ARTERIAL PRESSURE. THE PULSE. 243 



enced by the blood in its flow from the ventricles to the 

 veins, and (2) the elasticity of the larger arteries which allows 

 the blood to accumulate in them under a high pressure, in 

 consequence of this resistance. 



The Arterial Pressure. This cannot be directly meas- 

 ured with accuracy in man, but from measurements made on 

 other animals it is calculated that in the human aorta its 

 average is equal to that of a column of mercury 200 milli- 

 meters (8 inches) high. During the systole it rises about 5 

 millimeters (| inch) above this and during the pause falls the 

 same amount below it. The pressure in the venae cavae on the 

 other hand is often negative, the blood being, to use ordinary 

 language, often " sucked " out of them into the heart, and it 

 rarely rises above 5 millimeters (^ inch) of mercury except 

 under conditions (such as powerful muscular effort accom- 

 panied by holding the breath) which force blood on into the 

 venae cavae and, by impeding the pulmonary circulation, in- 

 terfere with the emptying of the right auricle. Hence to 

 maintain the flow from the aorta to the vena cava we have 

 an average difference of pressure equal to 200 5 195 

 millimeters (7| inches) of mercury, rising to 205 5 = 200 

 mm. (8 inches) during the cardiac systole and falling to 

 195 5 = 190 mm. (7-f inches) during the pause; but the 

 slight alterations, only about -^ of the whole difference of 

 aortic and vena-cava pressures which maintain the blood- 

 flow, are too small to cause appreciable changes in the rate of 

 the current in the capillaries. The pressure on the blood in 

 the pulmonary artery is about of that in the aorta. 



Since the blood flows from the aorta to its branches and 

 from these to the capillaries and thence to the veins, and 

 liquids in a set of continuous tubes flow from points of 

 greater to those of less pressure, it is clear that the blood- 

 pressure must constantly diminish from the aorta to the 

 right auricle; and similarly from the pulmonary artery to 

 the left auricle. At any point, in fact, the pressure is pro- 

 portionate to the resistance in front, and since the farther 

 the blood has gone the less of this, due to impediments at 

 branchings and to internal friction, it has to overcome in 

 finishing its round, the pressure on the blood diminishes as 

 we follow it from the aorta to the venae cavae. In the larger 

 arteries the fall of pressure is gradual and small, since the 

 amount of resistance met with in the flow through them is 



