A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against the pO₂ . This curve is called:

Oxygen dissociation curve.

The correct answer is Option (2) - Oxygen dissociation curve.

Haemoglobin is a red coloured iron containing pigment present in the RBCs. O2 can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O2 . Binding of oxygen with haemoglobin is primarily related to partial pressure of O2 . Partial pressure of CO2 , hydrogen ion concentration and temperature are the other factors which can interfere with this binding. A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2 . This curve is called the Oxygen dissociation curve and is highly useful in studying the effect of factors like pCO2 , H+ concentration, etc., on binding of O2 with haemoglobin. 

                                 Oxygen dissociation curve