COURSE OF THE BLOOD 



159 



One blood vessel leads directly into another. The structure of 

 their walls varies, however. The small, muscular-walled arteries 

 lead into the membranous capillaries which, in turn, lead into the 

 smallest of the veins. 



squeeze the veins as they contract. This, too, forces the blood 

 onward. If a person is inactive for a long time, the blood in 

 the veins becomes sluggish. When we sit still for too long a time, 

 we say a foot has 

 " gone to sleep." 

 Other signs of 

 discomfort may, 

 also, be evident 

 because the skele- 

 tal muscles are 

 not propelling 

 the blood through 

 the veins. 



Course of the 

 blood. The sys- 

 temic circulation. 

 The circulation of the blood through the body is easily under- 

 stood and remembered if we keep in mind that the sequence 

 of organs is from the left ventricle, arteries, capillaries, veins, 

 and right auricle. Blood never returns directly to the same 

 side of the heart it left and the auricles do not connect with 

 each other, nor do the ventricles connect with each other. 



The left ventricle contracts and sends the blood past the valves 

 into the large artery called the aorta. This, in turn, contracts 

 and sends blood through smaller and smaller arteries to all parts 

 of the body except the lungs. As the arteries get in among the 

 tissues of the body, the muscular tissue decreases until the walls 

 of the blood vessels consist of a single layer of cells. These ves- 

 sels are the arterioles which terminate in the capillaries. After 

 giving up the needed materials to the cells through the walls of 

 the capillaries and collecting excretions from the cells, the blood 

 passes into tiny veins. As the small veins lead from the tissues 

 they join and increase in size until they form the two largest 



