Haemoglobin (Hb) accounts for >90% of the dry mass of the red blood cells (RBCs, erythrocytes), about 29 pg in each RBC, and about 98% of the cytoplasmic protein content. In an adult man the total amount of haemoglobin in the circulation is about 900 g.

Haemoglobin is a tetrameric 65 kDa protein with almost spherical shape. It can bind four O2 molecules, with affinity that strongly depends on local oxygen partial pressure (PO2). Oxygen binding to haemoglobin can be described by the Adair equation, but in Hb-O2 saturation (SO2) range 20-80% the much simpler Hill equation provides an accurate dissociation curve:

P50 and n are dependent on the blood pH, temperature, and concentrations of CO2 and 2,3-DPG (Dash & Bassingthwaighte, 2010). In human subjects, P50 is 3.5±0.1 kPa in arterial blood, and 3.7±0.2 kPa in venous blood (Siggaard-Andersen et al., 1990). In a larger group, arterial P50 was found to vary between 2.15 and 6.44, with median 3.44 kPa (Gøthgen et al., 1990). P50 and n may be different in trained and untrained people (Braumann et al., 1979).

See also:


Adair GS, Bock AV, Field H Jr. The hemoglobin system: VI. The oxygen dissociation curve of hemoglobin. J Biol Chem. 1925; 63: 529-545.

Gell DA. Structure and function of haemoglobins. Blood Cells Mol Dis. 2017 (in press). doi: 10.1016/j.bcmd.2017.10.006.

Pittman RN. Oxygen transport in the microcirculation and its regulation. Microcirculation 2013; 20(2): 117-137. doi: 10.1111/micc.12017.

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Created at: 2017-03-30
Updated at: 2018-04-11
Written by: Vesa Oikonen