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INTRODUCTION TO ALVEOLAR GAS DIFFUSION

Learning Objectives

  • The student will be able to define the terms governing alveolar O2 diffusion, and the circumstances when O2 uptake is limited by perfusion or diffusion.

  • The student will be able to calculate the percentage of Q̇ comprising physiological shunt by using appropriate patient data and knowledge of HbO2 interactions in blood.

  • The student will be able to summarize processes in CO2 excretion and the manner by which blood CO2 acts to maintain normal blood pH.

  • The student will be able to use patient data to distinguish among respiratory acidosis, metabolic acidosis, respiratory alkalosis, and metabolic alkalosis.

Once gases reach the alveolar parenchyma by ventilation, absorption into the blood or excretion from it occurs not by convection, but by molecular diffusion according to a physiological restatement of Ohm’s law (Fig. 9.1). Diffusion of any gas through the septal barrier and into the blood is proportional to the alveolar epithelial surface area (SA) and the alveolar capillary endothelial surface area (SC) comprising the membrane available for such exchange. Quantitative measurements on electron photomicrographs of normal lung have estimated SA and SC to each be 50-70 m2. This symmetry of SA and SC dimensions is perhaps not surprising, given the importance placed earlier on good V̇A/Q̇ matching. Alveolar diffusion of any gas is inversely proportional to septal barrier thickness, estimated as its harmonic meanS) to emphasize statistically the thinnest regions where diffusion is presumably favored.

FIGURE 9.1

Alveolar diffusion as described by a physiological version of Ohm’s law: Flow = (P1 − P2)/R, where P1 and P2 are the "source" and "sink" gas partial pressures in two adjacent compartments. Conductance (C) is the inverse of resistance R, and thus Flow = C • (P1 − P2). C is proportional to the average of all alveolar and capillary surface areas [(SA + SC)/2] in the septa available for diffusion, and inversely proportional to the harmonic mean thickness of those septal barriers (τs). C is also proportional to Krogh’s coefficient of diffusivity (D) for each gas, which is in turn the ratio of its solubility in saline divided by the molecular weight (MW) of each gas.

Given these lung anatomical features that affect all gases, other factors will determine whether diffusional equilibrium is achieved between an alveolar airspace and the blood flowing through its capillaries. Again by analogy to Ohm’s law, each gas diffuses proportionally to the pressure gradient between its "source" (P1, the higher value) and its "sink" (P2, the lower value). For O2, P1 = PAo2 (Chap. 8) and ...

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