ARTERIES 141 



Their numerous branches differ from them mainly in 

 being smaller. 



Functions of arteries. The arteries form smooth 

 channels through which the blood is guided to all parts 

 of the body. When the left ventricle contracts and 

 pushes the blood into the aorta, the walls of the aorta 

 are stretched out by the extra supply of incoming blood. 

 The elastic recoil of the walls squeezes the blood forward 

 through the branches of the arteries. Before the arteries 

 have a chance to push all the blood that is in them for- 

 ward, they are again filled by the contraction of the left 

 ventricle. The right ventricle also contracts at the same 

 instant and sends its blood into the pulmonary artery, 

 which by its elastic contraction forces it through the 

 lungs. 



Structure of arterial wall. The elastic recoil of the 

 walls of the larger arteries is due to a special form of 

 highly elastic tissue which resembles rubber in its ac- 

 tion. Thin overlapping layers of this material lie in 

 the arterial walls. "When the arteries are stretched out 

 by the blood pumped into them, the elastic tissue makes 

 them recoil so that they press upon the blood and thereby 

 push it along. Obviously, the pressure exerted upon the 

 blood is greater when they are distended than when 

 they are but partially filled. As we shall see, this is 

 an important factor in the circulation of the blood. 



Control of blood supply. In addition to the elastic 

 tissue, the walls of the arteries also contain encircling 

 muscular fibres, which are especially numerous in the 

 very small arteries, the arterioles. These muscular fibres 

 are all of the non-striated variety and are slow in con- 

 traction.* In the arterioles, their function is to control 

 the amount of blood passing through them to the tissues. 

 For example, if a tissue is active, these small muscle 



