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Pulmonary Circulation: Gas Exchange in the Lungs

Deoxygenated blood (high CO₂, low O₂) travels via the pulmonary arteries to the pulmonary capillaries surrounding the alveoli.

Gas Exchange Steps

  1. Diffusion Gradients: Alveolar PO₂ (~100 mmHg) > Capillary PO₂ (~40 mmHg) → O₂ enters blood. Capillary PCO₂ (~45 mmHg) > Alveolar PCO₂ (~40 mmHg) → CO₂ exits blood.
  2. Structural Optimization: Alveolar-capillary membrane thickness ~0.5 μm; surface area ~70 m²; capillary transit time ~0.75 sec (sufficient for equilibration).
  3. Hemoglobin Binding: O₂ binds cooperatively to Hb (sigmoidal curve); CO₂ transported as bicarbonate (70%), carbamino-Hb (23%), dissolved (7%).
  4. Ventilation-Perfusion (V/Q) Matching: Regional regulation ensures perfusion matches ventilation; hypoxic pulmonary vasoconstriction diverts blood from poorly ventilated alveoli.

Output

Oxygenated blood (PO₂ ~100 mmHg, PCO₂ ~40 mmHg) returns via pulmonary veins to left atrium → systemic circulation.

Pathology (pneumonia, edema, embolism, fibrosis) disrupts diffusion, V/Q matching, or perfusion → hypoxemia ± hypercapnia.

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