SECTION II 



THE BLOOD PRESSURE AT DIFFERENT PARTS 

 OF THE VASCULAR CIRCUIT 



THE ARTERIAL BLOOD PRESSURE. The arterial blood pressure 

 as recorded by a mercurial manometer exhibits a series of pulsa- 

 tions corresponding to each heart-beat (Fig. 372). These pulsations 

 are due to the fact that the artery becomes fuller each time the 



ventricle forces more blood into it during 

 its systole. Between the beats of the heart, 

 i.e. during diastole, the aortic valves are 

 closed, and blood escapes from the arteries 

 into the capillaries and veins, so that the 

 blood pressure falls. The mercurial mano- 

 meter does not register these rapid changes 

 [A of pressure in the artery with any accuracy. 

 The inertia of the mercury is such that it 

 takes some time to be set into movement by 

 FIG. 372. Blood-pressure the rise of pressure in the artery, and 



tracing taken with mer- i ) pf OTp u v. aq oftoinpf] its fill] height the 

 curial manometer (from n61 8 n 



carotid of rabbit). pressure in the artery has already begun to 



A> ^nopressure! 116 falL With a very wide-tubed manometer 



the oscillations may be almost imperceptible 



owing to the mass of mercury that has to be moved at each 

 heart-beat. Such a manometer gives a true record of what is 

 known as the ' mean arterial pressure.' In order to determine the 

 true course of the pressure in the heart it is necessary to diminish 

 to the utmost possible extent the inertia of the moving parts of the 

 recording instrument, and to employ some manometer such as that 

 of Hiirthle or of Frank, in which the pressure is measured by recording 

 the stretching of an elastic membrane. Such instruments will be 

 described later in dealing with the changes of pressure in the ventricle 

 during contraction. 



In the living animal the variation in the arterial pressure at each 

 heart-beat is much greater than would be anticipated from an 

 inspection of the tracing given by the mercurial manometer. The 

 highest pressure which occurs while blood is passing from the heart 

 into the aorta is called the systolic arterial pressure ; the pressure 



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