146 THE HUMAN MECHANISM 



of the heart beat is to keep the arterial reservoir overfilled 

 or distended, so that the elastic reaction of its walls is brought 

 into play; and it is this elastic reaction of the arterial walls 

 which is the immediate cause of the steady outflow through 

 the small arteries and capillaries. 



The force of compression, or pressure, exerted by the 

 clastic arterial walls is primarily exerted upon the blood 

 within them ; and the more the arteries are distended the 

 greater will be the pressure exerted on the blood. A liquid 

 thus under pressure tends to find an outlet; should any 

 part of the arterial wall be weak, as sometimes happens in 

 diseased conditions, it is bulged outward ; and, for the same 

 reason, a flow of blood will take place through such outlets as 

 are presented by the smaller arteries and capillaries. More- 

 over, the greater the pressure of the blood in the arteries, the 

 more rapid will be the flow into the capillaries. Hence it is 

 customary to use the arterial blood pressure as a measure of 

 the force of elasticity exerted by the distended arterial wall. 



The veins, on the other hand, are less elastic than the 

 arteries ; they are, indeed, more like mere conducting tubes 

 through which the blood can flow back to the heart. They 

 are not overfilled (since, for one reason, there is no resistance 

 to the flow of blood out of them into the heart) and hence 

 venous blood pressure is low. 



Thus we have the conditions favorable for the flow from 

 the aorta to the great veins a high pressure in the arte- 

 rial reservoir and a low pressure in the venous reservoir. 

 It is the function of the heart, by continually pumping the 

 blood from the veins into the arteries, to keep the arterial 

 reservoir distended, thus maintaining a difference of pressure 

 in the two reservoirs. It is this difference of pressure which 

 drives the blood through the organs. 



11. The distribution of the blood among the organs. Some 

 organs require more blood than others, and the same organ 

 often requires more blood at one time than at another. Thus 



