It is determined by the concentration of haemoglobin, the affinity of oxygen to haemoglobin and the amount of oxygen dissolved in the plasma.
The haemoglobin level (normal range: 12-15g /100mL blood) is regulated by erythropoetin, a glycoprotein mainly produced by the kidneys. It is secreted into the plasma in response and in direct proportion to the degree of hypoxia.
The relationship between pO2 and the degree of saturation of haemoglobin with oxygen (sO2) is non-linear which is of fundamental biological importance. The flat part of the sigmoid-shaped oxyhaemoglobin dissociation curve favours binding of O2 to haemoglobin over a wide range of pO2 at the pulmonary level, whereas oxygen dissociates well from haemoglobin in the steep part of the curve in a small range of low oxygen tensions at the tissue level favouring its diffusion into the tissues.
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