There are four iron atoms in a hemoglobin molecule and each can bind one molecule of oxygen in a reversible interaction. As hemoglobin binds oxygen it changes its conformation and takes up oxygen more readily. This accounts for the sigmoid shaped hemoglobin dissociation curve (Fig. 8.9). As demonstrated by the curve, hemoglobin binds oxygen rapidly to a PaO2 of 8 kPa (60 mmHg) and then levels off, requiring higher partial pressures of oxygen to remain fully saturated. Another way of looking at it is that a PaO2 of 8 kPa is required to maintain saturations above 90%. This is important physiologically, because it allows for the easy off-loading of oxygen in the peripheral capillary beds where oxygen is needed (i.e. if the PaO2 is low in the periphery, hemoglobin gives up its oxygen molecule easily). Factors such as acidosis, increased temperature, and increased 2,3-dihydroxyphosphoglycerol (2,3-DPG) shift the hemoglobin dissociation curve to the right resulting in easier unloading of oxygen for a given PaO2.
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