Gas Transport in the Blood

1. Gas Transport in the Blood Dr Shihab Khogali Ninewells Hospital & Medical School, University of Dundee

2. What is This Lecture About? • Understand the effect of partial pressure onO2 and CO2 carriage in the blood • Understand the means of O2 carriage in the blood • Understand the oxygen-haemoglobin dissociation curve and the significance of its sigmoid shape • Know the Bohr effect and its significance in O2liberation at tissue level • Understand the means of CO2 carriage in the blood • Know the Haldane effect and its significance in the uptake of CO2 and CO2generated H+at tissue level; and CO2 liberation at the lungs See blackboard for detailed learning objectives

3. Atmospheric air Alveoli O2 Picked up by blood at the lungs must be transported to the tissues for cellular use Pulmonary circulation Systemic circulation CO2 produced at tissues must be transported to the lungs for removal from the body

4. Air Gas Pulmonary Capillary Arterial Oxygen Partial Pressures around the System 20 PO2 kPa Diffusion 10 Atmosphere Tissues

5. Henry’s Law • Gaseous Phase • Liquid Phase (gas in solution) The amount of a given gas dissolve in a given type and volume of liquid (e.g. blood) at a constant temperature is: proportional to the partial pressure of the gas in equilibrium with the liquid What is the Effect of Partial Pressure on Gas Solubility? • This means that if the partial pressure in the gas phase is increased the concentration of the gas in the liquid phase would increase proportionally • The partial pressure of a gas in solution is its partial pressure in the gas mixture with which it is in equilibrium

6. Dissolved Oxygen • The O2 amount dissolved in blood is proportional to the partial pressure (Henry’s Law) • 3ml O2 per litre of blood at a PO2 of 13.3 kPa • Under Resting conditions (cardiac output 5L/min): 15 ml/min of O2 is taken to tissues as dissolvedO2 • Even at strenuousexercise (cardiac output of 30 L/min): 90 ml/min would be taken to tissues as dissolvedO2 • RestingO2consumption of our body cells is about 250ml/min • O2consumption may increase up to 25 folds during strenuousexercise • Clearly, another mechanism is involved in O2transport in the blood.

7. Oxygen Transport in the Blood • Most O2 in the blood is transported bound to haemoglobin in the red blood cells • Normal O2 concentration in the arterial blood is about 20 ml/100 ml (200 ml per litre) at a normal arterial PO2 of 13.3 kPa and a normal haemoglobin concentration of 15 grams/100 ml • Percentage of O2 carried bound to haemoglobin = 98.5% • Percentage of O2carried in the dissolved form = 1.5% (3 ml per litre at a PO2 of 13.3 kPa ) • O2is present in the blood in two forms: (1) bound to haemoglobin (2) physically dissolved (very little O2)

8. Oxygen binding to haemoglobin • Haemoglobin can form a reversible combination with O2 • Each Hb molecule contains 4 haem groups • Each haem group reversibly binds to one O2 molecule • Haemoglobin is considered fully saturated when all the Hb present is carrying its maximum O2 load • The PO2is the primary factor which determine the percent saturation of haemoglobin with O2

9. Oxygen Haemoglobin Dissociation Curve % Haemoglobin Saturation O2 concentration ml/100 ml 13.3 5.3 8.0 Blood PO2 (kPa)

10. Oxygen Haemoglobin Dissociation Curve 100 Total O2 20 O2 combined with Hb O2 concentration (ml/100 ml) % Hb saturation Dissolved O2 0 0 0 13 PO2 (kPa)

11. 100 Hb =20 100 Hb =15 % Hb saturation 100 Hb =10 0 0 0 Saturation 20 O2 concentration (ml/100 ml) 0 0 13 PO2 (kP)

12. Oxygen binding of haemoglobin • Binding of one O2 to Hb increases the affinity of Hb for O2 • co-operativity • Sigmoid • Flattens where all sites are becoming occupied

13. % Haemoglobin Saturation O2 concentration ml/100 ml 13.3 5.3 8.0 Blood PO2 (kPa) Significance of Sigmoid • Flat upper portions means that moderate fall in alveolar PO2will not much affect oxygen loading • Steep lower part means that the peripheral tissues get a lot of oxygen for a small drop in capillary PO2

14. A shift of the curve to the right:- The Bohr Effect Increased release of O2by conditions at the tissues PCO2 [H+] Temperature 2,3-Biphosphoglycerate Bohr Effect 100 % Hb saturation 0 PO2

15. Additional O2 given up Off-loading of O2 at Tissues Curve in arterial conditions 20 Curve in tissue conditions 10 O2 content (ml/10mls) Arterial O2 Tension Tissue O2 Tension 100 13.3 60 8.0 80 10.6 0 0 20 2.6 40 5.3 PO2 (mm Hg, kP)

16. Means of CO2 Transport in the Blood • Solution (10%) • As Bicarbonate (60%) • As Carbamino compounds (30%)

17. (1) CO2 in Solution • Henry’s Law • Carbon dioxide about 20 times more soluble than oxygen • About 10% of carried CO2 is in solution

18. CA Carbonic Anhydrase Bicarbonate: Most CO2 is transported in the blood as bicarbonate Bicarbonate is formed in the blood by:- H2CO3 H+ + HCO-3 CO2 + H2O Occurs in red-blood cells

19. CA H+ + Hb HbH Bicarbonate Formation Chloride shift Capillary wall Cl- CO2 H++ HCO3- H2CO3 H2O + Red blood cell

20. (3) Carbamino Compounds • Carbamino compounds formed by combination of CO2 with terminal amine groups in blood proteins. • Especially globin of haemoglobin to give carbamino-haemoglobin • Rapid even without enzyme • Reduced Hb can bind more CO2 than HbO2

21. 5.3 13.3 - v a CO2 Dissociation Curve PO2 55 PO2 CO2 concentration (ml/100ml) a = CO2 content in arterial blood v- = CO2 content in mixed venous blood 45 5.3 6.6 CO2 partial pressure (kP)

22. The Haldane Effect Removing O2from Hb increases the ability of Hb to pick-up CO2 and CO2 generated H+ • The Boher effect and the haldane effect work in • synchrony to facilitate: • O2 liberation and uptake of CO2 & CO2 generated H+ at tissues

23. Summary of CO2 Transport in the Blood