Move a slider and watch hemoglobin change its mind about oxygen. The tool shows the Severinghaus curve shifting in real time as you adjust temperature, pH, PaCO₂, 2,3-DPG, and carboxyhemoglobin — with live P50, oxygen delivery calculation, and nine clinical scenario presets.
Drag anywhere on the chart to read a point.
Nothing yet — every condition is at its reference value. The curve sits on the standard line with a P50 near 26.9 mmHg.
The black tick marks what this same hemoglobin would release on a normal curve (4.8 mL/dL). Delivery is near normal.
Resting arterial blood at sea level. Everything sits at reference.
The shape change, not the crowding. Hemoglobin flips between a relaxed form that binds oxygen easily and a tense form that releases it. H⁺, CO₂, and 2,3-DPG bind at allosteric sites away from the heme and lock in the tense form, so affinity falls and the curve moves right. Temperature acts more directly, loosening the oxygen–heme bond itself.
Why the shape helps. The flat top means a falling PaO₂ costs almost no saturation until about 60 mmHg. Below that the curve turns steep. That steep segment sits exactly at tissue-level pressures, where small drops in PO₂ release large amounts of oxygen.
Carbon monoxide is a special case. CO occupies binding sites outright (ceiling falls) and locks the remaining sites into the high-affinity form (left shift). Oxygen content falls while PaO₂ and standard pulse oximetry stay normal — co-oximetry is what detects it.
Time matters for 2,3-DPG. Bohr effects from pH and CO₂ act in seconds. 2,3-DPG changes take hours to days — which is why chronic hypoxemia raises DPG, stored blood loses it, and rapidly correcting a chronic acidosis can leave a patient transiently left-shifted.
Curve generated with the Severinghaus (1979) equation and standard virtual-shift corrections for temperature, pH, and PaCO₂. The 2,3-DPG, carboxyhemoglobin, and fetal hemoglobin terms are teaching approximations calibrated to published P50 values, not literature-derived constants. Built for study and teaching, not for clinical decisions.
P50 is the partial pressure of oxygen at which hemoglobin is exactly 50% saturated. It is the single number that summarises where the curve sits. A normal P50 is about 26–27 mmHg. A higher P50 means a right-shifted curve (lower affinity, releases O₂ more readily); a lower P50 means a left-shifted curve (higher affinity, holds O₂).
Increased temperature, decreased pH (acidosis), increased PaCO₂, and increased 2,3-DPG all shift the curve right by stabilising the tense (T-state) form of hemoglobin. This is the Bohr effect — the same conditions present in exercising or septic tissue, where releasing more oxygen is exactly what is needed.
CO binds to heme with roughly 250× the affinity of oxygen, removing binding sites from circulation — this lowers the maximum achievable saturation (the ceiling effect). Simultaneously, CO locks the remaining heme sites into the high-affinity R-state, shifting the curve left. The result: content falls and the remaining oxygen is less readily delivered. Pulse oximetry cannot detect this because it cannot distinguish oxyhemoglobin from carboxyhemoglobin.
HbF (γ-chains instead of β-chains) binds 2,3-DPG poorly. Because DPG normally promotes the T-state (right shift), the absence of DPG binding leaves HbF in the high-affinity R-state. The resulting left shift means fetal blood at the same PO₂ carries more oxygen than maternal blood — creating the gradient that drives placental oxygen transfer.
2,3-DPG rises with chronic hypoxemia (altitude, COPD, cyanotic heart disease) and anaemia as a compensatory right shift. It falls in stored banked blood (negligible by 3 weeks), hypophosphataemia, and septic shock. Changes take hours to days, which is why rapidly correcting a chronic acidosis can transiently left-shift the curve before DPG catches up.
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