In medical contexts, blood pressure is often given simply as numbers without units.

Question

(a) In taking blood pressure, what physical quantity is actually being measured?
(b) What are the units in which blood pressure is typically reported?
(c) Is the reported blood pressure an absolute pressure or a gauge pressure?
(d) Suppose an ambitious zookeeper measures the blood pressure of a standing adult male giraffe (18 feet tall) in its front leg, just above the hoof, and in its neck, just below the jaw. By about how much are the two readings expected to differ?
(e) What happens to the blood pressure in a giraffe’s neck when it stoops to drink?
(f) What adaptations do giraffes have that allow them to accommodate pressure differences related to their height?

Answer 1

a. “Blood pressure” refers to the pressure exerted by the blood on the interior walls of the major arteries. The higher number (systolic pressure) represents the maximum pressure during the cardiac cycle, and the lower number (diastolic pressure) represents the minimum pressure during the cardiac cycle. In certain situations where patients require continuous, accurate, and rapid blood pressure monitoring, it is measured directly. This can be done by simply inserting a cannula (big needle) into a large artery and connecting it to a pressure transducer. This is much the same way that one measures pressure in a piece of chemical process equipment. Ordinarily, the blood pressure is measured indirectly using a sphygmomanometer. This is the familiar device with the inflatable cuff. What is

measured is the pressure inside the cuff that is needed to stop the flow of blood through the artery it is squeezing (usually the brachial artery in the upper arm). The systolic pressure is the lowest pressure at which flow is completely stopped throughout the cardiac cycle, and the diastolic pressure is the pressure at which the flow is not stopped at any point in the cardiac cycle. Blood flow is detected by a health care practitioner listening for it using a stethoscope, or by a small transducer built into the inflatable cuff.



b. Although they are often omitted, the units for blood pressure are mm of mercury. In SI units, a blood pressure of 120 over 80 would be a systolic pressure of 16000 Pa over 10700 Pa.



c. It is a gauge pressure, measured and reported as the difference in pressure between the pressure in the blood vessels and the ambient pressure. For example, when a scuba diver descends 30 feet below the surface, her blood pressure remains the same, even though the ambient pressure has roughly doubled from its value at the surface.



d. To a first approximation, we expect the pressure difference to just be the hydrostatic pressure difference due the difference in elevation between the hoof and the head. Taking this to be 17 feet (5.2 m) and taking the density of giraffe blood to be the pressure difference should be roughly bar. The pressure in the giraffe’s hooves is about half an atmosphere higher than that in its head (when it is standing).



e. It will fairly quickly increase by half an atmosphere, at least relative to the pressure in the giraffe’s hooves. The actual increase is not quite so big, because the giraffe’s heart moves to a slightly lower level when it stoops, and there accompanying changes in the giraffe’s pulse rate, etc. that have some effect.



f. The biggest problem would occur when the giraffe raises its head. Without special adaptations, the rapid change in pressure would drain all of the blood out of its head and it would faint (compare to the head rush you may sometimes get when changing the elevation of your head by just 3 feet or so). However, giraffes have a collapsible jugular vein and unusually strong ability to constrict the veins in their neck. This allows them to dramatically increase the flow resistance in their veins as they raise their heads, which prevents all of the blood from draining out. It seems that through differential contraction of different arterial paths, they can divert blood flow around the brain, or force a larger fraction through the brain. Overall, they have a much more sophisticated arrangement for controlling

cranial blood pressure than most other animals.