THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 



93 



ments, are ordinarily destroyed by the phagocytes before they can be 

 carried into the blood stream. 



THE MECHANICS OF CIRCULATION 



We have seen that it is the powerful contraction of the ventricles that 

 pumps the blood out along the arteries. When the ventricles contract, 

 50 to 100 cc. of blood is forced into the aorta and an equal amount into 

 the pulmonary artery. The passage of the blood through the smaller 

 arteries, especially through the fine capillaries, encounters marked fric- 

 tional resistance, so that the blood in the arteries is always under pres- 

 sure, which somewhat distends the elastic walls of these vessels. The 

 elasticity of the arteries continues to force the blood into the capillaries, 

 even in the intervals between heartbeats. Each beat of the heart sud- 

 denly pumps another 50 to 100 cc. of blood into the aorta and pulmonary 

 artery, and the increased pressure causes a short section of the elastic 

 artery walls to expand. The wave of pressure travels rapidly out over 

 the arteries and their branches at a rate 10 to 15 times as fast as the flow 

 of the blood itself and is gradually extinguished in the smaller arteries. 

 It is the succession of such impulses, or pressure waves, produced by the 

 rhythmic contractions of the ventricles and accompanied by a bulging 

 of the arterial wall, that constitutes the pulse, by means of which the 

 rate of heartbeat can be determined. It is also this series of repeated 

 impulses from the heart that produces an alternating maximal (systolic) 

 and minimal (diastolic) pressure in the arteries. 



The average normal blood pressure increases slowly with age. This is 

 often attributed to the loss of elasticity and tone of the arteries, but the 

 cause is somewhat obscure. According to studies published in 1950, the 

 range of normal variation is much greater than is commonly assumed. The 

 following table gives this range for different ages, in terms of millimeters 

 of mercury: 



Owing to the damping out of the pulse by friction and the elasticity 

 of the arterial walls, blood enters the capillaries in an almost steady 



