THE CIRCULATION OF THE BLOOD AND LYMPH 105 



June 28 



A" * 9 

 Aug. 3 



ii 7 



- 126 130 mm. of mercury. 



- 126 136 ,, 



- 132144 i. 

 134140 



. I? 6 144 (ZADEK.) 



Such measurements on man show that the mean blood- 

 pressure under similar conditions in one and the same 

 artery, and in one and the same individual, may vary for a 

 considerable time only within comparatively narrow limits. 



This relative constancy of the general arterial pressure is the result 

 of a delicate adjustment between the work of the heart, the resistance 

 of the vessels, and the volume of the circulating liquid. The 

 quantity of the blood is tolerably steady in health, and considerable 

 changes may be artificially produced in it (p. 165) without affecting 

 the pressure in any great degree. On the other hand, the work of 

 the heart and the peripheral resistance are highly variable and vastly 

 influential. A narrowing of the arterioles throughout the body or in 

 some extensive vascular tract increases the peripheral resistance; 

 and if the heart continues to beat as before, the pressure must rise. 

 If the arterioles are widened, while the heart's action remains un- 

 changed, the pressure must fall. In like manner an increase or a 

 decrease in the activity of the heart, in the absence of any change in 

 the peripheral resistance, will cause a rise or a fall in the blood- 

 pressure. But if a slowing of the heart is accompanied by an increase 

 in the peripheral resistance, or a dilatation of the arterioles by an 

 increase in the activity of the heart, the one change may be partially 

 or completely balanced by the other, and the pressure may vary 

 within narrow limits or not at all. 



Not only is the mean pressure, as measured in a large artery, 

 tolerably constant, but if recorded simultaneously in two arteries at 

 different distances from the heart, it is seen to decrease very gradually 

 so long as the arteries remain large enough to hold a cannula. It is 

 nearly as high, for instance, in the crural artery of a dog as in the 

 carotid. It is easy to see that this must be so, for the resistance of 

 the arteries between the point where the arterioles are given off and 

 the heart is only a small fraction of the total resistance of the vascular 

 path; and we have said (p. 73) that the lateral pressure at any cross 

 section of a system of tubes through which liquid is flowing is pro- 

 portional to the resistance still to be overcome. This is also the 

 reason why the pressure is always much lower in the pulmonary 

 artery and right ventricle than in the aorta and left ventricle (only one- 

 third to one-sixth as great), for the total resistance of the vascular 

 path through the lungs is much less than that of the systemic circuit. 



The Velocity-pulse. We have seen that the blood is pro- 

 pelled through the arteries in a series of waves that travel 

 from the heart towards the periphery. The particles in the 

 front of the pulse-wave are constantly changing, but since 

 every section of the arterial tree is successively distended, 



