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The carriage of respiratory gases Slide 0

The carriage of respiratory gases Slide 0. The carriage of respiratory gases. Click mouse button or use and keys esc to end. The carriage of respiratory gases Slide 1 Basics 1. The composition of air. (%) N 2 O 2 CO 2 Atmosphere 79 21 0.03

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The carriage of respiratory gases Slide 0

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  1. The carriage of respiratory gases Slide 0 The carriage of respiratory gases Click mouse button or use and keys esc to end

  2. The carriage of respiratory gases Slide 1 Basics 1. The composition of air (%)N2 O2 CO2 Atmosphere 79 21 0.03 Alveoli 79 14 6 Exhaled air is therefore a mixture of these values, typically 79 16 4 but varies according to physiological state.

  3. The carriage of respiratory gases Slide 2 Basics 2. Atmospheric and partial pressures Partial Pressure is the pressure exerted by one gas in a mixture of gases. Atmospheric pressure (at sea level) is about 100 kPa. One fifth of the air is oxygen. So one fifth of atmospheric pressure is due to oxygen. Therefore the PARTIAL PRESSURE OF OXYGEN is about 20 kPa. (Partial pressure of oxygen can be written as ppO2)

  4. The carriage of respiratory gases Slide 3 Basics 3. Partial pressures of oxygen Some key values for ppO2 are: Atmospheric 20 kPa Alveolar 13 kPa And a typical value for blood flowing through tissues of the body: 5kPa

  5. The carriage of respiratory gases Slide 4 4. Oxygen transport Oxygen is picked up by the blood in the lungs and transported in the circulation. As the blood passes through the capillary beds of the tissues, a proportion of the oxygen is lost from the blood to the tissues. Oxygen is hardly soluble in plasma; it is carried by HAEMOGLOBIN in the red blood cells. Thus haemoglobin loads oxygen in the lung capillaries and unloads it in the tissue capillaries.

  6. loading unloading The carriage of respiratory gases Slide 5 5. Oxygen loading So to function efficiently, haemoglobin must attract oxygen under certain conditions, but lose it under other conditions. Loading/unloading is a reversible reaction, in which one haemoglobin (Hb) can attract up to 4 oxygen molecules: Hb + 4O2 HbO8

  7. The carriage of respiratory gases Slide 6 6. A reminder of the structure of a haemoglobin molecule • Haemoglobin is a conjugated quaternary protein comprising: • two alpha-globulins • two beta-globulins • four haem groups, each with an iron atom at its core • It is the iron atoms which bind oxygen.

  8. The carriage of respiratory gases Slide 7 7. Haemoglobin structure

  9. The carriage of respiratory gases Slide 8 8. Loading and saturation Hb loads/unloads one O2 at a time, so can exist as Hb, HbO2, HbO4, HbO6, or HbO8. There are billions of Hb molecules in the blood. The term %saturation refers to the overall degree of loading of Hb with oxygen.

  10. loading unloading The carriage of respiratory gases Slide 9 9. So what factors affect the loading/unloading equilibrium? Hb + 4O2 HbO8 • oxygen levels, i.e. ppO2 • carbon dioxide levels, i.e. ppCO2 • blood pH • temperature

  11. The carriage of respiratory gases Slide 10 10. Loading loading Hb + 4O2HbO8 unloading relatively high (13 kPa) relatively low relatively high relatively low WHY? • Loading occurs at the alveolar exchange surface, where: • ppO2 is • ppCO2 is • blood pH is therefore • and temperature is

  12. The carriage of respiratory gases Slide 11 11. Unloading loading Hb + 4O2 HbO8 unloading relatively low (5 kPa) relatively high relatively lower relatively higher WHY? • Unloading occurs in the tissue capillaries, where: • ppO2 is • ppCO2 is • blood pH is therefore • and temperature is

  13. loading unloading The carriage of respiratory gases Slide 12 12. Saturation and oxygen dissociation Hb + 4O2 HbO8 In fact, blood passing through the tissues never totally unloads all its oxygen. Here, blood oxygen saturation may fall to around 50%, in contrast to 100% saturation in the lung capillaries. OXYGEN DISSOCIATION CURVES are graphs which show the relationship between the ppO2 and the degree of saturation.

  14. The carriage of respiratory gases Slide 13 13. Oxygen dissociation curve 100 80 60 What happens in the lungs and in the tissues? Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  15. The carriage of respiratory gases Slide 14 14. Oxygen dissociation curve explained 100 In the upper part of the graph Hb is loaded with oxygen. 80 lungs 60 Saturation of blood with O2 (%) 40 In the lower part Hb is unloading its oxygen. 20 tissues 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  16. The carriage of respiratory gases Slide 14a 14a. Effect of changes in ppO2 100 What happens to %saturation when there is a small change in ppO2 a) in the lungs? b) in the tissues? See how actively respiring tissues promote more O2 unloading. 80 60 Saturation of blood with O2 (%) 40 20 tissues lungs 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  17. The carriage of respiratory gases Slide 15 15. Oxygen dissociation curve 100 • How do actively respiring tissues affect other factors: • ppCO2 • pH • temp? • And how do these affect loading / unloading? 80 60 Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  18. The carriage of respiratory gases Slide 16 16. REMEMBER: • Unloading occurs in the tissue capillaries, where: • ppO2 is relatively low (5 kPa) • ppCO2 is relatively high • blood pH is therefore relatively lower • and temperature is relatively higher

  19. The carriage of respiratory gases Slide 17 17. Shift to the right - the Bohr effect of higher ppCO2 100 Increased ppCO2, lower pH and increased temp all have the effect of pushing the curve to the right. See how this decreases Hb’s affinity for O2, so more is unloaded. 80 higher CO2 normal CO2 60 Saturation of blood with O2 (%) 40 Higher affinity lower affinity 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  20. The carriage of respiratory gases Slide 18 18. Shift to the left 100 What are the consequences of a shift to the left? Is Hb’s affinity for oxygen more or less? Under what circumstances might this graph apply? 80 60 Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  21. The carriage of respiratory gases Slide 19 19. Questions 100 80 If the black curve is ‘normal’, which curve represents the effect of elevated levels of carbon dioxide? 60 Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  22. The carriage of respiratory gases Slide 20 20. Questions 100 80 If the black curve is human haemoglobin, which curve represents bird Hb and which lugworm Hb? 60 Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  23. The carriage of respiratory gases Slide 21 21. Questions 100 80 If the black curve is human maternal haemoglobin, which curve represents foetal Hb? 60 Saturation of blood with O2 (%) 40 20 0 2 4 6 8 10 12 14 16 partial pressure of oxygen (kPa)

  24. The carriage of respiratory gases Slide 22 22. The carriage of CO2 CO2 is rather more soluble than O2. It is transported from tissues to lungs in three ways: • dissolved directly in plasma (about 5%) • combined with the polypeptides of haemoglobin (forming carbamino-haemoglobin) (about 10%) • as HCO3- in the plasma, following a series of reactions within the red blood cells These reactions will explain why there is a direct relationship between the level of CO2 and the degree unloading of O2 by haemoglobin:

  25. The carriage of respiratory gases Slide 23 23. Reactions inside the RBC - CO2 reacts with water carbonic anhydrase + H2O H2CO3 CO2 CO2 CO2 from respiring tissues enters the red blood cell and combines with water, forming carbonic acid. The reaction is accelerated by CARBONIC ANHYDRASE.

  26. The carriage of respiratory gases Slide 24 24. Reactions inside the RBC - dissociation of carbonic acid carbonic anhydrase CO2 CO2 + H2O H2CO3 HCO3- H+ Cl- Carbonic acid dissociates and HCO3- is transported out of the RBC, in exchange for Cl-. (This is the CHLORIDE SHIFT)

  27. The carriage of respiratory gases Slide 25 25. Reactions inside the RBC - unloading of oxygen carbonic anhydrase CO2 CO2 + H2O H2CO3 HCO3- H+ Cl- HHb HbO2 O2 H+ displaces O2 from haemoglobin, forming HHb - reduced haemoglobin. The O2 is liberated to the tissues.

  28. The carriage of respiratory gases Slide 26 26. Reactions inside the RBC - explanation of the Bohr effect carbonic anhydrase CO2 CO2 + H2O H2CO3 HCO3- H+ Cl- HHb HbO2 O2 The more CO2 , the greater the displacement of O2 from Hb. CO2 reduces the affinity of Hb for O2 . This explains the Bohr effect.

  29. The carriage of respiratory gases Slide 27 27. Reactions inside the RBC - reversal in the lungs. carbonic anhydrase CO2 CO2 + H2O H2CO3 HCO3- H+ Cl- HHb HbO2 O2 All these reactions are reversible. In the lung capillaries ppO2 is higher and ppCO2 is lower and so this happens:

  30. The carriage of respiratory gases Slide 28 28. Reactions inside the RBC - release of CO2. carbonic anhydrase H2CO3 CO2 CO2 + H2O HCO3- H+ Cl- HHb HbO2 O2 In the lung capillaries ppO2 is higher and ppCO2 is lower, so now O2 binds to Hb and this results in the release of CO2 Don’t worry - you don’t have to learn these reactions!

  31. The carriage of respiratory gases Slide 29 29. Summary • Most oxygen is transported bound to haemoglobin, as oxyhaemoglobin • The oxygen saturation of haemoglobin is affected by: • ppO2 • ppCO2 • pH • temperature • CO2 is transported in three ways: • dissolved in plasma • bound to haemoglobin as carbamino-haemoglobin • converted to hydrogencarbonate ions in the red cells • High levels of CO2 facilitate O2 unloading from haemoglobin through the formation of hydrogen ions. The effect of increased CO2 / decreased pH on O2 unloading is called the Bohr effect. • High levels of O2 facilitate CO2 unloading from the blood. • • • end • • •

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