Hematosis: Pulmonary Gas Exchange Explained

Hematosis (also called pulmonary gas exchange or external respiration) is the process by which oxygen enters the blood and carbon dioxide leaves it across the alveolar-capillary membrane.

Mechanism

1. Ventilation: Air reaches alveoli; PO₂ ~100 mmHg, PCO₂ ~40 mmHg.
2. Diffusion: O₂ moves down its gradient from alveolar air (100 mmHg) → capillary blood (40 mmHg). CO₂ moves from capillary blood (45 mmHg) → alveolar air (40 mmHg).
3. Perfusion: Oxygenated blood travels via pulmonary veins → left heart → systemic circulation.

Key Factors

• Surface area: ~70 m² from ~300 million alveoli
• Membrane thickness: ~0.5 µm (type I pneumocytes + capillary endothelium)
• Partial pressure gradients maintained by ventilation and perfusion matching (V/Q ratio ~0.8)

Clinical Relevance

Impaired hematosis causes hypoxemia (low PaO₂) and/or hypercapnia (high PaCO₂), seen in ARDS, pulmonary edema, fibrosis, COPD, and pulmonary embolism. Pulse oximetry and ABG assess adequacy.