Oxygen dissociation curve: haemoglobin, the Bohr shift with CO₂, pH and temperature, and lung volumes

What it does

Red blood cells carry oxygen bound to haemoglobin. Each haemoglobin has four places for oxygen. How full it is (its saturation) depends on how much oxygen is around it, measured as the partial pressure PO₂. Plotting saturation against PO₂ gives an S-shaped oxygen dissociation curve: haemoglobin fills up easily in the lungs (about 100 mmHg) and lets oxygen go quickly once PO₂ drops in the tissues (about 40 mmHg at rest, lower in working muscle).

• Slide PO₂ and see the saturation and a picture of one haemoglobin with its four sites.
• Change the conditions. More carbon dioxide, more acid (lower pH) and warmth, as in working muscle, move the curve right: haemoglobin holds oxygen less tightly and gives more to the tissues. This is the Bohr effect. Cool, alkaline blood with little CO₂ moves it left.
• Fetal haemoglobin holds oxygen more tightly than an adult’s, so a baby can take oxygen from its mother’s blood.
• Lung volumes: type tidal volume, the reserves and the residual volume to draw a spirogram and work out the vital capacity, total lung capacity and the air breathed each minute.

It fits Class 11 Biology: breathing and exchange of gases (respiratory volumes and capacities, transport of oxygen, the dissociation curve and the factors that shift it). It is a teaching model. The heart that moves this blood is in cardiac cycle; blood pH and CO₂ are read in blood gas interpreter; oxygen use is measured with respirometry calculator and VO2max estimator.

How to use it

1. Slide PO₂ from 0 to 120 mmHg. The label says when you are near lung or tissue values.
2. Pick conditions: Normal blood, Working muscle (pH 7.2, CO₂ 60 mmHg, 39 °C) or Cold, low CO₂, or move the pH, CO₂ and temperature sliders yourself. The dashed line keeps the normal curve for comparison.
3. Read the facts: P50 (the PO₂ where haemoglobin is half full), saturation in the lungs and tissues, and how much oxygen is given up per 100 mL of blood.
4. Lung volumes: change any value; the spirogram shows two quiet breaths, the deepest breath in and out, then quiet breathing.
5. Download both graphs and the numbers as an image, PDF or Word file. Check yourself gives fresh questions every time.

Worked example

At normal conditions and PO₂ 40 mmHg the page shows 75 % saturated. In the lungs (100 mmHg) it is 97.7 %, so the blood gives up 22.8 % of its load, about 4.8 mL of oxygen per 100 mL of blood. Press Working muscle: P50 rises from 26.9 to 37.0 mmHg, the curve moves right by 10.1 mmHg, and at 40 mmHg haemoglobin is only 55.3 % full, so it gives up 42.5 % of its load, nearly twice as much. For the lungs, with tidal volume 500 mL, inspiratory reserve 2750 mL and expiratory reserve 1050 mL, the vital capacity is 1050 + 500 + 2750 = 4300 mL, and with 1150 mL residual volume the total lung capacity is 5450 mL.

FAQ

Why is the curve S-shaped? When one oxygen binds, haemoglobin changes shape and the next ones bind more easily (cooperative binding). So the middle of the curve is steep, just where tissue PO₂ lies.

Why can’t you breathe out the residual volume? The lungs and airways never collapse completely; that air stays to keep gas exchange going between breaths.

Is this accurate for a real person? It follows a well-known equation for average adult blood, but people differ, and it is not a medical tool.

Last updated 2026-11-09.