Oxyhaemoglobin Dissociation curve

Normally 97% of the oxygen transported in the blood is carried in chemical combination with Hb; the remaining 3% is in the dissolved state in the water of plasma (at 37 deg C)

Each molecule of Hb combines with 4 molecules of oxygen [as have 4 polypeptide chains each bound to a Fe-containing heme group]. When all 4 molecules of oxygen are bound to the Hb, the Hb is said to be fully saturated.

A ‘cooperation’ exists between the 4 polypeptides of the Hb. After the first O2 combines with a Fe-heme, the remainder of the Hb molecule undergoes a conformational change thereby , and progressively more so, increasing the affinity for each subsequent molecule of oxygen. Similarly, unloading of one oxygen molecule enhances the unloading of the next. Thus the affinity of Hb for O2 changes with its state of O2 saturation. [block of 4 postage stamps analogy]. The 4th molecule of oxygen binds 300x more tightly than the 1st

HHb + O2 HbO2 + H+

Deoxyhemoglobin oxyhemoglobin

The rate at which Hb reversibly binds or releases O2 is regulated by: PO2; pH; PCO2; DPG

Hb & PO2

not a linear relationship; see S-shaped oxygen-Hb dissociation curve: —> steep slope between 10 -50 mmHg PO2, plateaus between 70 - 100 mmHg
At 70 mm Hg is almost completely saturated and further increases in PO2 produce only small increases in oxygen binding
Most oxygen unloading occurs in the steep part of the curve, where the partial pressure changes only little. Only 20-25% of the bound oxygen is unloaded per cycle: still see substantial amounts of oxygen still available in venous blood (venous reserve) for any increased demands in O2 requirements eg exercise.

Hb & Temp

Increased temp —> affinity of Hb for O2 declines —> less oxygen binds & O2 unloading is enhanced
Therefore increased release of O2 in exercising muscles
[note]: reverse occurs in chilled tissues; see little unloading of oxygen —> red cheeks

Hb & pH

increased H+ —> weaken Hb-O2 bond [bohr effect]
decreased blood pH —> decreased affinity of Hb for O2 —> increased O2 unloading to tissues
Therefore increased release of O2 in exercising muscles

Hb & DPG

rbc lack mitochondria —> anaerobic metabolism —> byproduct: DPG
increased metabolic rate rbc (eg fever, exercise) —> increased DPG prod —> DPG bind to Hb —> lowers affinity of Hb for O2 —> increased O2 release in tissues

Max oxygen carried by Hb of blood:

15 gms Hb in each 100 ml blood
1 gm Hb combines with max 1.34 ml O2 —>100 ml blood carries 20 ml blood when 100% saturated

Amount oxygen released from Hb in tissues

100 ml blood carries 19.4 ml blood when 97% saturated [arterial]
100 ml blood carries 14.4 ml blood when 75% saturated [venous] —>5 ml oxygen transported to tissues by each 100 ml blood/cycle