Oxygen-Hemoglobin Dissociation Calculator
Calculate hemoglobin oxygen saturation at any partial pressure using the Hill equation.
Adjust P50 and cooperativity to model shifts from pH, temperature, and 2,3-DPG.
Hemoglobin carries oxygen from the lungs to tissues. Unlike a simple carrier molecule, hemoglobin shows cooperative binding: once one oxygen molecule binds, subsequent binding becomes progressively easier. This cooperativity produces the characteristic S-shaped dissociation curve.
The Hill equation:
Y = pO2^n / (P50^n + pO2^n)
where Y is fractional saturation (0 to 1), pO2 is partial pressure of oxygen in mmHg, P50 is the pO2 at 50% saturation (normally about 26-27 mmHg in humans), and n is the Hill coefficient.
The Hill coefficient n measures cooperativity. Pure non-cooperative binding gives n = 1 (a hyperbolic curve). Hemoglobin has n ≈ 2.7 for adult HbA. The theoretical maximum for a 4-subunit protein is n = 4.
P50 shifts. The curve shifts right (higher P50, lower affinity) with:
- Rising temperature
- Falling pH (Bohr effect — acidosis in exercising tissues)
- Rising CO2
- Rising 2,3-DPG
These shifts enhance oxygen unloading at active tissues where it is needed most.
The curve shifts left (lower P50, higher affinity) with alkalosis, hypothermia, fetal hemoglobin (HbF), and carbon monoxide poisoning. A left-shifted curve loads oxygen well but releases it poorly to tissues.
At arterial pO2 (~100 mmHg), saturation is about 98%. At venous pO2 (~40 mmHg), it drops to about 75%, delivering roughly 25% of its oxygen load to tissues per pass.