Gas Diffusion and Alveolar Ventilation Mechanics

Vocabulary flashcards covering capillary pressures, diffusion gradients, normal blood gas values, anatomical shunts, and the alveolar air equation.

Oncotic Pressure

A pulling pressure created by protein molecules inside the capillary that acts like a vacuum to retain water and fluid within the vessel, moving fluid against the pressure gradient from areas of low to high pressure.

Hydrostatic Pressure

The pushing pressure inside the capillary that forces fluid out of the vessel into the interstitial space from areas of high pressure to low pressure.

Pulmonary Edema

A clinical condition where an imbalance of pressures—such as an elevation in hydrostatic pressure from congestive heart failure—causes fluid to seep out of the capillary into the interstitial space and eventually flood the alveoli.

Alveolar Capillary (AC) Membrane

The structural boundary separating air in the alveoli from blood in the pulmonary capillaries across which oxygen and carbon dioxide diffuse.

Dalton's Law

A physical law stating that the total barometric pressure of a gas mixture equals the sum of the individual partial pressures exerted by each gas in the mixture.

Partial Pressure of Inspired Oxygen ($P_{\text{I}}\text{O}_2$)

The partial pressure exerted by inspired oxygen, calculated by multiplying the fraction of inspired oxygen ($F_{\text{I}}\text{O}_2$) by the barometric pressure ($P_B$); on room air ($21\%$) at sea level ($760\,\text{mmHg}$), it equals approximately $160\,\text{mmHg}$.

Hyperbaric Oxygen Therapy

A therapeutic modality that increases barometric pressure (such as in a monoplace chamber at multiple atmospheres) to markedly raise $P_{\text{I}}\text{O}_2$ and displace carbon monoxide from hemoglobin.

Simple Diffusion

The net movement of gas molecules from an area of higher partial pressure to an area of lower partial pressure across a membrane until equilibrium is reached, resulting from the random high-speed motion of molecules.

Water Vapor Pressure at Body Temperature

The partial pressure exerted by water vapor once inspired air reaches $100\%$ saturation at body temperature ($37^\circ\text{C}$) past the isothermic saturation boundary (ISB), which is a constant $47\,\text{mmHg}$.

Normal Alveolar Oxygen Tension ($P_{\text{A}}\text{O}_2$)

The partial pressure of oxygen in the alveoli, which normally measures approximately $100\,\text{mmHg}$ when breathing room air at sea level due to dilution by water vapor and competition with carbon dioxide.

Normal Arterial Oxygen Tension ($P_{\text{a}}\text{O}_2$)

The partial pressure of oxygen dissolved in arterial blood, normally ranging between $80\,\text{mmHg}$ and $100\,\text{mmHg}$ in a healthy adult breathing room air.

Oxygen Consumption ($\dot{V}\text{O}_2$)

The volume of oxygen consumed by systemic tissues per minute to sustain metabolic pathways like the Krebs cycle, with a normal value averaging $250\,\text{mL/min}$.

Total Oxygen Delivery

The total volume of oxygen delivered through systemic arteries to the tissues, measuring approximately $1000\,\text{mL/min}$, of which the tissues consume roughly $25\%$ ($250\,\text{mL/min}$) while $75\%$ returns to the venous system.

Venous Oxygen Tension ($P_{\bar{\text{v}}}\text{O}_2$)

The partial pressure of oxygen present in deoxygenated systemic venous blood returning to the pulmonary artery, normally measuring $40\,\text{mmHg}$.

Oxygen Diffusion Gradient

The pressure difference between the alveolus ($100\,\text{mmHg}$) and mixed venous blood entering the pulmonary capillary ($40\,\text{mmHg}$), creating a normal driving gradient of $60\,\text{mmHg}$ for oxygen diffusion.

Carbon Dioxide Production ($\dot{V}\text{CO}_2$)

The rate at which carbon dioxide is generated by cellular metabolism and released into the circulation, normally averaging $200\,\text{mL/min}$.

Venous Carbon Dioxide Tension ($P_{\bar{\text{v}}}\text{CO}_2$)

The partial pressure of carbon dioxide in mixed venous blood returning from the tissues to the pulmonary capillaries, normally measuring $46\,\text{mmHg}$.

Carbon Dioxide Diffusion Gradient

The pressure gradient driving carbon dioxide diffusion from the pulmonary capillary blood ($46\,\text{mmHg}$) into the alveolus ($40\,\text{mmHg}$), which normally equals $6\,\text{mmHg}$.

Alveolar-Arterial Oxygen Tension Difference ($P(\text{A-a})\text{O}_2$)

The clinical calculation determined by subtracting arterial oxygen tension ($P_{\text{a}}\text{O}_2$) from alveolar oxygen tension ($P_{\text{A}}\text{O}_2$); normal on room air is $10\text{ to }25\,\text{mmHg}$, whereas values exceeding $350\,\text{mmHg}$ indicate critical shunting where oxygen is failing to participate in gas exchange.

Anatomical Shunts

Normal vascular routes—specifically the bronchial veins and Thebesian veins—that deliver deoxygenated venous blood directly into oxygenated pulmonary veins and the left heart, naturally creating a difference between alveolar and arterial oxygen tensions.

Thebesian Veins

Small veins of the coronary venous system that empty directly into the left side of the heart, contributing to the normal anatomical shunt.

Alveolar Air Equation

The mathematical formula used to calculate alveolar oxygen tension: $P_{\text{A}}\text{O}_2 = [F_{\text{I}}\text{O}_2 \times (P_B - 47)] - \left(\frac{P_{\text{a}}\text{CO}_2}{R}\right)$.

Respiratory Quotient ($R$)

The ratio of carbon dioxide production to oxygen consumption ($\frac{\dot{V}\text{CO}_2}{\dot{V}\text{O}_2} = \frac{200\,\text{mL/min}}{250\,\text{mL/min}}$), which yields a normal clinical constant of $0.8$.

High $F_{\text{I}}\text{O}_2$ Rule

The clinical modification to the alveolar air equation applied when a patient receives an $F_{\text{I}}\text{O}_2$ of $0.60$ ($60\%$) or higher, where the respiratory quotient ($R$) becomes $1$ and dividing $P_{\text{a}}\text{CO}_2$ by $0.8$ is eliminated.

Ventilation-Perfusion ($V/Q$) Matching

The relationship and alignment between the amount of gas reaching the alveoli ($V$) and the blood flow reaching the surrounding pulmonary capillaries ($Q$).