Lecture 16 – Alveolar Gases and Diffusion

Vocabulary flashcards covering convection, diffusion, Fick's law, diffusion parameters, alveolar gas exchange, ventilation components, and related concepts from the lecture notes.

Convection (bulk flow)

Mass transfer by bulk motion from high pressure to low pressure; occurs in larger tubes and conducting airways; moves O2/CO2 long distances; flux = velocity × concentration.

Diffusion

Movement of molecules down their concentration gradient; follows Fick's first law; occurs at gas-exchange sites (alveoli and capillaries) over short distances.

Fick's first law of diffusion

Rate of diffusion increases with concentration (partial pressure) gradient and diffusivity; decreases with thickness; increases with surface area.

Partial pressure gradient

Difference in partial pressures across a barrier (P1 − P2); for gases in lungs, the driving force for diffusion is the partial pressure gradient.

Surface area for diffusion

Area available for diffusion; in lungs ~50–100 m²; ~300 million alveoli in 6 L.

Diffusion distance/thickness

Thickness of the diffusion barrier; greater thickness decreases diffusion rate.

CO2 diffusivity relative to O2

CO2 is about 20× more diffusible than O2 due to higher solubility; facilitates rapid equilibration in healthy lungs.

Trachea (convection)

Small cross-sectional area with high velocity; air transport by convection; relies on pressure gradient.

Respiratory bronchioles (diffusion)

Large cross-sectional area with low velocity; gas exchange by diffusion; relies on the partial pressure gradient.

Alveoli in walls of bronchioles

Alveoli are located in the walls of respiratory bronchioles, enabling diffusion with blood.

RBC transit time in pulmonary capillary

Time red blood cells spend in contact with alveolar gas for exchange; ~0.75 s at rest; sets window for diffusion.

Diffusion reserve during exercise

Increased cardiac output reduces contact time; diffusion reserve allows some gas exchange to occur; equilibrium may be reached later (approx. 0.25 s).

Alveolar gas composition

Ventilation and metabolism balance determine alveolar O2 and CO2; O2 is added by ventilation and CO2 removed by ventilation; water vapor reduces available partial pressures.

Dead space volume (VD)

Volume of air with no gas exchange; includes anatomic dead space and (if present) alveolar dead space; typically about 150 ml.

Alveolar volume (VA)

Volume of air in alveoli that participates in gas exchange; typically about 350 ml.

Tidal volume (VT)

Total volume of air moved per breath; VT = VD + VA; example given is 500 ml.

Alveolar ventilation vs gas exchange

Only alveolar volume (VA) contributes to gas exchange; dead space ventilation does not participate in metabolism-supporting gas exchange.

PAO2 and PACO2 relationship

Alveolar O2 partial pressure (PAO2) and CO2 partial pressure (PACO2) are inversely related; increasing ventilation raises PAO2 and lowers PACO2.

Water vapor pressure in humidified air

Humidification adds water vapor (~6 kPa), occupying part of total pressure and reducing the portion available for O2.