The Diffusion of Pulmonary Gases

Vocabulary practice flashcards covering Chapter 4: The Diffusion of Pulmonary Gases, focusing on gas laws, partial pressures, alveolar-capillary membrane diffusion, and perfusion- versus diffusion-limited gas transfer.

Gas Diffusion

The passive movement of individual gas molecules from an area of high pressure (high concentration) to an area of low pressure (low concentration) driven by kinetic energy.

Pressure Gradient

The difference in total gas pressure that drives the bulk movement of a gas volume in one direction—from an area of high pressure to an area of low pressure—serving as the primary mechanism for pulmonary ventilation.

Dalton's Law

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

Partial Pressure

The force exerted by an individual gas within a mixture of gases or in a liquid, which is directly proportional to the percentage of that gas in the mixture.

Torr

A unit of pressure named in honor of Evangelista Torricelli, defined as exactly equal to $1\,\text{mm Hg}$.

Diffusion Gradient

The individual partial pressure difference of a specific gas across a membrane that permits gases to move independently of one another and in opposite directions.

Hyperbaric Oxygen Therapy (HBOT)

The therapeutic application of oxygen at pressures greater than $1\,\text{atm}$ (typically $1.5$ to $3$ times atmospheric pressure) inside a sealed chamber to increase dissolved oxygen in the blood.

Henry's Law

The physical law stating that the amount of a gas that dissolves in a liquid at a given temperature is directly proportional to the partial pressure of that gas.

Solubility Coefficient

The volume of a gas that can be dissolved by $1\,\text{mL}$ of a given liquid at standard pressure ($760\,\text{mm Hg}$) and a specified temperature, varying inversely with temperature.

Solubility Coefficient of Oxygen ($\text{O}_2$)

The measure of oxygen solubility in water at $37^\circ\text{C}$ and $760\,\text{mm Hg}$, equal to $0.0244\,\text{mL/mm Hg/mL}\,\text{H}_2\text{O}$.

Solubility Coefficient of Carbon Dioxide ($\text{CO}_2$)

The measure of carbon dioxide solubility in water at $37^\circ\text{C}$ and $760\,\text{mm Hg}$, equal to $0.592\,\text{mL/mm Hg/mL}\,\text{H}_2\text{O}$, making it $24$ times more soluble than oxygen in liquid.

Molecular Water (Water Vapor)

Water present in its gaseous state that behaves according to the gas laws and exerts its own partial pressure ($P_{\text{H}_2\text{O}}$).

Alveolar Absolute Humidity

The mass of water vapor contained in alveolar gas that is fully saturated ($100\%$ humidified) at normal body temperature ($37^\circ\text{C}$), equal to $44.0\,\text{mg/L}$.

Alveolar Water Vapor Pressure ($P_{\text{H}_2\text{O}}$)

The partial pressure exerted by water vapor in fully saturated alveolar gas at $37^\circ\text{C}$, constantly equal to $47.0\,\text{mm Hg}$ regardless of the humidity of inspired ambient air.

Ideal Alveolar Gas Equation

The clinical formula used to calculate alveolar oxygen tension: $P_{\text{A}\text{O}_2} = [P_{\text{B}} - P_{\text{H}_2\text{O}}]F_{\text{I}\text{O}_2} - P_{\text{a}\text{CO}_2}(1.25)$.

Respiratory Exchange Ratio (RR)

The ratio of the volume of oxygen entering the pulmonary capillary blood ($\approx 250\,\text{mL/min}$) to the volume of carbon dioxide diffusing into the alveoli ($\approx 200\,\text{mL/min}$), normally approximating $0.8$.

Alveolar-Capillary Membrane (AC-Membrane)

The anatomical barrier measuring between $0.2$ and $2.5\,\mu\text{m}$ in thickness, comprising nine distinct layers through which gases must diffuse between alveolar gas and red blood cells.

Capillary Transit Time

The total duration required for venous blood to pass through the alveolar-capillary system, averaging approximately $0.75\,\text{second}$ under normal resting conditions.

Diffusion Equilibrium Time

The normal time required for oxygen and carbon dioxide partial pressures to equilibrate across the alveolar-capillary membrane, which is approximately $0.25\,\text{second}$ (one-third of resting transit time).

Fick's Law

The physical law stating that gas diffusion rate across tissue is directly proportional to surface area ($A$), diffusion constant ($D$), and pressure gradient ($P_1 - P_2$), and inversely proportional to tissue thickness ($T$).

Graham's Law

The law stating that the rate of diffusion of a gas through a liquid is directly proportional to the gas's solubility coefficient and inversely proportional to the square root of its gram-molecular weight ($\sqrt{\text{GMW}}$).

Relative Diffusion Rate of $\text{CO}_2$ to $\text{O}_2$

The combined effect of Henry's and Graham's laws demonstrating that carbon dioxide diffuses through the alveolar-capillary membrane approximately $20$ times faster than oxygen due to its much higher liquid solubility.

Oxygen Toxicity

A pathological state developing within $24\,\text{hours}$ from high inspired oxygen partial pressures ($P_{\text{O}_2}$) and exposure to $F_{\text{I}\text{O}_2} > 0.50$, affecting the pulmonary system and central nervous system.

Perfusion-Limited Gas Flow

A condition where the transfer of gas across the alveolar wall depends entirely on the volume of blood flowing past the alveoli because the gas rapidly reaches partial pressure equilibrium with blood plasma.

Nitrous Oxide ($\text{N}_2\text{O}$)

A gas used to demonstrate perfusion limitation because it does not chemically combine with hemoglobin, causing its blood partial pressure to equilibrate with alveolar gas within one-tenth of capillary transit time.

Diffusion-Limited Gas Flow

A condition where the movement of gas across the alveolar wall is restricted solely by the physical barriers and structural integrity of the alveolar-capillary membrane, rather than the rate of blood flow.

Carbon Monoxide ($\text{CO}$)

A gas used to evaluate diffusion limitation because its affinity for hemoglobin is approximately $210$ times greater than that of oxygen, preventing it from generating an appreciable partial pressure in plasma.

Diffusing Capacity of Carbon Monoxide ($D_{\text{L}\text{CO}}$) Test

A diagnostic pulmonary function test measuring the volume of carbon monoxide crossing the alveolar-capillary membrane per minute per unit pressure gradient, with a normal resting value of $25\,\text{mL/min/mm Hg}$.

Bullae

Large air spaces formed in emphysematous lungs when adjacent alveolar and pulmonary capillary walls are destroyed and merge together, drastically diminishing alveolar surface area.

Blanketing Barrier

A protective gaseous layer of an inert gas, such as nitrogen or argon, used to displace oxygen from an enclosed volume to interrupt or prevent oxidation.