EXCHANGE OF RESPIRATORY GASES

1. EXCHANGE OFRESPIRATORY GASES DrKalpana.B

2. Blood-gas exchange depends upon – Diffusion capacity of the gas Solubility of the gas Affinity of the gas with Hemoglobin

3. RESPIRATORY UNIT

4. Diffusion capacity of Gas(Dc) • Amount of a gas that crosses the alveolar • Capillary membrane/min/mm Hg difference in partial pressure of gas on the two sides of the membrane • Normal Dc for O2(DO2)=20-30ml/mi/mm Hg at rest.

5. RESPIRATORY MEMBRANE

6. 1.A layer of fluid lining the alveolus and containing the surfactant 2. The alveolar epithelium composed of thin epithelial cells 3. An epithelial basement membrane 4. A thin layer of interstitial space 5. A capillary basement membrane 6. The capillary endothelial membrane

7. Factors affecting diffusion capacity • Pressure gradient • Solubility of gas in fluid medium • Molecular weight of the gas • Thickness of the respiratory membrane • Total surface area of the respiratory membrane

9. DC is directly proportional to solubility of gas • solubility of CO2>O2>CO • DO2=1.23*DCO • DCo2=24.6*Dco • DCO2= 20.7*DO2

10. Oxygen transport

12. Oxygen transport • 2 forms • Physical form –Dissolved form in plasma as simple solution • Chemical form –Combination with Hemoglobin

13. Physical form (or) Dissolved form in plasma • Account for about 1 to 2 % of O2 transport • Solubility coefficient of O2 = 0.003 ml/dl/mm Hg • O2 content in arterial blood (PO2104mm Hg) = 0.3ml/dl (15 ml in whole blood of 5Liter) • O2 content in venous blood (PO2 40 mm Hg) =0.12 ml /dl (6 ml in whole blood)

14. Chemical form (oxy-hemoglobin) • Normal Hb level = 15gm/dl • With full saturation (100%) O2 carrying capacity of Hb = 1.34 ml/ gm of Hb • O2 carrying capacity of 100ml blood = • 1.34 x 15 = 20.1ml/dl (with 100% Hb sat.) • Arterial blood (PO2 of 97mm Hg) with 97.5% Hb saturation (due physiological shunt) = 19.8 ml/dl

15. Hemoglobin (Hb) • Molecular weight = 68,000 • Consists of 4 Heme moieties (with a ferrous ion in each) & conjugate with 4 polypeptide chains • Each ferrous ion combine with a molecule of Oxygen • Each Hb molecule carry 4 molecule of O2 (Hb4O8)

16. Diagrammatic representation of a molecule of hemoglobin A, showing the four subunits

17. Combination of O2 with Hb is oxygenation (not oxidation) • This oxygenation & de oxygenation are so rapid require <0.01sec

18. The iron stays in the ferrous state, the reaction is oxygenation. • Hb4 + O2 Hb4O2 • Hb4O2 + O2 Hb4O4 • Hb4O4 + O2 Hb4O6 • Hb4O6 + O2 Hb4O8 • The reaction is rapid, requiring less than 0.01s.

19. Combination of O2 with Hb depend on PO2. • Oxygen can be readily released from hemoglobin when it is needed

20. Hemoglobin accepts oxygen readily whenever the partial pressure of oxygen is more • Hemoglobin gives out oxygen whenever the partial pressure of oxygen in the blood is less

21. The curve obtained by plotting the Hb saturations against their various PO2 is known as– Oxygen dissociation curve Or Oxy-Hb dissociation curve Or Hb dissociation curve

23. O2 dissociation curve • Sigmoid in shape • Having three phases– • Phase 1 -- Slow ascend (between 0 to 10 mm Hg of PO2.) • Phase 2 --Steep ascend (between 10 to 50 mm Hg) & • Phase 3 -- Plateau (between 70 to 100 mm Hg of PO2.)

25. 3 important points in O2 dissociation curve are -- • Arterial point -- PO2 of 100 mm Hg & Hb saturation of 97,5% • Venous point --PO2 of 40 mm Hg & Hb saturation of 75% • P50 --PO2 of 28 mm Hg & Hb saturation of 50% • P50 – is the partial pressure at which 50% Hb saturation occur • When P50 – increases O2 affinity decreases

26. Effect of configuration change of Hb • The configuration of Hb changes due to oxygenation & deoxygenation • In deoxygenated state –Hb is in tight form (& expel O2) –Hence called “T” Hb • In oxygenated state – Hb is relaxed form (accommodate more O2) –called “R” Hb

27. However all the 4 atoms of the Hb do not combine with oxygen simultaneously. • Stepwise process • Shifting affinity

28. Advantages of sigmoid shape curve • Plateau phase – Helps to tide over atmospheric pressure variations in different environmental conditions • Steep phase –Helps to deliver more O2 in case of body’s requirement

29. Saturation of hemoglobin with oxygen depends on the partial pressure of oxygen • When the partial pressure of oxygen is more, hemoglobin accepts oxygen • When the partial pressure of oxygen is less hemoglobin releases oxygen

30. The oxyhemoglobin dissociation curve is shifted either left or right by various factors • Shift of the curve to left indicates acceptance of oxygen by hemoglobin • Shift of the curve to right indicates dissociation of oxygen from hemoglobin

31. Factors shifting O2 dissociation curve to right In the blood level -- • Increase in temperature • Increase in H+ ion concentration (& pH) • Increase in partial pressure of CO2 • Increase in 2,3-DPG (2,3-BPG) level • Decrease in partial pressure of oxygen

32. Factors shifting O2 dissociation curve to left 1. In the fetal blood – fetal hemoglobin has got more affinity for oxygen than adult hemoglobin 2. Decrease in hydrogen ion concentration and increase in pH (alkalinity)

33. Bohr’s effect • Decrease O2 affinity (shifting of O2-dissociation curve to right) due to decrease in pH of blood is called Bohr’s effect • Normally decrease in pH of blood occur due to increase in CO2 content of blood – Hence, increase in blood CO2 shifts the O2 dissociation curve to right is called Bohr’s effect.

34. In the tissues, due to the continuous metabolic activities • The partial pressure of carbon dioxide is very high and the partial pressure of oxygen is low • Due to the pressure gradient carbon dioxide enters the blood and oxygen is released from the blood to the tissues • The presence of carbon dioxide decreases the affinity of hemoglobin for oxygen and enhances further release of oxygen to the tissues • oxygen dissociation curve is shifted to the right

36. Significance of Bohr’s effect • Takes place at tissuelevel • Helps in unloading & O2 delivery to tissue • Most of Hb unsaturation (O2 delivery) to tissue occur due to decrease in PO2. • Extra 1 to 2% of unsaturation of Hb (and resulting O2 delivery) is due to increase in PCO2

37. Factors affecting the affinity of hemoglobin for oxygen

41. TISSUE PO2 = 40mm Hg DIFFUSION OF OXYGEN FROM BLOOD INTO THE TISSUES O2 Venous end Arterial end Capillary PO2 = 40mm Hg O2 Content = 14ml% PO2 = 95 mm Hg O2 Content = 19ml%

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