Fig. 25.—Observation by the auscultatory method and a mercury instrument. One hand regulates the stop cock which releases air gradually.
Fig. 26.—Observation by the auscultatory method and a dial instrument. The right hand holds the bulb and regulates the air valve.
Arterial Pressure
The arterial pressure in the large arteries undergoes extensive fluctuations with every heart beat. The maximum pressure produced by the systole of the left ventricle of the heart is known as the maximum or systolic pressure. It practically equals the intraventricular pressure. The minimum pressure in the artery, the pressure at the end of diastole, is called the diastolic pressure. The difference between the systolic and diastolic pressures is known as the pulse pressure. There is yet another term known as the mean pressure. For convenience, this may be said to be the arithmetical mean of the systolic and diastolic pressures. Actually, however, this can not be the case, owing to the form of the pulse wave, which is not a uniform rise and fall—the upstroke being a straight line, but the downstroke being broken usually by two notches. We do not make use of the mean pressure in recording results. It is of experimental interest and needs only to be mentioned here.
Fig. 27.—Schema to illustrate the gradual decrease in pressure from the heart to the vena cava: (a), arteries; (c), capillaries; (v), veins; (A), aorta, pressure 150 mm.; (B), brachial artery, pressure 130 mm.; (F), femoral vein, 20 mm.; (IVC), inferior vena cava, 3 mm. (Modified from Howell.)