175 



304. Exchange of oxygen and carbonic acid in the capil- 

 laries. The blood in a capillary is separated from a living 

 cell of the body by a wall so thin that it is no hindrance 

 to the passage of oxygen from the red blood cells. In 

 return for the oxygen received from the blood, the body 

 cells give out carbonic acid gas, which passes through the 

 capillary walls into the blood as readily as the oxygen 

 passes in the opposite direction. A given particle of 

 blood remains in a capillary only a second at most, and 

 in that time there occurs an exchange of oxygen and 

 nutritive matter between the blood and the body cells. 

 Arteries are simply tubes which conduct blood to the 

 capillaries, where all the actual work of nourishing the 

 cells is performed. 



305. Veins. The network of capillaries at the end of 

 each artery unites to form a single tube, called a vein. 

 Each vein unites with others again and again, to form 

 larger tubes which run alongside of each artery, and finally 

 all unite to form two main veins. One vein, called the 

 descending vena cava, returns blood from the head and 

 arms ; the other, called the ascending vena cava, returns 

 blood from the lower extremities and trunk. Each opens 

 into the right auricle. The veins have about three times 

 the capacity of the arteries. Their walls are composed of 

 the same material, but are very much thinner, for they do 

 not have to stand much pressure of blood. The blood 

 current is correspondingly slow. The veins have valves 

 at intervals which permit of a free flow toward the heart, 

 but oppose its passage backward, so that when a vein is 

 pressed the blood is forced only towards the heart. The 

 contraction of the muscles pressing upon the veins is 

 thus a great aid to the flow of blood. The flow of blood 

 is also aided by the movements of the chest in breathing, 



