Gas Exchange, Ventilation, and Perfusion of the Lungs

Vocabulary practice flashcards covering pulmonary gas exchange, diffusion versus perfusion limitation, ventilation equations and compartments, and regional pulmonary perfusion dynamics.

Fick's Law of Diffusion

A physical principle stating that the net rate of gas transfer ($\dot{V}_{O2}$) across a membrane is proportional to the diffusing capacity ($D_{L,O2}$) and the partial pressure gradient between alveolar gas and capillary blood ($P_{AO2} - P_{cO2}$) at a single point in space and time.

Diffusing Capacity ($D_L$)

A composite physiological coefficient in Fick's law that incorporates barrier surface area ($A$), gas solubility ($s$), barrier thickness ($a$), and gas molecular weight ($MW$) into the net rate equation $\dot{V}_{net} = D_L \cdot (P_1 - P_2)$.

Henry's Law

A gas law stating that the concentration of a dissolved gas in liquid ($[O_2]_{\text{Dis}}$) is directly proportional to its partial pressure ($P_{O2}$) and solubility coefficient ($s$), defined by the formula $[O_2]_{\text{Dis}} = s \cdot P_{O2}$.

Type I Pneumocytes

The extremely thin epithelial cells that make up the vast majority of the alveolar wall barrier, providing an expansive surface area of approximately $100\,\text{m}^2$ (roughly half the size of a tennis court) to optimize passive gas diffusion.

Perfusion-Limited Gas Exchange

A condition of pulmonary gas transfer in which the total uptake of a gas depends strictly on pulmonary blood flow ($\dot{Q}$) rather than diffusion capacity ($D_L$), identified when partial pressure in the capillary reaches complete equilibration with alveolar gas before the end of the capillary.

Diffusion-Limited Gas Exchange

A condition of pulmonary gas transfer in which net gas uptake is restricted by the physical properties of the blood-gas barrier ($D_L$), characterized by capillary partial pressure failing to reach alveolar partial pressure by the end of the pulmonary capillary.

Carbon Monoxide ($CO$) Uptake

A classic model of diffusion-limited gas transfer; because carbon monoxide binds with high affinity to hemoglobin, capillary $P_{CO}$ remains near zero, maintaining a continuous gradient where total uptake depends entirely on diffusing capacity ($D_L$).

Nitrous Oxide ($N_2O$) Uptake

A classic model of perfusion-limited gas transfer; because nitrous oxide does not bind to hemoglobin, capillary partial pressure rapidly equilibrates with alveolar partial pressure at the very beginning of the capillary, making uptake dependent entirely on blood flow ($\dot{Q}$).

Pulmonary $O_2$ Uptake at Rest

A perfusion-limited process in healthy individuals at sea level, in which capillary $P_{O2}$ rises from mixed venous levels ($40\,\text{mm\,Hg}$) to reach complete equilibration with alveolar $P_{AO2}$ ($100\,\text{mm\,Hg}$) approximately $1/3$ of the way along the capillary length.

Effect of Vigorous Exercise on $O_2$ Uptake

A physiological response where cardiac output increases 3-fold ($\dot{Q} = 3$), which increases total $O_2$ uptake 3-fold; uptake remains perfusion-limited at sea level due to the lung's large capillary transit time reserve.

Effect of High Altitude on Resting $O_2$ Uptake

A physiological state where low atmospheric barometric pressure reduces alveolar $P_{AO2}$, diminishing the initial alveolar-capillary gradient from $\approx 60\,\text{mm\,Hg}$ to $\approx 30\,\text{mm\,Hg}$ and slowing diffusion, though equilibration is still achieved and uptake remains perfusion-limited at rest.

Exercise at High Altitude

A scenario where a reduced alveolar-capillary gradient ($\approx 30\,\text{mm\,Hg}$) combines with shortened capillary transit time from elevated cardiac output, preventing capillary $P_{cO2}$ from reaching alveolar $P_{AO2}$ and rendering oxygen uptake diffusion-limited in healthy individuals.

Pulmonary $CO_2$ Release

The transfer of carbon dioxide from pulmonary blood to alveolar air, which is normally perfusion-limited but can become diffusion-limited under pathological conditions such as pulmonary fibrosis, COPD, or severe anemia.

Total Ventilation ($\dot{V}_T$)

The total volume of gas entering or leaving the respiratory tract per minute (clinically called minute ventilation), calculated as the product of tidal volume and respiratory rate: $\dot{V}_T = V_T \cdot f = (0.5\,\text{L}) \cdot (12/\text{min}) = 6\,\text{L/min}$.

Anatomic Dead Space ($V_D$)

The volume of the conducting airways (airway generations 0 through 16) that does not contain gas-exchanging alveoli, measuring approximately $150\,\text{mL}$ in an average adult.

Dead-Space Ventilation ($\dot{V}_D$)

The volume of air per minute shuttled back and forth through non-gas-exchanging conducting airways, calculated as $\dot{V}_D = V_D \cdot f = (0.150\,\text{L}) \cdot (12/\text{min}) = 1.8\,\text{L/min}$, accounting for approximately $30\%$ of total ventilation.

Alveolar Ventilation ($\dot{V}_A$)

The volume of fresh inspired air that reaches the respiratory zones and participates in gas exchange per minute, calculated as $\dot{V}_A = (V_T - V_D) \cdot f = (0.500\,\text{L} - 0.150\,\text{L}) \cdot (12/\text{min}) = 4.2\,\text{L/min}$.

Alveolar Dead Space

The volume of gas residing within alveoli that are ventilated with fresh air but receive no capillary blood perfusion, preventing gas exchange.

Physiological Dead Space

The total volume of gas in each respiratory cycle that does not participate in gas exchange, calculated as the sum of anatomic dead space and alveolar dead space.

Alveolar Ventilation Equation

A fundamental equation demonstrating that alveolar ventilation is inversely proportional to alveolar carbon dioxide partial pressure: $\dot{V}_A = 0.863 \cdot \frac{\dot{V}_{CO2}}{P_{ACO2}}$, where $\dot{V}_{CO2}$ is metabolic carbon dioxide production.

Hyperventilation

An increase in alveolar ventilation ($\dot{V}_A$) beyond metabolic needs that dilutes alveolar $CO_2$, driving arterial $P_{aCO2}$ down toward low levels, which leads to cerebral vasoconstriction, reduced brain perfusion, and dizziness.

Hypoventilation

A reduction in alveolar ventilation ($\dot{V}_A$) below metabolic requirement that causes alveolar and arterial $P_{O2}$ to drop, while alveolar and arterial $P_{CO2}$ double or rise proportionally due to diminished carbon dioxide excretion.

Alveolar Gas Equation

An equation defining the relationship between inspired oxygen, carbon dioxide excretion, and alveolar $P_{AO2}$, expressed in the general case as $P_{AO2} = P_{IO2} - P_{ACO2} \left(F_{IO2} + \frac{1 - F_{IO2}}{RQ}\right)$.

Physiological Non-Uniformity of Ventilation

The regional variation in airflow caused by gravity in an upright individual, where the base of the lung has lower resting volume and higher compliance than the apex, resulting in greater alveolar ventilation per unit volume at the base.

Pulmonary Vascular Resistance ($R_{\text{pulmonary}}$)

The low hydrodynamic resistance of the pulmonary bed, calculated as $\Delta P / \dot{Q} = 7\,\text{mm\,Hg} / (83\,\text{mL/s}) = 0.08\,\text{PRU}$ compared to systemic resistance of $1.1\,\text{PRU}$, which protects alveolar air spaces from hydrostatic fluid transudation.

Physiological Non-Uniformity of Perfusion

The gravitational and hydrostatic pressure gradient in an upright lung that causes pulmonary blood flow per unit volume to be markedly higher at the base than at the apex.

West Zone 1

An apex lung zone where alveolar pressure exceeds pulmonary arterial and venous pressures ($P_A > P_{PA} > P_{PV}$), collapsing capillaries and preventing blood flow; absent in normal upright individuals but seen during positive-pressure ventilation or hemorrhage.

West Zone 2

A middle lung zone where pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure exceeds venous pressure ($P_{PA} > P_A > P_{PV}$), producing a waterfall effect where blood flow is determined by the arterial-alveolar pressure gradient ($P_{PA} - P_A$).

West Zone 3

A dependent lung zone where pulmonary arterial and venous pressures both exceed alveolar pressure ($P_{PA} > P_{PV} > P_A$), keeping capillaries distended throughout and driving continuous flow governed by the arteriovenous pressure gradient ($P_{PA} - P_{PV}$).

West Zone 4

The extreme base of the lung where blood flow decreases slightly due to elevated intrapleural pressure ($P_{ip}$), partial compression of extra-alveolar vessels, and reduced regional volume causing lower mechanical radial traction.