 



Aorta and its branches to 



All organs of the body (except the lungs) to 



Veins which unite to form the venae cavae to 



Right auricle to 



Right ventricle 



Thus the blood which leaves the left ventricle flows to 

 the different organs of the body (except the lungs) and 

 returns by way of the veins to the right side of the heart; 



Systemic circulation 



