Sixty-six times: oxygen held by haemoglobin
At full saturation, the haemoglobin of a reference adult can bind 20.1 millilitres of oxygen in each decilitre of blood. Clinical formulas use dissolved-oxygen coefficients of 0.0030–0.0031 millilitres per decilitre per millimetre of mercury: 0.30–0.31 millilitres at 100 millimetres of mercury. The ratio is 64.8–67.0, or about 66. In the reference case, mixed-venous saturation is 75 percent, while about 76.4 percent of arterial oxygen content returns to the lungs. Without haemoglobin, even the impossibly favourable assumption of complete extraction would require about 81–83 litres a minute at resting oxygen consumption.
The dissociation kymograph
Drag on the smoked drum to choose a partial pressure. The pale trace is adult haemoglobin, computed from Severinghaus (1979) and shifted with the Bohr factor. The line at 100 millimetres of mercury is the lung; the line at 40 millimetres of mercury is resting tissue.
The four pockets fill with saturation. Allosteric cooperation makes the second and third oxygen molecules bind more readily than the first, producing a sigmoid curve.
Why dissolved oxygen is not enough
Oxygen is poorly soluble in warm water. Published clinical formulas use rounded coefficients of 0.0030–0.0031 millilitres per decilitre per millimetre of mercury. At 100 millimetres of mercury that gives 0.30–0.31 millilitres dissolved in a decilitre. An adult at rest consumes about 250 millilitres of oxygen a minute. If the heart carried only this dissolved fraction and the tissues extracted all of it, the required output would be 80.6–83.3 litres a minute, or about 82. A resting output of 5.0 litres a minute would carry only 15–15.5 millilitres of dissolved oxygen.
Haemoglobin repairs the deficit without enlarging the pump. Hüfner’s number, 1.34 millilitres of oxygen per gram, times 15.0 grams of haemoglobin in a decilitre, yields 20.1 millilitres bound at full saturation. The ratio against 0.30–0.31 is 64.8–67.0, or about 66 times. The plateau keeps that cargo nearly full between 80 and 100 millimetres of mercury; the steep limb hands it over between 20 and 40, where muscle is working.
Two columns, the same blood
The left stack is oxygen clipped to iron at the kymograph’s current saturation. The right stack is oxygen dissolved at the same pressure. The ratio updates when you move the cursor, pH, temperature or haemoglobin.
Without the left-hand column, even complete extraction of dissolved oxygen would require at least 82 litres a minute at a consumption of 250 millilitres. This theoretical minimum depends on a physiologically unattainable state.
Three-quarters come back
In the reference case, blood leaving the lung is 97.5 percent saturated, while mixed-venous blood returning from all organs is 75 percent saturated. These percentages describe haemoglobin saturation. The classical Fick pair uses a content difference of 5.0 millilitres per decilitre: 5.0 litres a minute × 50 millilitres per litre = 250 millilitres a minute.
The full calculation, with saturations of 97.5 / 75 and the midpoint coefficient 0.00305, gives a difference of 4.7 millilitres per decilitre. Tissues extract 23.6 percent of arterial content, while 76.4 percent returns to the lungs. The 75 percent SvO₂ reference and the 76.4 percent returned-content fraction are therefore close in value but different in meaning.
That reserve is why you can stand up before breathing speeds. Muscle lowers local PO2, drops pH and raises temperature: the curve shifts right, P50 climbs from 26.8 to about 33.4 millimetres of mercury at pH 7.20, and the same haemoglobin unloads more without waiting for a faster lung.
Myoglobin steals, the fetus steals
Myoglobin has a single iron pocket, so its curve is hyperbolic. Its P50 sits at 2.8 millimetres of mercury, ten times further left than adult haemoglobin. In exercise, when muscle PO2 falls, myoglobin pulls oxygen off the tetramer in the capillary. Fetal haemoglobin has a P50 around 19 millimetres of mercury: at the same placental PO2, fetal saturation is higher than the mother's, and oxygen moves toward the child.
| Carrier | P50 | Shape | Role |
|---|---|---|---|
| Adult HbA | 26.8 mmHg | sigmoid, n ≈ 2.8 | transport between 100 and 40 mmHg |
| Fetal HbF | 19 mmHg | sigmoid, left-shifted | takes oxygen from maternal HbA |
| Myoglobin | 2.8 mmHg | hyperbola, n = 1 | muscle store, steals from HbA |
More on iron than in the lungs
With one arterial litre at 19.9 millilitres per decilitre and four venous litres at 15.2 millilitres per decilitre, blood holds about 807 millilitres of oxygen. A functional residual capacity of 2.5 litres, at an alveolar fraction of 0.15, holds 375 millilitres. The ratio is 2.2: more oxygen sits on iron than in lung air. That is why a short pause in breathing is tolerable. The blood store buys more time than alveolar volume alone.
Start with mass and pulse
Enter mass and pulse for a resting one-MET scenario. The calculation fixes cardiac output at the teaching reference of 5.0 litres a minute, haemoglobin at 15.0 grams per decilitre, SaO₂ at 97.5 percent and PaO₂ at 100 millimetres of mercury. Mass sets conventional oxygen consumption; pulse shows the stroke volume implied by the fixed output. The scenario illustrates Fick's relation and does not estimate your physiological values.
Method note and biophysical references
The figures 66 and 82 are rounded centres of the ranges given by the clinical coefficients 0.0030–0.0031: a ratio of 64.8–67.0 and a theoretical output of 80.6–83.3 L/min. The instrument uses the midpoint, 0.00305. The reference adult has 15.0 g/dL haemoglobin, 5.0 L of blood, 5.0 L/min of output and 250 mL/min of consumption. These are teaching magnitudes, not personal values.
- Hüfner, G. (1894). Capacity 1.34 mL STPD / g; theoretical 1.39 mL/g for mass 64,458 g/mol and four irons.
- NCBI Bookshelf, Oxygen Transport and modern dissolved-oxygen transport data. Rounded clinical coefficients: 0.0030–0.0031 mL/dL/mmHg.
- Severinghaus, J. W. (1979). J Appl Physiol 46:599–602. S = 1 / (1 + 23,400 / (P³ + 150P)).
- Bohr, C., Hasselbalch, K. and Krogh, A. (1904). The effect of carbon dioxide on oxygen binding; the model uses a Bohr coefficient of −0.48 for its teaching scenario.
- Hill, A. V. (1910). Coefficient n ≈ 2.8 for the adult tetramer.
- NCBI Bookshelf, Oxygen Transport. Practical capacity of 1.34 mL O₂/g Hb and rounded dissolved-oxygen coefficient of 0.003 mL/dL/mmHg.
- NCBI Bookshelf, Oxygen Transport in Normal and Pathological Situations. Resting pair: 5 L/min output, 20 mL/dL arterial content, 15 mL/dL venous content and 250 mL/min consumption.
- NCBI MeSH, Metabolic Equivalent. One MET is the conventional reference of about 3.5 mL O₂/kg/min; individual values vary.
- NCBI Bookshelf, Oxygen Saturation. Fick: V̇O2 = Q × (CaO2 − CvO2).
CaO2 ≈ (1.34 × [Hb] × SO2) + (0.0030–0.0031 × PO2), with [Hb] in g/dL, SO2 expressed as a fraction and PO2 in mmHg; the result is in mL/dL. The 81–83 L/min range assumes complete extraction of dissolved oxygen. Because real extraction cannot be complete, the range is a theoretical lower bound on the flow required in this thought experiment.
This material is educational. It does not replace a consultation, a blood count or a blood gas.