O2 Dissociation Curve Calculator

Calculate hemoglobin oxygen saturation from pO2 using Hill equation and identify Bohr effect shifts.

26.7 = normal adult HbA
Cooperativity of O2 binding
Bohr effect: lower pH → right shift
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Why the Curve Is Sigmoid

Haemoglobin has four subunits, and binding oxygen to one increases the affinity of the others. This cooperativity produces the S-shape, and the Hill coefficient of about 2.7 quantifies it.

SO2 = pO2n / (P50n + pO2n)
pO2SO2Region
100 mmHg97%Flat — arterial
60 mmHg90%Still adequate
40 mmHg75%Steep — venous
27 mmHg50%P50 by definition

The two regions do different jobs. The flat upper portion means arterial saturation stays near 97% even if pO2 falls from 100 to 60 — a substantial safety margin for lung disease or altitude. The steep lower portion means a small further drop in tissue pO2 unloads a large amount of oxygen.

Myoglobin, with one subunit and no cooperativity, gives a hyperbolic curve and P50 near 2.8 mmHg. It therefore holds oxygen tightly until tissue pO2 falls very low — ideal for a storage molecule but useless for transport.

What Shifts the Curve

Shift right (unload more)Shift left (hold tighter)
Increased CO2, decreased pHDecreased CO2, increased pH
Increased temperatureDecreased temperature
Increased 2,3-BPGDecreased 2,3-BPG
—Fetal haemoglobin, carbon monoxide

Every right-shifting factor is a marker of metabolically active tissue — warm, acidic, CO2-rich. The Bohr effect therefore delivers oxygen preferentially where it is being consumed, without any regulation being required.

Fetal haemoglobin shifts left because it binds 2,3-BPG poorly, giving it higher affinity than maternal haemoglobin — which is precisely how oxygen transfers across the placenta.

Carbon monoxide is doubly dangerous: it occupies binding sites and also shifts the remaining sites left, impairing release of what oxygen is still carried. Pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so readings appear falsely normal.

Worked Examples

Example 1: Arterial blood: pO2=95, P50=26.7, n=2.7
SO2=95^2.7/(26.7^2.7+95^2.7)
Result: SO2=96.9% — near flat part of curve
Small pO2 changes → small saturation changes
Example 2: Venous: pO2=40mmHg
SO2=40^2.7/(26.7^2.7+40^2.7)
Result: SO2=74.9% — steep part of curve
Large O2 unloading from small pO2 change
Example 3: Why the flat portion matters
pO2 falling from 100 to 60 mmHg
Result: Saturation only falls 97% to 90%
The flat upper region provides a large safety margin. Substantial lung impairment produces little desaturation until pO2 drops below about 60.
Example 4: Why fetal haemoglobin binds tighter
HbF binds 2,3-BPG poorly
Result: Left-shifted curve
Higher affinity than maternal haemoglobin allows oxygen to transfer across the placenta down an affinity gradient rather than a pressure gradient alone.

Common Mistakes

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Reading pulse oximetry in carbon monoxide poisoning

Standard oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation appears normal. Co-oximetry is required.

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Assuming saturation tracks pO2 linearly

The curve is sigmoid. Saturation barely changes between 100 and 60 mmHg but falls steeply below 60.

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Forgetting the curve shifts

Acidosis, fever and raised 2,3-BPG all shift right, unloading more oxygen at a given pO2. A single curve does not describe all conditions.

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Confusing content with saturation

Saturation is the percentage of available binding sites occupied. A severely anaemic patient can have 100% saturation and dangerously low oxygen content.

Frequently Asked Questions

Physiological significance of sigmoid shape?
Steep middle: efficient O2 loading in lungs (pO2 95mmHg→97% sat) AND unloading in tissues (pO2 40→73% → ~25% O2 released). Flat upper: pO2 variations at altitude don't dramatically affect loading. Cooperative binding creates ideal O2 carrier.
P50 shifts?
Right shift (P50↑, unloads more O2): low pH (Bohr effect), high CO2, high temperature, high 2,3-BPG. Exercise conditions: acidosis + heat → right shift → more O2 to muscles. Left shift (P50↓, holds O2 more tightly): fetal Hb (HbF), low 2,3-BPG, alkalosis, CO poisoning (HbCO).
Why is the oxygen dissociation curve sigmoid?
Because haemoglobin's four subunits bind cooperatively — oxygen binding to one raises the affinity of the others. The Hill coefficient is about 2.7.
What does a right shift mean?
Reduced affinity, so more oxygen is released at a given pO2. Acidosis, raised CO2, fever and increased 2,3-BPG all shift right.
Why is fetal haemoglobin left-shifted?
It binds 2,3-BPG poorly, giving higher oxygen affinity than maternal haemoglobin and allowing transfer across the placenta.
Why is pulse oximetry unreliable in carbon monoxide poisoning?
Carboxyhaemoglobin absorbs light similarly to oxyhaemoglobin, so saturation reads falsely normal. Co-oximetry is needed.