THE SYSTEMIC ClECtTLATlON 197 



and larger veins and finally bring back the oxygen laden 

 blood to the left auricle by four veins (two from each lung). 

 These veins are called the right and left pulmonary veins 

 respectively. From the left auricle the blood passes into 

 the left ventricle, and in this manner the transfer of blood 

 from the right to the left side of the heart is brought about. 

 In our study of the lungs we shall discuss more in detail the 

 manner in which it loses its carbon dioxide and takes on 

 oxygen. 



In this circulation it will be noticed that it is the pul- 

 monary artery which contains the carbon dioxide laden 

 blood, while the pulmonary veins contain the bright red, 

 oxygen laden blood. Owing to this fact the only universal 

 definition that will apply to all arteries and veins is one 

 which is based on the direction of blood flow. Arteries 

 convey blood away from the heart, veins toward the heart. 



The systemic circulation. — From the left ventricle this 

 oxygen laden blood is pumped into a large artery called 

 the aorta. The aorta subdivides into many branches 

 which carry the blood to all parts of the body. In the 

 tissues these arteries subdivide into capillaries, and these, 

 in turn, collect into veins through which the blood finally 

 enters two large veins (the superior and mferior vencB 

 carce) and is returned to the right auricle by them. In 

 this system the blood is transferred over the body and 

 from the left to the right side of the hear,t. 



Since it is this system that carries food and oxygen to 

 the tissues and transfers their wastes and carbon dioxide 

 to the excretory organs, the relations of the branches is 

 important to a clear understanding of the blood flow of 

 this system. As it leaves the heart, the great aorta bends 

 first to the left, then downward through the rear part of 



