The sigmoid nature of Hb-02 dissociation curve is because of

A i.e. Binding of one 02 molecule increase affinity for the next 02 molecule Oxygen - hemoglobin dissociation curve is sigmoid (or S) shaped because binding of one oxygen molecule to heme increases the affinity of second heme molecule for oxygenQ an so on. The relationship between paial pressure of 02 and the percentage saturation of hemoglobin with 02 can be explained graphically. And the graph is called 0,- hemoglobin dissociation Curve. It represents the affinity of hemoglobin for 02 Normal curve S- shaped or sigmoid shapedQ due to interconversion of hemoglobin from low affinity tightly bound Tense configuration (T state) to high affinity released configuration (R state) as more 02 molecules are bound. - Relaxed configuration exposes more oxygen binding sites & significantly increases 02 affinity. - Combination of the first heme with 02 increase the affinity of second heme for 02 (due to TR interconversion) and oxygenation of second increases the affinity of the third and so on. * P,0 - The paial pressure of 02 at which Hb saturation is 50% Q. The normal value is 26.6 mmHg * Myoglobin has hyperbolic 02-binding curve. Bohr effect: The presence of CO2 (4, pH) decreases the affinity of Hb for 02, this enhances fuher release of 02 to the tissue and the 02 dissociation curve is shifted to right. This is known as Bohr effect. So decrease in 02 affinity of hemoglobin when pH of blood falls is called Bohr effectQ. And it is caused by the fact that deoxygenated hemoglobin binds H+ more actively than does oxyhemoglobin. All the factors, which shift the 02- dissociation curve to the right enhances the Bohr effect. Binding of 02 to hemoglobin reduces its affinity for CO2(2 or in other words CO2 disociation curve shifts to right (i.e. blood begins to lose some CO2 as it becomes oxygenated) is called Haldane - Christian - Douglas effect