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Physiology Objectives 39
Properties of gases: remember the ideal gas law: PV = nRT. From this, we can
tell everything else: if everything is held constant except pressure and volume,
we get P1V1 = P2V2 (Boyle’s Law) and similarly, if volume and temperature are
variable, V1/T1 = V2/T2 (Charle’s Law)
 Note: Henry’s Law states that the gas content in a solution is equal to its
solubility multiplied by its partial pressure. With regards to oxygen, there
is a higher O2 content in the blood than in water because blood has
hemoglobin which greatly increases O2 solubility.
Dalton’s Law of Partial Pressures: the partial pressure exerted by a gas in a
mixture is directly proportional to its concentration
Calculation of oxygen in different areas: oxygen composes 21% of
atmospheric air, and therefore is present at 21% of 760 mmHg (atmospheric
pressure) or 159 mmHg partial pressure. In the human body, the air is
humidified, and “loses” 47 mmHg due to water vapor pressure. Thus, the new
partial pressure of oxygen inside the respiratory system is 21% of 713 mmHg or
149 mmHg.
Minute ventilation: ventilation flow into the lungs per minute
Gas exchange: by measuring inspired and expired gas concentrations and
volumes, one can calculate the gas being exchanged; with oxygen inspired and
expired concentrations and volumes are calculated, with carbon dioxide, since
none is being inspired, only expired concentration and volume are necessary.
Respiratory quotient: determined by taking a tissue sample and measuring CO2
production and O2 consumption. The quotient is CO2 produced/O2 consumed.
Experiment to measure O2 uptake, CO2 output, and respiratory exchange
ratio: have patient breathe into latex bag for a minute, and measure the volume
inspired. Then, measure the volume of the bag, and determine the concentrations
of both O2 and CO2 in the bag. From this, we can use equations to determine the
oxygen uptake and carbon dioxide elimination; the respiratory exchange ratio is
simply the carbon dioxide elimination/oxygen uptake.
Dead space: gas not participating in gas exchange
Anatomic: gas in the conduction zone; gas not able to exchange due to its
location anatomically in the body
 Calculation: usually assumed to be about 2.2 ml/kg; can be
determined by inhaling pure oxygen and measuring amount of gas
with little nitrogen, and half of the gas with increasing amounts of
Physiologic: gas in unperfused alveoli; gas not able to exchange in a
location anatomically designed for perfusion
 Calculation: measure tidal volume, exhaled partial pressure of CO2,
and arterial partial pressure of CO2. Then, use Bohr equation to find
physiologic dead space (note also that alveolar volume is tidal volume
minus physiologic dead space)
Alveolar ventilation and alveolar gas composition: as alveolar ventilation
increases, arterial CO2 partial pressure decreases
Alveolar gas equation:
PAO2 = FIO2 (Barometric pressure – water vapor pressure) – (PaCO2/R)
PAO2 = alveolar partial pressure of O2
FIO2 = [O2] in inspired air
PaCO2 = arterial partial pressure of CO2
R = respiratory quotient
Barometric pressure is normally 760 mmHg
Water vapor pressure is normally 47 mmHg