236 THE HUMAN BODY. 



lates in them until the elastic reaction of the stretched ar- 

 teries is able to squeeze in a minute through the capillaries 

 just so much blood as the left ventricle pumps into the 

 aorta, and the right into the pulmonary artery, in the same 

 time. Accordingly in a living animal a pressure-gauge 

 connected with an artery shows a much higher pressure 

 than one connected with a vein, and this persistent differ- 

 ence of pressure, only increased by a small fraction of the 

 whole at each heart-beat, keeps up a steady flow from the 

 arteries to the veins. The heart keeps the arteries stretched 

 and the stretched arteries maintain the flow through the 

 capillaries, and the constancy of the current in these de- 

 pends on two factors: (1) the resistance experienced 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 conse- 

 quence 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 

 millimeters (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 vena? 

 cavse 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 mus- 

 cular effort accompanied by holding the breath) which 

 force blood on into the venae cavae and, by impeding the- 

 pulmonary circulation, interfere 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. (7f inches) of mercury, 

 rising to 205 5 = 200 mm. (8 inches) during the cardiac 

 systole and falling to 195 5 = 190 mm. (7| inches) dur- 

 ing the pause; but the slight alterations, only about -fa of 

 the whole difference of aortic and vena cava pressures which 

 maintain the blood-flow, are too slight to cause appreciable 



