Pulmonary Gas Exchange: How Blood Releases CO2 and Absorbs O2 in the Lungs
Deoxygenated blood travels from the right ventricle via the pulmonary arteries to the alveolar capillaries. There, across the respiratory membrane:
- CO₂ diffuses out (from blood → alveolus) down its partial pressure gradient.
- O₂ diffuses in (from alveolus → blood) down its gradient, binding to hemoglobin.
Oxygenated blood then returns via pulmonary veins to the left atrium, then systemic circulation through the aorta.
Why the Lung Circuit Is Essential
- Systemic venous blood has low pO₂ (~40 mmHg) and high pCO₂ (~45 mmHg).
- Alveolar air has high pO₂ (~100 mmHg) and low pCO₂ (~40 mmHg).
- Diffusion equilibrates these gradients in ~0.25 seconds at rest.
Without this circuit, tissues would lack oxygen for aerobic metabolism and accumulate toxic CO₂.
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