Mechanism of Blood Oxygen Transport
How oxygen moves from lungs to tissues — and the molecular machinery that makes it efficient.
Two Parallel Transport Systems
Primary 98.5%
Reversible binding to hemoglobin. Oxygen binds to the iron-containing heme groups inside hemoglobin (Hb) tetramers. Each Hb carries up to four O2 molecules. In pulmonary capillaries (high PO2), Hb becomes oxyhemoglobin (HbO2); in tissues (low PO2), O2 dissociates and is released.
Binding is cooperative: the first O2 makes subsequent binding easier, producing the characteristic sigmoid saturation curve.
Secondary 1.5%
Physical dissolution in plasma. A small fraction of O2 dissolves directly into plasma per Henry’s law. This dissolved pool is what creates the PO2 gradient that drives diffusion into and out of red blood cells — and it is the immediately accessible oxygen for tissues before hemoglobin unloads.
The Complete Pathway
| Stage | Location | What happens |
|---|---|---|
| Loading | Lungs (pulmonary capillaries) | Alveolar O2 diffuses across the respiratory membrane; Hb saturation rises to ~98% |
| Transport | Systemic arteries | Oxygen circulates primarily as HbO2 |
| Unloading | Active tissues | Lower PO2, higher CO2, lower pH, and higher temperature shift the dissociation curve right (Bohr effect), reducing Hb affinity and releasing O2 |
Regulatory Factors
Bohr effect — Increased CO2 and decreased pH promote O2 release where metabolic demand is highest.
- Temperature: Hyperthermia favors unloading (active muscles get more O2).
- 2,3-BPG: A metabolic byproduct in RBCs that stabilizes deoxygenated Hb, reducing O2 affinity. Critical at high altitude or during chronic hypoxia.
- Cooperativity: Conformational change after the first O2 binds increases affinity for the remaining three sites.
Bottom Line
Hemoglobin is a high-capacity, finely regulated molecular shuttle: it loads in oxygen-rich environments and unloads precisely where metabolic demand is highest. The small dissolved fraction maintains the diffusion gradient that makes the entire system work.