ARTERIES, CAPILLARIES, AND VEINS 111 



and down, carrying the float and pen with it, and are thus 

 recorded. By this method it is found that the blood in an 

 artery exhibits at least two regularly recurring changes of 

 pressure, which take the form of smaller waves superimposed 

 upon larger ones. The latter are due to respiratory move- 

 ments, while the former are due to heart beats (Fig. 5). The 

 mean blood pressure is the average pressure during any arbi- 

 trarily chosen length of time. This in man is about 150 mm. 

 Hg or more in the aorta; from 10 to 70 mm. Hg in the capil- 

 laries. In the external jugular and in the veins near the heart 

 the pressure becomes negative. From the aorta through the 

 capillaries and veins back to the heart there is a continuous 

 decline in pressure. 



FIG. 5 



Tracing of arterial pressure with a mercury manometer (Foster). The 

 smaller curves, p p, are the pulse curves. The space from r to r embraces 

 a respiratory undulation. The tracing is taken from a dog, and the irregu- 

 larities visible in it are those frequently met with in this animal. 



The cause of the high pressure in the aorta is the intermittent 

 entrance into it of jets of blood, the resistance offered by the 

 peripheral capillaries and the elasticity of the vessel walls. 

 Each volume of blood forced into the aorta from the heart 

 distends the wall of the vessel, which, through its elasticity, 

 tends gradually to return to its normal size during every dias- 

 tole of the heart. It is at all times, however, stretched, and 

 therefore, always exerts a pressure upon the blood within. 

 Under normal conditions the amount of blood accommodated 

 by the yielding artery during each systole is equal to the amount 

 that passes from the arteries and to the capillaries during the 

 diastole of the heart. Each increase of pressure caused by the 

 heart beat is propagated in the form of a wave through the 

 arterial system, and constitutes what is called the pulse. 



The pressure in the capillaries and veins is caused by the 

 sjiine factors that are present in the aorta power of the heart, 



