CIRCULATION 397 



Probably the blood is actually purest when leaving the kidneys, 

 though it is still dark colored, due to lack of oxygen. It is not 

 correct to speak of " dark blood " as always being " impure blood." 



The Heart. The heart is a hollow, cone-shaped muscle, located 

 behind the breast bone, between the lungs, nearly on the center 

 line of the body; the point is downward and lies between the fifth 

 and sixth ribs a little to the left. Since the " beat " is strongest 

 near the tip it has given the idea that the whole heart is on the left 

 side, which is not true. The heart consists of two entirely separate 

 halves, right and left, each of which consists of a thin- walled auricle 

 and a thick muscular ventricle. The auricles act as reservoirs for 

 the incoming blood and permit a steady flow and rapid filling of 

 the ventricles. The ventricles, by alternate expansion and con- 

 traction, force the blood into the arteries and so around the body. 

 Between each auricle and its ventricle are valves which allow blood 

 to enter the ventricle but prevent its exit, except by the arteries, 

 and at the base of each artery are valves preventing the blood from 

 flowing back into the ventricles. 



Action of Heart. The right auricle receives de-oxygenated blood 

 from the veins through which it has been collected from the whole 

 body. This passes through the valve into the right ventricle, which, 

 when it contracts, forces it to the lungs, via the pulmonary arteries. 

 In the lungs, the blood receives a new load of oxygen, unloads some 

 carbon dioxide and water, and returns via the pulmonary veins to 

 the left auricle. From here it passes through the valves into the 

 left ventricle and is thence forced out through the aorta to all parts 

 of the body. The ventricles contract and expand together so 

 there are two waves of blood sent out at each beat, one to the lungs 

 and one to the general circulation. While the ventricles are con- 

 tracting and forcing out their blood, both auricles have been filling 

 so there is no stop in the flow. 



Rate of Beat. The rate of heart beat is normally 72 times per 

 minute in man; 80, in women; much higher in young children 

 and in very old persons, reaching the average at about twenty 

 years of age. Naturally, the amount of blood needed is affected 

 by exercise, temperature, food, excitement, pain, etc., and so all 



