Chapter 4: The Diffusion of Pulmonary Gases

Vocabulary terms and definitions regarding the physics of gas partial pressures and diffusion in the context of respiratory physiology.

Dalton’s Law

The law of partial pressures stating that the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted independently by each gas in the mixture.

Partial Pressure

The pressure exerted by each individual gas in a mixture, which is directly proportional to the percentage of that gas in the gas mixture.

Earth’s Atmospheric Gases

Consist of nitrogen ($N_2$), oxygen ($O_2$), carbon dioxide ($CO_2$), and other trace gases like argon.

Normal Barometric Pressure ($P_B$) at sea level

A pressure of $760\,mmHg$.

Atmospheric $PO_2$ (at sea level)

Calculated as $0.21 \times 760 = 159.6\,mmHg$, based on oxygen making up approximately $21\%$ of the atmosphere.

Altitude and Atmospheric Pressure

Atmospheric pressure decreases with an increase in altitude because the density of the different gases surrounding the earth decreases.

Gas Concentration at Elevation

Even though barometric pressure decreases with altitude, the concentration (percentage) of atmospheric gases remains the same; for example, oxygen remains at approximately $21\%$ at the top of Mt. Everest.

Water Descent and Pressure

Atmospheric pressure increases by $1$ atmosphere ($760\,mmHg$) for every $33$ feet of descent in water below sea level.

Pressure Gradient

The movement of gas from an area of high pressure (high concentration) to an area of low pressure (low concentration); it is the primary mechanism for moving air in and out of the lungs during ventilation.

Gas Diffusion

The movement of "individual gas molecules" from an area of high pressure to an area of low pressure, where each gas moves independently.

Alveolar Oxygen Tension ($PAO_2$)

The partial pressure of oxygen in the alveoli, calculated using the ideal alveolar gas equation: $PAO_2 = [P_B - PH_2O] FIO_2 - PaCO_2(1.25)$.

$PH_2O$ (Water Vapor Pressure)

The partial pressure of water vapor, which is $47.0\,mmHg$ in Alveolar Gas, Arterial Blood, and Venous Blood.

Arterial Blood $PO_2$

The partial pressure of oxygen in arterial blood, which is normally $95.0\,mmHg$.

Venous Blood $PO_2$

The partial pressure of oxygen in venous blood, which is normally $40.0\,mmHg$.

Alveolar Gas $PO_2$

The partial pressure of oxygen in the alveoli, which is normally $100.0\,mmHg$.

Arterial Blood $PCO_2$

The partial pressure of carbon dioxide in arterial blood, which is normally $40.0\,mmHg$.