Dissociation Curve

• In the lungs ppO_ 2 is high and so Hb is 95-97% saturated with O_2, but in respiring tissues ppO_2 is low so Hb is 20-25% saturated with O_2: as ppO_2 increases, so does the % saturation of Hb but not directly proportional

  
• At low ppO_2, Hb does not readily take up O_2 as it is difficult for O_2 molecules to reach the haem groups within the Hb molecules
• As ppO_2 increases, diffusion gradient increases, when one O_2 molecule binds; there is a conformational change in the Hb enabling the 2nd and 3rd O_2 molecules to bind easily; seen in the initial steep region
• When each Hb has 3x O_2 molecules bound, it is more difficult for the 4th O_2 molecule to bind; seen on the curve as the more sloped region (explains why Hb doesn’t achieve 100% saturation)