HOW DOES THE AIR PRESSURE INSIDE THE BLADDER PROVIDE
INFORMATION ABOUT THE BLOOD PRESSURE?
Paragraph 5-1 stated that blood pressure is the force with which the blood
pushes against the walls of the blood vessel. However, paragraph 5-3a(5) states that
the gauge on the sphygmomanometer measures the air pressure inside the bladder!
a. Indirect Measurement. Some things cannot be measured directly without
difficulty. For example, the height of a building can be measured by climbing to the top
of the building, holding on to one end of a very long tape measure, and dropping the
other end to a friend on the ground who reads off the height. This method may work for
a building that is not very high, but is not recommended for determining the height of the
Empire State Building. The height of a building, however, can be determined indirectly,
such as by measuring its shadow. (Method: Put a stick in the ground so that it is
straight up and down. Measure the height of the stick, the length of the stick's shadow,
and the length of the building's shadow. The height of the building is equal to the length
of the building's shadow times the height of the stick divided by the length of the stick's
b. Blood Pressure Measurement. Just as the height of the building was
determined by measuring something else (its shadow), the pressure of the blood at its
highest (systolic) and normal (diastolic) levels can be determined by measuring the air
pressure in the bladder.
(1) When the bladder is first placed around the arm and not inflated, the
artery beneath the bladder functions normally (figure 5-5 A ).
(2) When the bladder is inflated, the bladder squeezes the arm. If the
bladder is inflated to a pressure greater than the systolic pressure of the artery, the
artery beneath the bladder will collapse (figure 5-5 B ). The artery will remain collapsed,
thus shutting off blood flow below the bladder, even when there is a heartbeat. Thus,
when there is no blood flow in the artery below the bladder, you know that the air
pressure in the bladder is greater than the systolic blood pressure.
(3) When the bladder is inflated to a pressure that is less than the systolic
pressure but greater than the diastolic pressure, blood will flow beneath the bladder only
when the (blood) pressure within the artery is greater than the (air) pressure within the
bladder. This occurs when the force of the heartbeat increases the pressure within the
artery. Once the additional force of the heartbeat has passed (the artery returns to
diastolic pressure), the artery will collapse again (figure 5-5 C ). Thus, when blood
suddenly passes through the artery beneath the bladder, stops, starts again, and stops
again, you know that the pressure within the bladder is less than the systolic pressure
but more than the diastolic pressure.