Biomedical Engineering Reference
In-Depth Information
2.2.3 Pressure
Pressure is defined as the force per unit area. The SI unit of pressure is N/m
2
and
is called Pascal. Other units of pressure are atmosphere, bar, torr, millimeters of
mercury (1 atm
760 mmHg) and dynes per square-centimeter. Due to contraction
of the heart, the blood pressure in the aorta normally rises to 120 mmHg during
systole, whereas it falls to 80 mmHg during diastole. The pressure of arterial blood
is largely dissipated when the blood enters the capillaries. Although the diameter of
a single capillary is small (7-10
=
m), the number of capillaries supplied by a single
arteriole is so large that the total cross-sectional area available for the flow of blood
is significantly more. Hence, the pressure of the blood as it enters the capillaries
decreases. When blood leaves the capillaries and enters the venules and veins, little
pressure remains to force it along. Pressure in the veins is nearly one-tenth of that in
the arteries. Thus, blood in the veins below the heart is helped back up to the heart
by the squeezing effect of contracting muscles present on the veins. Veins also have
one-way valves called venous valves to prevent backflow within the veins. One of
the functions of the kidney is to monitor blood pressure and take corrective action
if it should drop by secreting the enzyme renin.
Pressure also arises due to Earth's gravity. The pressure at the bottom of a stag-
nant liquid of height (
μ
Δ
h
) is expressed in terms of the density (
ρ
) of the fluid (for
blood
=
1,050 kg/m
3
) as
Δ=
Pgh
ρ
Δ
(2.2)
c
where
g
c
is the gravitational constant and is 9.81 m/s
2
in SI units. This pressure is
also called hydrostatic pressure. If the person is lying down (called a supine posi-
tion), then the hydrostatic pressure is ignored. If blood pressure is measured at
the head height, the systolic/diastolic pressure readings are nearly 35 mmHg less
compared to readings taken at heart level, whereas at ground height the pressure
readings will be 100 mmHg greater. The pressure at any point in a fluid acts equally
in all directions. This is called Pascal's law of fluid pressure.
Blood pressure is monitored using a device called the sphygmomanometer,
which does not require surgical intervention. It consists of an inflatable cuff, a rub-
ber bulb to inflate the cuff, a valve to control pressure, and a pressure gauge such
as a monometer. The cuff is wrapped around the upper arm at roughly the same
vertical height as the location of the heart. Then the cuff is inflated by squeezing the
rubber bulb to a pressure higher (typically 180 mmHg) than the systolic pressure.
The pressure difference between the inside (
P
inside
) and the outside (
P
outside
) of a
vessel is called the transmural pressure, which regulates the shape of a vessel. When
the applied external pressure is higher than the systolic pressure, the artery col-
lapses and blood flow is temporarily halted. Then, the valve is opened to release the
pressure of the cuff very slowly. When the cuff pressure reaches systolic pressure,
the artery reopens allowing blood to pulse in the artery, which makes a whoosh-
ing sound due to vibrations against the artery walls. This sound is heard for the
first time with the help of a stethoscope. With the help of a monometer, the pres-
sure corresponding to that sound is recorded. The blood flow through the artery
increases steadily until the pressure of the cuff drops below the diastolic pressure.
