Close to the heart, the mean arterial pressure is about 00— 120 mm. It falls only slowly in the large arteries, but between the smaller arteries and the capillaries there is a very extensive fall of pressure, so that the capil lary pressure is only about io to 3o mm. of mercury; from the capillaries to the veins the blood pressure falls steadily, until in the large veins near the heart it may be negative.
The arterial blood pressure in man is determined by means of the Riva Rocci sphygmomanom eter (figure 19), which consists of a rubber bag that can be strapped round the upper arm. The bag is connected with a manometer, and the pressure in the bag is raised by pumping air into it. The pressure at which all pul sations disappear in an artery below the place of compression represents the systolic pressure. The diastolic pressure can also be determined by the same apparatus.
To determine the venous pressure in man, an apparatus is used which is constructed on the same principle as the sphygmomanom eter. The skin is greased, and an annular rubber bag is placed over a vein and cemented over with a glass plate (figure 2o). On blowing air into the bag, the pressure can be determined at which the vein collapses. The blood pressure in the capillaries is usually determined by the von Kries method (figure 21). A small glass plate is placed over the skin; attached to this plate is a small scale pan on which weights are placed until the pressure is just suffi cient to blanch the underlying skin.
The mean blood pressure in the circulatory system of a young adult man in the horizontal position was found to be as follows: From this we see that the largest drop in pressure occurs between the small arteries and the small veins. This shows that the main resistance of the vascular system is situated in the arterioles. The arterioles are always in a semi contracted state (tone), partly determined by impulses coming from the central nervous system, and partly by the properties of the plain muscles of which they are composed. Since the total vascular bed of the capillaries is very much larger than that of the arterioles, the main drop in the pressure must occur just past the arterioles. The mean arterial pressure depends on two factors : (a) the total resistance to the outflow of blood from the arterial system, i.e., the state of con
striction of the arterioles; (b) the output of the heart in a given time, which depends on the inflow of blood.
The Arterial Pulse.—Owing to the elasticity of the arteries, every systolic rise of the blood pressure produces an expansion of their walls, which can be felt by placing a finger on any super ficial artery. It is obvious that the nearer the artery is to the heart, the more pronounced will be the pulse. The rate of trans mission of this pressure wave will depend on the elasticity of the arteries. If they were rigid, no pulse could be recorded. The more elastic the arteries, the slower will be the transmission of pressure along them. Under normal conditions, the pulse wave in man is transmitted at the rate of about 7 meters per second, but if the arteries are initially stretched by high pressure their walls will tend to approximate to rigidity, and therefore the propagation of the pulse wave will be faster. It is important not to confuse the velocity of the pulse wave, which is simply a transmission of pres sure along the tubes, with the velocity of the blood flow. The latter is of the order of about 0.5 meters per second in the aorta, and considerably less in the smaller blood vessels.
The elasticity of the arterial system determines another im portant feature of the blood flow ; it is the cause of the continuity of the blood flow (except for slight increases during systole and de creases during diastole), in spite of an intermittent ejection of blood by the heart into the aorta. A sufficient amount of blood is accommodated in the arterial system to maintain a flow into the capillaries during the whole period of diastole. In the capillaries the pulse disappears, and the blood flow is continuous. Arterial pulse tracings recorded by means of sphygmographs (a system of levers which can be placed on a pulsating artery, the pulsation of which is thus registered on a moving plate with blackened sur face) show various secondary undulations, either in the ascending part of the wave (anacrotic pulse), or in the descending (cata crotic pulse). These are partly due to extra vibrations set up in the arterial wall by the inrushing blood, and partly to pressure waves reflected from the periphery.