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 pulsations corre- 

 sponding to each heart beat (Fig. 387). 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 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 the rise of 

 pressure in the artery, and before it has attained 

 its full height the pressure in the artery has FIG. 387. Blood-pressure' 

 already begun to fall. Witk a very wide-tubed Sw 8 ^,^^ 

 manometer the oscillations may be almost im- carotid of rabbit), 

 perceptible owing to the mass of mercury that A ' absci pre SS ure Une f n 

 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 Erank, 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 li ving 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 at the end of diastole, just before 

 the heart begins^to force a fresh quantity of blood into the aorta, is the 

 diastolic pressure ; and the range between these two extremes is known 



919 



