Inputs
Example Data Table
| Scenario | pH | Reference pH | phi | P50 ref (mmHg) | PO2 (mmHg) |
|---|---|---|---|---|---|
| Resting arterial reference | 7.40 | 7.40 | -0.48 | 26.6 | 100 |
| Working tissue, mildly acidic | 7.20 | 7.40 | -0.48 | 26.6 | 40 |
| More alkaline shift | 7.55 | 7.40 | -0.48 | 26.6 | 60 |
Formula Used
This calculator models the Bohr effect by shifting the oxygen affinity parameter, P50, as blood acidity changes. A common empirical form is:
Once the shifted P50 is known, hemoglobin oxygen saturation is estimated with a Hill model:
If you choose the chemistry mode, pH is approximated from bicarbonate and carbon dioxide using:
How to Use This Calculator
- Select a pH input mode: direct pH or chemistry-based pH.
- Choose your PO2 unit and enter a single PO2 value.
- Set the reference pH, reference P50, Bohr coefficient, and Hill coefficient.
- Define table start, end, and step to generate a curve dataset.
- Press Calculate to show results above the form.
- Use the download buttons to export CSV or PDF summaries.
Interpretation Tips
- Lower pH usually increases P50, shifting the curve right.
- At fixed PO2, a right shift often reduces saturation, aiding oxygen release.
- In alkaline conditions, P50 can decrease, increasing saturation at a given PO2.
- Use the table to compare shifted vs reference curves at multiple PO2 points.
Professional Article
1) What the Bohr effect represents
The Bohr effect describes how increased acidity (lower pH) and higher carbon dioxide reduce hemoglobin’s oxygen affinity. In practical terms, the oxygen dissociation curve shifts right, meaning a higher oxygen partial pressure is needed to reach the same saturation. This supports oxygen unloading in active tissues.
2) Why P50 is a useful summary metric
P50 is the PO2 at which hemoglobin is 50% saturated. Many adult references cite a baseline near 26–27 mmHg at pH 7.40, normal temperature, and typical 2,3‑BPG levels. A right shift increases P50; a left shift decreases it, altering delivery.
3) How this calculator models the shift
The calculator uses an empirical Bohr coefficient (phi) to scale changes in P50 with pH. A commonly used magnitude is around −0.4 to −0.6, but values depend on experimental conditions. You can change phi to match your dataset or lecture notes.
4) Linking P50 to saturation with the Hill equation
After computing a shifted P50, saturation is estimated using a Hill model with coefficient n. Adult hemoglobin is often approximated by n ≈ 2.7, capturing cooperative binding. The output saturation is a model-based estimate that is most useful for comparing scenarios rather than replacing laboratory measurement.
5) Using chemistry inputs to approximate pH
When bicarbonate and pCO2 are known, the calculator can estimate pH using the Henderson–Hasselbalch relationship for the bicarbonate buffer system. Typical arterial values are about HCO3− ≈ 24 mmol/L and pCO2 ≈ 40 mmHg. This option helps when pH is not directly available.
6) Choosing realistic PO2 points for interpretation
To mirror physiology, many users evaluate PO2 ≈ 100 mmHg to represent alveolar blood and PO2 ≈ 40 mmHg to represent resting tissue. By holding PO2 constant and varying pH, the calculator highlights how acidosis can reduce saturation and promote oxygen release where metabolism is elevated.
7) Unit handling and curve table generation
You may enter PO2 in mmHg or kPa; internally the model operates in mmHg and converts as needed (1 kPa ≈ 7.50062 mmHg). The PO2 table allows quick curve plotting in spreadsheets. Smaller step sizes yield smoother curves but can increase file size.
8) Practical cautions and recommended workflow
Oxygen affinity is influenced by temperature, 2,3‑BPG, fetal hemoglobin fraction, and other factors not explicitly modeled here. For best results, set a reference P50 consistent with your context, choose a phi from your source, then compare relative changes. Export CSV for full tables and archive PDF summaries for reports.
FAQs
1) What does a negative Bohr coefficient mean?
A negative phi means that decreasing pH increases P50, shifting the curve right. At a fixed PO2, saturation tends to drop, helping oxygen unload in tissues.
2) Why does the calculator report both shifted and reference saturation?
The reference saturation uses the reference P50, while shifted saturation uses the pH-adjusted P50. Comparing them at the same PO2 isolates the modeled Bohr effect.
3) Can I use kPa instead of mmHg?
Yes. Enter PO2 in kPa and the calculator converts internally. P50 values are handled in mmHg to keep the equations consistent, and the table is displayed in your chosen unit.
4) What PO2 should I use for tissue versus lung conditions?
A common teaching choice is about 40 mmHg for tissue and about 100 mmHg for alveolar blood. Use your scenario values if you have measured PO2.
5) How accurate is the pH from bicarbonate and pCO2 mode?
It is an approximation using the bicarbonate buffer model. It works best near physiological ranges. For clinical decisions, use measured blood gas values and validated models.
6) Why might my results differ from a textbook curve?
Textbook curves assume specific temperature, 2,3‑BPG, and hemoglobin type. Changing these factors shifts affinity. Adjust reference P50 and phi to match the conditions described in your source.
7) What should I export: CSV or PDF?
Use CSV for full curve data and plotting. Use PDF for a compact snapshot of key inputs and results. Many users keep both for reproducible reporting.