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Understanding Gaseous Exchange: Key Factors & Mechanisms

This tutorial covers general factors affecting gas flow, key formulas, the exchange of respiratory gases in lungs, diffusion rates, partial pressure, ventilation-perfusion ratio, and the role of diffusion in gaseous exchange. Learn about the relationship between ventilation and perfusion, the concept of physiologic shunt, and more.

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Understanding Gaseous Exchange: Key Factors & Mechanisms

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  1. Tutorial – Gaseous Exchange

  2. Gaseous Exchange • General factors affecting ‘flow’ • F = k * ΔP / R • K = constant • ΔP = delta P, ‘pressure gradient’ • R = ‘resistance’ • For exchange of respiratory gases in lungs… • R is related to cross-sectional area and thickness • K is related gas under consideration • Molecular weight • ‘velocity’ • Solubility

  3. Gaseous Exchange • D = (A * ΔP * s) / (d * sqrt(MW) ) • D = diffusion rate • A = diffusion area • ΔP = partial pressure difference • s = solubility • d = diffusion distance • MW = molecular weight

  4. Gaseous Exchange • Partial pressure • Pressure exerted by individual gas, independent of others • O2 = 20.93 % • If P = 760 mmHg, PO2 = 20.93/100 * 760 = 159 mmHg • Gas diffuses down a partial pressure gradient • When no gradient, no net (nett) movement • Gas can diffuse into water • By definition, when air/liquid in equilibrium (ie equal diffusion in and out), the partial pressures are the same • When partial pressures differ, the gas will move down its partial pressure gradient • Partial pressure should not be confused with concentration • Conc a function of both partial pressure & ‘solubility’

  5. Gaseous Exchange • A = 50 square metres • over which ~ 100 ml blood is ‘smeared’ – a monolayer! • d = down to 0.5 microns • Diffusing capacity of respiratory membrane… • Oxygen ~ 21 ml / min / mmHg • Normal pressure gradient is 11 mmHg  230 ml/min at rest • Carbon dioxide • Difficult to measure but >400 ml / min / mmHg • Can increase diffusing capacity by… • Increasing perfusion of under-perfused parts of lungs • Increasing CO (blood normally equilibrates 1/3rd along cap)

  6. Gaseous Exchange • Some parts of lungs very poorly perfused at rest • Do not need that capacity, can exchange all the gases we need over a smaller area • Link between ventilation (V) and perfusion (Q) • V/Q (or V/P) ratio • Approx 0.85 • Large V/Q means more air ventilated than needed to service the blood flowing through the lungs • Wasted ventilation • Low V/Q means more blood flowing than needed • Incomplete oxygenation, so wasted cardiac output • Called a ‘physiologic shunt’ (as if part of CO not put through lungs)

  7. Gaseous Exchange • ‘Ventilation’ of respiratory membrane a bit of a myth • As airways divide, each gets smaller… • But cross-sectional area gets greater • Thus, velocity of air gets smaller the further down the respiratory ‘tree’ • ‘Bulk flow’ approaches zero at the alveoli • Helps keep alveolar composition fairly constant and exchange with blood fairly constant in face of intermittent breathing • Significant proportion of exchange is by diffusion • Alveoli small enough that O2 in middle can diffuse • Similar to a river  wide delta (eg Nile)

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