Respiratory Physiology: Alveolar Blood Gas Exchange

Flashcards covering the mechanisms of alveolar blood gas exchange, partial pressure calculations, diffusion laws, and clinical pathologies affecting lung capacity.

Partial pressure of a gas ($P_{gas}$)

The pressure caused by a specific gas alone in a mixture, which is directly proportional to its concentration and is calculated as Total pressure $\times$ Fractional gas concentration ($F_{gas}$).

Mean sea-level atmospheric pressure ($P_{atm}$ or $P_{B}$)

$760\,mmHg$ or $1\,atm$.

Partial pressure of water vapor ($P_{H_{2}O}$)

Approximately $47\,mmHg$ at $37^\circ\text{C}$, which humidifies dry room air in the tracheobronchial tree.

Inspired Oxygen Partial Pressure ($PI_{O_{2}}$) calculation

$PI_{O_{2}} = (P_{atm} - P_{H_{2}O}) \times Fi_{O_{2}}$. At sea level on room air, this equals $(760 - 47) \times 0.21 = 150\,mmHg$.

Alveolar Gas Equation ($PA_{O_{2}}$, $R = 0.8$)

$PA_{O_{2}} = PI_{O_{2}} - \frac{PA_{CO_{2}}}{R}$, which predicts Alveolar $PO_{2}$ depending on alveolar $PCO_{2}$ and inspired oxygen.

Respiratory Exchange Ratio ($R$)

The ratio of $CO_{2}$ production to $O_{2}$ consumption ($\frac{\dot{V}CO_{2}}{\dot{V}O_{2}}$), which is normally $0.8$ at rest ($200\,mL/min / 250\,mL/min$).

Physiological Shunt (Venous Admixture)

The roughly $2\%$ of systemic cardiac output that bypasses pulmonary circulation, causing systemic arterial $PO_{2}$ to be slightly lower than alveolar $PO_{2}$.

Bronchial circulation (as a shunt source)

Deoxygenated bronchial blood emptying directly into pulmonary veins.

Thebesian veins (as a shunt source)

Deoxygenated blood from coronary circulation emptying directly into the left heart chambers.

Alveolar-systemic arterial difference ($\text{A-a gradient}$)

The difference between alveolar and arterial oxygen partial pressure; in a young healthy non-smoker it is $5$-$10\,mmHg$, and generally increases with age, remaining less than $(\frac{Age}{4} + 4)$.

Henry's Law

States that the Concentration of a gas in solution = Partial pressure ($P_{gas}$) $\times$ Solubility.

Solubility of $O_{2}$ in blood

$0.003\,mL\,O_{2}/100\,mL$ blood per $mmHg$.

Solubility of $CO_{2}$ in blood

$0.07\,mL\,CO_{2}/100\,mL$ blood per $mmHg$.

Fick's Law of Diffusion (formula)

Rate of Diffusion ($\dot{V}_{gas}$) = $\frac{A}{T} \times D \times (P_{1} - P_{2})$, where $A$ is surface area, $T$ is membrane thickness, and $D$ is the diffusion coefficient.

Lung Diffusion Capacity ($DL$)

The ratio of the rate of diffusion to the partial pressure gradient: $DL = \frac{\dot{V}_{gas}}{(P_{1} - P_{2})}$.

Diffusing Capacity of Lungs for Carbon Monoxide ($DLCO$)

A pulmonary function test measuring gas-exchange capability; calculated as $DLCO = \frac{\dot{V}CO}{PA_{CO}}$ because capillary partial pressure of CO ($Pc_{CO}$) is considered zero.

Hemoglobin affinity for Carbon Monoxide ($CO$)

Carbon monoxide binds to hemoglobin $200$-$250$ times more strongly than oxygen, keeping free plasma $CO$ levels near zero during a $DLCO$ test.

Recruitment (during exercise)

The opening of dormant capillaries and dilatation of existing ones, which increases surface area ($A$) and Lung Diffusion Capacity ($DL$).

Emphysema (effect on diffusion)

Destruction of alveoli and capillaries leading to a decrease in effective surface area ($A$) and a decrease in Lung Diffusion Capacity ($DL$).

Pulmonary Fibrosis (effect on diffusion)

Thickening and scarring of lung tissues which increases the diffusion distance ($T$) and decreases Diffusion Capacity.

Pulmonary Edema (effect on diffusion)

Presence of fluid in the interstitial space which increases diffusion distance ($T$) and decreases Diffusion Capacity.

Anemia (effect on diffusion)

Decrease in $RBCs$ (and $Hb$) which reduces the $O_{2}$ carrying capacity of pulmonary capillary blood and decreases overall diffusion capacity.

Pneumonia (effect on gas diffusion)

Infection causing consolidation (alveoli filled with fluid or pus), which increases thickness (diffusion distance) and decreases effective surface area and ventilation.

Perfusion Limited Gas Exchange

Gas transfer characterized by complete partial pressure equilibrium being reached early in the capillary; transfer only increases if blood flow increases (e.g., Nitrous oxide, normal $O_{2}$, $CO_{2}$).

Diffusion Limited Gas Exchange

Gas transfer where partial pressure equilibrium is never reached; transfer is limited by membrane properties (e.g., Carbon Monoxide, and $O_{2}$ in fibrosis or strenuous exercise).

Nitrous oxide ($N_{2}O$) transfer type

Perfusion limited, because it doesn't bind to hemoglobin and dissolves freely, causing its partial pressure to rapidly equilibrate with the alveoli.