Oxygen Transport: From Alveoli to Tissues via Hemoglobin

1. Inhalation & Alveolar Diffusion

• Air enters alveoli (PO₂ ~100 mmHg).
• O₂ diffuses across alveolar-capillary membrane (type I pneumocyte, basement membrane, capillary endothelium) into pulmonary capillary blood.

2. Hemoglobin Binding

• Each Hb tetramer binds 4 O₂ molecules cooperatively (sigmoidal dissociation curve).
• Forms oxyhemoglobin (HbO₂).
• P₅₀ (PO₂ at 50% saturation) ~26.6 mmHg; shifted right by ↑H⁺, ↑CO₂, ↑2,3-BPG, ↑temp (Bohr effect).

3. Bloodstream Transport

• RBCs carry HbO₂ through pulmonary veins → left heart → systemic arteries → arterioles → capillaries.
• Convective flow (cardiac output × arterial O₂ content) delivers O₂ to tissues.

4. Tissue Diffusion & Utilization

• At capillaries, PO₂ drops (~40 mmHg in resting muscle, <20 mmHg in active muscle).
• O₂ dissociates from Hb, diffuses into cells → mitochondria → oxidative phosphorylation (ATP).
• Venous blood returns (PO₂ ~40 mmHg, SvO₂ ~75%).

Key Determinants of O₂ Delivery

DO₂ = CO × CaO₂ = CO × (1.34 × Hb × SaO₂ + 0.003 × PaO₂). Anemia, low CO, or hypoxemia each reduce DO₂.