SEQUENCE OF PULMONARY GAS EXCHANGE

A. Oxygen

1. driving force: alveolar-blood DP_O2
2. rate of equilibration determined by D_O2
3. for normal D_O2, oxygen equilibration requires 0.25 seconds
4. since blood requires 0.75 seconds to pass though the pulmonary capillaries, normally oxygen equilibration is complete
5. CO₂ loss and pH increase result in greater O₂ uptake by blood

B. Carbon Dioxide

1. driving force: alveolar-blood DP_CO2
2. high D_CO2 almost always permits complete equilibration of CO₂ between alveolar gas and pulmonary capillary blood
3. O₂ uptake aids CO2 loss from blood

4. Chloride Shift: as a red cell passes through the lungs, it loses Cl^- to plasma

Note:  Chloride Shift: In pulmonary capillaries, Cl^- leaves the red cell and moves into the plasma in exchange for HCO₃; in the systemic capillaries, the reverse occurs.

Notes:

(1) carbonic anhydrase localized in erythrocyte but not present in plasma or interstitial fluid
(2) erythrocyte membrane permeable to small negative ions but not small positive ions
(3) Hb in erythrocyte helps to buffer H⁺