PulmTools learning lab · Free to explore
Explore the oxyhemoglobin curve. Compare oxygen affinity, P50, and left or right shifts.
60mmHg
90.6%
Illustrative adult curves. Shift presets show affinity changes, not patient-specific predictions.
The physiology behind the interaction
The oxyhemoglobin (oxygen–hemoglobin) dissociation curve relates oxygen tension (PO₂) to hemoglobin saturation. Moving the point changes PO₂ along a fixed curve. Selecting a left or right shift changes oxygen affinity while keeping your chosen PO₂ constant. The faint reference line helps you compare like with like.
The S-shaped curve reflects cooperative oxygen binding. Over the steeper region, a small change in PO₂ produces a larger change in saturation. On the plateau, the same change in PO₂ has a smaller effect. Try the 40, 60, and 100 mmHg landmarks and follow the point.
These are reference relationships for learning. They do not define patient-specific oxygen targets.
P50 is the PO₂ at 50% saturation. The reference model is approximately 26.9 mmHg. A lower P50 indicates higher oxygen affinity and a left shift; a higher P50 indicates lower affinity and a right shift.
At a fixed tissue PO₂, a right shift favors oxygen unloading. Actual oxygen delivery also depends on hemoglobin concentration, cardiac output, and the clinical context.
Higher temperature, lower pH, higher PCO₂, and increased 2,3-DPG reduce oxygen affinity. The right-shift preset makes that direction visible. Its P50 of 32 mmHg is an illustrative teaching choice, not a calculated response to a specific physiologic change.
Lower temperature, higher pH, lower PCO₂, and reduced 2,3-DPG increase oxygen affinity. The left-shift preset uses an illustrative P50 of 22 mmHg. Higher saturation at the same PO₂ reflects greater affinity and does not by itself establish better tissue oxygen delivery.
The reference adult HbA curve uses the simplified Severinghaus equation: S = (PO₂³ + 150 × PO₂) / (PO₂³ + 150 × PO₂ + 23,400), with S as fractional saturation. The displayed percentage is 100 × S. Reference conditions are pH 7.40, 37°C, and PCO₂ 40 mmHg.
Reference P50 is the positive solution of PO₂³ + 150 × PO₂ = 23,400. For the two illustrative shifts, the same equation is evaluated at effective PO₂ = selected PO₂ × reference P50 / preset P50. This preserves the 50% crossing at the selected P50.
The presets do not quantitatively model a particular pH, temperature, PCO₂, or 2,3-DPG concentration. They do not represent fetal hemoglobin, carbon monoxide exposure, methemoglobinemia, pulse-oximeter behavior, or patient-specific oxygen loading and delivery. This educational explorer is not a blood-gas conversion tool or treatment calculator.
It relates oxygen partial pressure (PO₂) to the percentage of hemoglobin binding sites occupied by oxygen. Its S shape reflects cooperative oxygen binding. It does not directly show oxygen content, blood flow, or tissue oxygen delivery.
Higher temperature, lower pH, higher PCO₂, and increased 2,3-DPG favor a right shift. Oxygen affinity falls and P50 rises. At the same PO₂, saturation is lower and unloading is more favored. The simulator uses an illustrative P50 of 32 mmHg for this direction, not a patient-specific prediction.
Lower temperature, higher pH, lower PCO₂, and reduced 2,3-DPG favor a left shift. Affinity increases and P50 falls. The illustrative left-shift preset uses P50 22 mmHg. It is an adult reference curve rescaled for teaching, not a fetal-hemoglobin or dyshemoglobin model.
P50 is the oxygen tension at which hemoglobin is 50% saturated. A commonly used reference adult value is approximately 26–27 mmHg under standard conditions. This Severinghaus reference model gives about 26.9 mmHg at pH 7.40, 37°C, and PCO₂ 40 mmHg. A higher P50 indicates lower affinity; a lower P50 indicates higher affinity.
On a standard adult curve, 60 mmHg corresponds to roughly 90% saturation; this model gives about 90.6%. This is an approximate relationship under reference conditions, not a universal conversion or an oxygen-treatment target. Affinity shifts change the relationship.
No. This is a modeled equilibrium hemoglobin saturation derived from PO₂ and an affinity preset. SpO₂ is an estimate obtained from a pulse oximeter. The model does not simulate pulse-oximeter performance or account for carboxyhemoglobin, methemoglobin, fetal hemoglobin, or an individual patient's physiology.
No. Changing PO₂ while keeping affinity fixed moves the point along the same curve. A shift means the saturation-versus-PO₂ relationship has changed because affinity changed. The prediction questions help you practice this distinction.