PSIO 532 Study Guide - Midterm Guide: Pulmonary Artery, Fraction Of Inspired Oxygen, Partial Pressure
PSL – Respiratory Physiology
GAS EXCHANGE
o Pressure is proportional to the average force exerted by molecules colliding with
the walls of a container. Sea level = 760 mmHg; the partial pressure of oxygen at
sea level is 159.2 mmHg; 5,500 meters = 380 mmHg
o Pressure is usually measured relative to the atmosphere. If the pressure inside
a container = 755 mmHg and the pressure outside = 760 mmHg, then the relative
pressure is -5 mmHg.
o Pressure is also proportional to kinetic energy; therefore, pressure is proportional to
temperature
o PIO2 = FIO2 x PB → when using the barometric pressure to plug into this equation, you
have to subtract the water vapor pressure; but it is taught to students as dry oxygen
(absence of H20)
o In the image, they did not subtract the water vapor from the barometric pressure
o When water is exposed to air, water molecules leave the liquid and become water
vapor.
o Vapor pressure depends on temperature, and is independent of barometric pressure.
o Inspired air is rapidly warmed and saturated with water. The vapor pressure of water at
37oC
is 47 mmHg.
o The PO2 of dry air at sea level is 159 mmHg (760 * 0.2093), but the PO2 at 37oC is 149
mmHg
[(760 - 47) * 0.21]
o As you go up in altitude, the fractional concentration of oxygen in the atmosphere
does not change, it is ALWAYS 0.2093; what is changing is the barometric pressure
o Because 47 mmHg is a constant at 37oC, you must subtract 47 from the barometric
pressure to
calculate PIO2
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