Example data table
Typical curve example using P50 = 26.6 mmHg and n = 2.7.
| PO2 (mmHg) | Estimated SaO2 (%) |
|---|---|
| 20 | 31.65 |
| 40 | 75.05 |
| 60 | 89.99 |
| 80 | 95.13 |
| 100 | 97.28 |
Formula used
1) Hill saturation model
The oxygen saturation of hemoglobin binding sites is estimated by:
- PO2: oxygen partial pressure (mmHg internally).
- P50: PO2 where saturation reaches 50%.
- n: Hill coefficient controlling curve steepness.
2) Optional P50 shifting (approximate)
If modifiers are enabled, the calculator shifts the curve by adjusting P50 using a simplified logarithmic model:
This is an educational approximation and is not a clinical gas analyzer.
3) Dyshemoglobins and fractional oxygenation
COHb and MetHb reduce the fraction of hemoglobin available for oxygen. The calculator reports:
- Site SaO2 (functional): saturation of available oxygen-binding sites.
- Total O2Hb (fractional): Site SaO2 × (1 − COHb − MetHb).
4) Optional oxygen content
If hemoglobin concentration is provided:
How to use this calculator
- Select a mode: compute saturation from PO2, or invert for PO2.
- Enter PO2 (or target saturation) and choose the unit.
- Set Hill coefficient and baseline P50 for your model.
- Optionally enable modifiers to shift P50 for conditions.
- Optionally add COHb, MetHb, and hemoglobin for CaO2.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF for a report.
Technical article
1) What hemoglobin saturation represents
Hemoglobin saturation is the fraction of oxygen-binding sites occupied by O2. In this calculator, “site SaO2” describes binding-site occupancy predicted from PO2, while “total O2Hb” scales that value by the remaining functional hemoglobin when COHb and MetHb are present. Outputs are unitless fractions shown as percentages.
2) Why PO2 and saturation are nonlinear
The PO2–saturation relationship is sigmoidal because binding is cooperative. Around the mid-range (roughly 20–60 mmHg), small PO2 changes cause large saturation changes. At higher PO2 (for example 80–100 mmHg), the curve flattens and saturation approaches a plateau, which stabilizes oxygen loading despite moderate pressure fluctuations.
3) P50 as an affinity marker
P50 is the PO2 where predicted saturation equals 50%. A typical baseline for adult hemoglobin is about 26–27 mmHg near pH 7.40 and 37 °C, with PCO2 near 40 mmHg and 2,3-DPG around 5 mmol/L. Lower P50 increases affinity (left shift); higher P50 decreases affinity (right shift).
4) Hill coefficient and curve steepness
The Hill coefficient n controls how sharply saturation rises near P50. Values near 2.7 often approximate adult hemoglobin behavior, while n=1 produces a simple hyperbola (no cooperativity). In this model, increasing n makes the transition region steeper, changing sensitivity to PO2 in the mid-range.
5) How modifiers shift the curve
When enabled, the calculator adjusts P50 using an empirical log model. Decreased pH (Bohr effect), increased temperature, increased PCO2, and increased 2,3-DPG tend to raise P50 (right shift). The opposite changes lower P50 (left shift). These adjustments approximate direction and scale rather than replacing laboratory calibration.
6) Functional vs fractional oxygenation
COHb and MetHb reduce the oxygen-carrying fraction of total hemoglobin. “Site SaO2” describes saturation of available oxygen-binding sites, while “total O2Hb” applies the availability factor (1 − COHb − MetHb). This separation helps compare an affinity estimate to an overall oxygenated fraction when dyshemoglobins are present.
7) Estimating oxygen content (CaO2)
If hemoglobin concentration is entered, the calculator estimates CaO2 in mL O2/dL using 1.34 mL O2 per gram Hb and dissolved oxygen 0.0031×PO2. Example: Hb 15 g/dL and total O2Hb 98% yields about 1.34×15×0.98 ≈ 19.7 mL/dL, plus ~0.31 mL/dL dissolved at PO2 100 mmHg.
8) Interpreting results responsibly
These outputs are model-based estimates for learning, simulation, and engineering comparisons. Real blood depends on hemoglobin variants, measurement technique, temperature at sampling, and instrument-specific calibration. For safety, treat results as approximate and avoid clinical decisions. Use consistent inputs when comparing scenarios or exporting reports.
FAQs
1) Which mode should I use?
Use “Saturation from PO2” when you know PO2 and want SaO2. Use “PO2 from saturation” when you know a target saturation and want the implied PO2 based on your P50 and Hill settings.
2) Can I enter PO2 in kPa?
Yes. Select kPa and enter your value. The calculator converts internally to mmHg (1 kPa ≈ 7.5006 mmHg) so the Hill equation and P50 calculations remain consistent.
3) What are typical starting values for P50 and n?
A common baseline is P50 ≈ 26.6 mmHg and Hill coefficient n ≈ 2.7. These values are convenient for educational curves; adjust them when modeling different hemoglobin states or assumed affinity.
4) Why does total O2Hb drop when COHb or MetHb rises?
COHb and MetHb reduce the fraction of hemoglobin that can carry oxygen. The calculator multiplies site saturation by the remaining available fraction (1 − COHb − MetHb), producing a lower “total O2Hb” percentage.
5) Does this replace a blood gas analyzer?
No. It is a physics-based estimator for learning and comparisons. Clinical instruments use measured samples, calibration standards, and additional corrections not captured by this simplified curve and modifier model.
6) How is CaO2 computed, and what matters most?
CaO2 uses hemoglobin concentration and total O2Hb, plus a small dissolved oxygen term. Hb and saturation dominate the result; the dissolved term (0.0031×PO2) is comparatively small at typical pressures.
7) Why do modifiers change saturation at the same PO2?
Modifiers shift P50, moving the curve left or right. At a fixed PO2, a higher P50 (right shift) yields lower predicted saturation, and a lower P50 (left shift) yields higher predicted saturation.