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Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

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Hint: Oxygen dissociation curve is an important tool which helps in understanding how our blood carries and releases oxygen. It describes the affinity of oxygen for hemoglobin. It describes how hemoglobin captures and releases oxygen molecules into the fluid that surrounds it.

Complete answer:
To solve this question, we must know about the oxygen dissociation curve and its pattern.
Oxygen dissociation curve is a curve in which the proportion of saturated haemoglobin is plotted on the vertical axis versus the oxygen tension which is plotted on the horizontal axis.
The function of haemoglobin is to carry oxygen in the blood. At tension 100mm of Hg or more, the haemoglobin is completely saturated.

At oxygen tension 50mm of Hg, the saturation declines slowly and the unloading of oxygen takes place.
When the oxygen tension is 40mm of Hg, it dissociates the oxy-hemoglobin complex which makes oxygen readily available to the cells.
The oxygen dissociation curve is sigmoidal in shape. The sigmoidal shape is because of the affinity of oxygen to haemoglobin. The affinity for oxygen increases with the subsequent binding of molecules. After the binding of the first oxygen molecule, it increases the affinity for the second molecule as a reason of which subsequent haemoglobin attracts more oxygen.

Note: Raised Temperature, pH, carbon dioxide concentration affects the oxygen-dissociation curve and shifts the curve towards right. These factors cause haemoglobin to give up oxygen more readily. Binding of carbon monoxide shifts the curve to the left.