Biomedical Engineering Reference
In-Depth Information
between two locations, measured in units of mm Hg.
6
Zero pressure is that observed in
a vacuum, and a pressure relative to 0 pressure is called absolute pressure. Unless stated
otherwise, pressure is given relative to standard atmospheric pressure. Systolic or peak
blood pressure is typically around 120 mm Hg relative to standard atmospheric pressure,
or 880 mm Hg in terms of absolute pressure. With regard to inspiration, the pressure within
the lungs is
4 mm Hg, a negative pressure (with respect to standard atmospheric pres-
sure) that allows air to move into the lungs.
Within the body, we identify relative pressures that drive solutes in or out of a compart-
ment. For example, the pressure inside a capillary on the arterial end is approximately
30 mm Hg and inside a capillary on the venous end is approximately 10 mm Hg. Thus, a
relative pressure difference of 20 mm Hg drives the blood from the arterial to the venous
end of the capillary. On the arterial end of the capillary, the pressure in the interstitial fluid
is approximately 17 mm Hg, with a relative pressure difference of 13 mm Hg that drives the
plasma through the capillary walls into the interstitial fluid. On the venous end of the cap-
illary, the pressure in the interstitial fluid is still approximately 17 mm Hg. Thus, a relative
pressure of 7 mm Hg drives the interstitial fluid through the capillary walls into the plasma.
For the cell, the pressure difference between inside and outside is zero. Any pressure dif-
ference across the cell membrane causes a flow of water from high pressure to low pressure
to equilibrate the pressure gradient.
As given by the ideal gas law, pressure is a function of temperature, volume, and the
number of atoms and molecules. The ideal gas law assumes that there is no energy between
atoms or molecules, such as attractive forces, and the only energy is kinetic. The motion of
atoms and molecules create pressure as they collide with each other and the walls of the
compartment—the faster the motion, the larger the pressure. Changes in temperature affect
the motion of the particles. Increasing the temperature increases the speed of the atoms and
molecules, which in turn, increases the pressure. Increasing the number of particles
increases the pressure, since more collisions are possible. Pressure is inversely proportional
to volume. Increasing the volume reduces the pressure, since there is more space for the par-
ticles to move, which reduces the number of collisions. Since temperature is highly regulated
in the body, the affects of temperature changes are not a major consideration in osmosis for
the body. To more fully appreciate osmosis, we first present two situations that are analyzed
qualitatively. Following this, we quantitatively analyze osmosis.
Consider Figure 7.5, with water on both sides of the membrane. The two pistons allow
pressures
p
1
>
p
2
, Piston 1 drives water
through the membrane from the left side to the right side. Suppose
p
1
and
p
2
to be applied to each compartment. If
p
1
¼
p
2
, and water is
mixed with a small amount of solute on the left side and water is mixed with a large
amount of solute on the right side. Osmosis causes water to move from the left side to
the right side until the water concentrations are the same on both sides of the membrane.
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In physiology and medicine, the unit for measuring pressure is mm Hg. This unit is defined as the height of
a column of mercury that can be supported for a given pressure. The pressure of atmosphere at sea level,
called the standard atmospheric pressure, is a commonly used reference for pressure measurements and
equals 760 mm Hg. The SI unit of pressure is the
1
m
, and the U.S. common unit of
Pascal
(Pa), where 1Pa
¼
lb
in
2
696
lb
in
2
pressure is
ð
psi
Þ:
At sea level, standard atmospheric pressure is 760 mm Hg
¼
101
:
325 k Pa
¼
14
:
:
