Live binding model

Oxygen-Hemoglobin Dissociation Curve

Change blood PO₂ to move along the curve. Change pH, temperature, or PCO₂, or compare 2,3-BPG states to move the curve itself and alter hemoglobin's oxygen affinity.

Hemoglobin saturation at equilibrium Standard curve Active curve

Hemoglobin saturation

71.7%

Blood PO₂

40 mmHg

P50

28.1 mmHg

Curve shift

right shift

Right shift: lower oxygen affinity

At 40 mmHg PO₂, hemoglobin holds less oxygen than it would under standard conditions, favoring unloading into tissue.

Blood oxygen pressure40 mmHg PO₂Moves the blood along the active curve10120

Shift the curve

pH7.35 Lower pH shifts right and favors unloading6.97.6Temperature37.0 °CHigher temperature shifts right3540Carbon dioxide pressure46 mmHg PCO₂Higher PCO₂ shifts right2060

2,3-BPG (2,3-DPG)Elevated after sustained altitude exposure shifts right

Plateau: protect loading

PO₂ above about 60 mmHg

Large changes in PO₂ produce smaller changes in saturation, helping preserve oxygen loading in the lungs.

Steep region: support unloading

PO₂ below about 60 mmHg

Small decreases in PO₂ release proportionally more oxygen from hemoglobin for diffusion into tissues.

2,3-BPG is a red-cell metabolite

It binds preferentially to deoxygenated adult hemoglobin and stabilizes the low-affinity state. This teaching state adds 1.5 mmHg to standardized P50, based on a four-day human exposure at 3,500 m. That favors tissue unloading, but can oppose oxygen loading in severely hypoxic lungs. In real altitude acclimatization, hyperventilation lowers PCO₂ and raises pH, often offsetting much of the BPG right shift.

Teaching model, not a pulse-oximeter or arterial-blood-gas calculator. Saturation is an equilibrium estimate for adult hemoglobin. The elevated-BPG state is a literature-calibrated teaching comparison, not an individual prediction, and the model does not include dyshemoglobins, fetal hemoglobin, diffusion limitation, shunt, or measurement error.

Model foundation

The active teaching curve uses HumMod's Hill equation and its documented Severinghaus corrections for pH, temperature, and PCO₂. JSim's EBTOW model supplies a mechanistic four-site Adair reference for cooperative oxygen binding. The elevated 2,3-BPG state uses a conservative standardized P50 difference from a four-day human hypobaric-chamber study.

Chapter 11: Respiration explains loading, unloading, the steep region, and rightward shifts during exercise.
JSim EBTOW oxygen and water exchange model implements equilibrium hemoglobin binding with the four-step Adair equation.
HumMod hemoglobin properties reference records the Hill model and Severinghaus shift coefficients used here.
Human hypobaric-chamber BPG study compares standardized and actual-condition P50 after 96 hours at 3,500 m.
Five-day high-altitude P50 study shows why elevated BPG need not produce the same net shift under in-vivo acid-base conditions.