Which is greater, blood pressure or atmospheric pressure? It shouldn't be difficult to answer this question if we consider the actuality of blood escaping once a vessel is severed. Blood pressure, in this regard, is therefore greater than atmospheric pressure.
The rate and force at which blood escapes is an approximate indication of blood pressure. To a great extent, blood pressure exhibits divergence in the different areas within the circulatory system.
With each heartbeat, fluctuations in arterial pressure take place. These fluctuations are correlated with the intermittent gushing forth of blood from a severed artery, as well as with the arterial pulse. These fluctuations can best be marked with the aid of a manometric instrument (specifically, a mercury manometer), in which it can be noticed that the pressure ascends during cardiac systole, when blood is ejected by the heart, and descends during diastole, when the semilunar valves close. These rising and falling of arterial pressure are what we refer to as the systolic and diastolic pressures.
In normal young adults, the normal systolic arterial pressure is about 120 mmHg (millimeter of mercury), while the diastolic pressure is about 80 mmHg. The arterial pressure fluctuates between these two points during each heart cycle. This fluctuation, which is better known as pulse pressure, normally measures about 40 mmHg. However, a lot of normal young adults may have lower pulse pressure than that mentioned here; it may increase appreciably with exercise, excitement, or such other normal conditions.
Two factors can be considered significant contributors to the pulse pressure phenomenon: the action of the semilunar valves (a set of three crescent-shaped cusps between the heart and the aorta, and another set of three between the heart and the pulmonary artery) and the springiness of the arteries. In systole during ejection of blood from the heart, the springiness of the vessel walls reduces the ascent in aortic pressure. The vessels expand, in the process providing a greater volume of channel for the conveyance of the ejected blood.
Similarly in diastole, the springy reaction of the vessels keeps the aortic and arterial pressure from descending rapidly. Likewise, as the heart slows up, the same pressure in the aorta and arteries is kept from descending to the low point reached within the heart itself in diastole because of the closure of the semilunar valves. Passing from the aorta on to the arterioles (the small terminal twigs of an artery), the pulse pressure becomes less and less, this being the result of arterial elasticity that progressively reduces both the ascent of pressure in systole and the descent of pressure in diastole.
Meanwhile, pulse pressure has totally vanished in the capillary bed. This merely supports the fact that blood constantly flows in the capillaries, unlike in the aorta where blood flow is intermittent.
The arterial pulse pressure, therefore, is made more intense with the hardening of the arteries, when the springiness of the vessels is greatly diminished. Further, when damage to the semilunar valves occurs, the diastolic pressure in the arteries may descend to such a low point, possibly nearing the diastolic pressure occurring within the ventricle.
This situation can heighten the arterial pulse pressure, usually to such an abnormal magnitude that it is already possible for it to be determined by way of the throbbing of an equally abnormal strong pulse in the wrist. The good thing about this is that the said throbbing of an abnormally strong pulse may help the doctor in detecting leakage or damage in the semilunar valves.
1. "Pulse Pressure" by Craig Weber, M.D., on About.com: High Blood Pressure (online).
2. "The Importance of Pulse Pressure" by Robert W. Griffith, MD, on HealthandAge.com (online).