72 PHYSIOLOGY 



and thinner-walled tubes which penetrate into every part 

 of the body. The finest of these are the capillaries. In 

 these the wall is made of a single layer of flat cells placed 

 edge to edge. It is so thin that the blood is brought in 

 close contact with the tissues and by osmosis gives up 

 readily the nourishing substances it contains, and takes 

 away the waste products. The capillaries unite to form 

 larger tubes called veins, that bring the blood back to the 

 heart. The walls of the veins are very thin, for little 

 work is required of them. The pressure is reduced in 

 the capillaries, and in the veins not enough is exerted to 

 urge the blood onward. It is sucked back into the heart 

 by the alternate change of pressure in the chest cavity 

 due to the movements of expiration and inspiration. 



The blood leaves the left ventricle through a large 

 artery called the aorta, goes throughout the tissues and 

 returns to the right auricle. From this it passes into 

 the right ventricle. It leaves the right ventricle through 

 the large pulmonary artery, goes to the lungs and thence 

 back to the left auricle (Fig. 48). From this it goes to 

 the left ventricle and is ready for another circuit. As 

 the circuit through the body is much longer than that 

 through the lungs, the left ventricle exerts more pressure 

 than the right ventricle and its walls are correspondingly 

 thicker. 



(4) Elasticity, The blood vessels are extremely 

 elastic. This quality causes the blood flow which in the 

 arteries responds to the intermittent heart beat, to be- 

 come steady by the time it reaches the capillaries. At 

 every contraction of the ventricles blood is pushed into 

 the two large arteries, which stretch to receive it. The 



