The Role of Haemoglobin in the Transport of Oxygen and Carbon

dioxide

Dissociation Curves

• Show the relationship between percentage saturation and partial pressure (conc.) of O2

• The greater the conc. of O2 , the more saturated Hb becomes with O2

• The degree to which the Hb is saturated at diff. O2 partial pressures can be measured

• THOUGHT: ↑ [O2] ↑ Hb saturation → Straight line graph NO S-shaped/Sigmoid → O2 dissociation curve

•~ 30 mm Hg O2 only 50% of Hb is present as oxyHb

•At a partial pressure of 0 no O2 is attached to Hb

•S- shape of the curve is physiologically important

•At the steep part of the curve: a small↓ [O2] in the environment (tissues) → large fall in the % change in Hb saturation ; O2 given up to tissues.

THE BOHR EFFECT

•In regions with an ↑ed [CO2], the O2 dissociation curve is shifted to the right → BOHR EFFCT•The vertical line ~ 29mmHg → 50% saturation of the 3 curves representing 3 diff. [CO2]

•Right curve (70mmHg): high [CO2] → lower saturation of Hb at an [O2] of 29mmHg

•At 15mmHg CO2 + 29mmHg O2→ higher saturation of Hb with O2

•→ the effect of ↑ed CO2 → O2 to be released from Hb

The Bohr Effect cont’d

• Remember CO2 is a product of respiration

• The faster respiration is occurring, the faster CO2 is produced

• High levels of respiration is associated with high [CO2]; this happens when O2 is most needed thus the ↑ed [CO2] makes Hb release O2

The Bohr Effect cont’d

• CO2 when dissolved forms a weak acid:

H2O + CO2 ↔ H2CO3 ↔ H+ HCO3-

carbonic acid

• The H+ combine with Hb and make it less able to carry O2

Transport of CO2

• CO2 is carried by the blood in 3 diff. ways: In solution – 5%Combined with protein – 10-20%As hydrogencarbonate – 85%

• CO2 must not be allowed to accumulate in the body as it forms an acid in sol. And could lead to fatal changes in blood pH.

Role of Hb in the Transport of CO2

• CO2 produced by tissues diffuses into the bloodstream and passes into RBCs.

• CO2 combines with H2O forming carbonic acid (H2CO3). Catalyst involved – nzym- carbonic anhydrase (RBCs).

• H2CO3 dissociates into H+ and HCO3-:

H2O + CO2 ↔ H2CO3 ↔ H+ HCO3-

Role of Hb in the Transport of CO2

• H+ displace O2 from Hb (basis of the Bohr effect)

• The deoxygenated Hb accepts H+ forming haemoglobinic acid (H.Hb).

• By accepting H+, Hb acts as a buffer molecule (prevents the blood from becoming acidic).

Role of Hb in the Transport of CO2

• HCO3- formed within RBCs diffuse into the

plasma and combine with Na forming NaHCO3.

• The loss of –vely charged HCO3- from

RBCs leaves them with a more +ve charge.

• This is however balanced by Cl- diffusing into RBCs → chloride shift

Role of Hb in the Transport of CO2

• When the RBCs reach the lungs, the partial pressure of O2 is high and the partial pressure of CO2 is low, the reverse occurs i.e O2 is taken up by RBCs and CO2 is released.

Assignment

• Why is the oxygen dissociation curve S-shaped rather than a straight line?