Explain the significance of the shape of the oxyhemoglobin dissociation curve.

The OS-HB dissociation curve is sigmoid shaped, flat at the top and bottom of the curve and steep and linear in the middle of the curve. This shape represents the unique kinetics of the binding of four molecules of oxygen to one molecule of Hb. The first molecule of O2 is slowly combined with the Hb molecule (first flat portion of the curve), after which the hemoglobin rotates to allow the next two molecules of O2 to combine very rapidly (the steep linear portion of the curve). The kinetics of the combination of the fourth O2 molecule with the Hb molecule is slow and results in the final flat portion of the curve. During the unloading of O2 from the Hb molecule, the reverse of the loading process occurs. The Hb molecule has the ability to release the O2 rapidly in response to the decreases in tissue PO2; that is, the steep linear portion of the curve is a reflection of the increasing difference between PaO2 and PtO2 . In addition, other properties of the Hb molecule related to metabolism assist in this rapid unloading of oxygen.

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