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

(N/A) The oxygen dissociation curve is a graph showing the percentage saturation of oxyhaemoglobin at various partial pressures of oxygen $(pO_2)$.
The curve represents the equilibrium between oxyhaemoglobin and haemoglobin at different partial pressures.
In the lungs,the partial pressure of oxygen is high,which facilitates the binding of oxygen to haemoglobin to form oxyhaemoglobin.
In the tissues,the partial pressure of oxygen is low,causing oxyhaemoglobin to dissociate and release oxygen.
The sigmoidal ($S$-shaped) pattern of the curve is due to the cooperative binding of oxygen to haemoglobin. When the first oxygen molecule binds to one of the four haem groups of the haemoglobin molecule,it induces a conformational change in the protein structure. This change increases the affinity of the remaining haem groups for subsequent oxygen molecules,making it easier for them to bind.