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Gas Exchange, O 2 and CO 2 Transport

Gas Exchange, O 2 and CO 2 Transport. Fiona Gilmour SHO 20/05/04. Gas Exchange. For adequate gas exchange Alveoli must be ventilated Capillaries must be perfused Gases normally exchanged are CO 2 and O 2 Normal O 2 consumption 250-300ml/min Normal CO 2 production 200-250ml/min

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Gas Exchange, O 2 and CO 2 Transport

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  1. Gas Exchange, O2 and CO2 Transport Fiona Gilmour SHO 20/05/04

  2. Gas Exchange • For adequate gas exchange • Alveoli must be ventilated • Capillaries must be perfused • Gases normally exchanged are CO2 and O2 • Normal O2 consumption 250-300ml/min • Normal CO2 production 200-250ml/min • Tidal volume is 500mls, 2/3 used for gas exchange • 1/3 physiological dead space

  3. Ratio of CO2 produced to O2 consumed is the respiratory exchange ratio. In steady state this equals the repiratory quotient RQ 0.82 • R = VCO2 / VO2 • Gas consumption or production can be calculated • VO2 = VE (FIO2 – FEO2) • VCO2 = VE x FECO2 • O2 extraction coefficient • VO2 / VI x FIO2 x 100 • At rest this is 15-20%

  4. Gas Exchange • Alveolar gas composition determined by • inspired gas composition • alveolar ventilation • Metabolism • PAO2 = PIO2 – PACO2 / R • Mean alveolar PO2 = 100mmHg • Mean alveolar PCO2 = 40mmHg • PAO2decreases if PIO2 decreases • PACO2 decreases in hyperventilation and increases in hypoventilation

  5. Fick’s Law of Diffusion • Gas movement across blood/gas barrier and blood/tissue barrier occurs by diffusion down partial pressure gradient • depends on • Molecular weight and solubility of the gas • surface area available • thickness of barrier (distance travelled)

  6. Diffusion • Lung is ideal for diffusion • Large surface area (50-100 sqm) • Very thin blood gas barrier (0.3microm) • Sufficient contact time occurs in alveoli to permit capillary blood to have same partial pressure as alveolar gas • CO2 diffuses 20X more rapidly than O2 • Higher solubility but similar molecular weight

  7. Oxygen Transport • 99% total O2 carried in RBC • 1% in physical solution • for each mmHg PO2 there is 0.03ml O2/L blood • normal arterial blood (100mmHg) contains 3mlO2/L • Amount of O2 per litre of blood depends on Haemoglobin concentration and degree of saturation which depends on PO2 • PO2 95mmHg Hb 97% saturated • PO2 40mmHg Hb 70% saturated

  8. Haemoglobin • Globular protein • 4 subunits • Haem group in polypeptide chain • 4 polypeptide chains = globulin • Each molecule of Hb can bind 4 molecules of O2 • Oxyhaemoglobin is red • Deoxyhaemoglobin is purple

  9. Haemoglobin • Attachment of 1 O2 facillitates uptake of subsequent O2 • Haem-haem interaction • Amount of O2 bound to Hb determined by PO2 • PCO2, pH, 2,3BPG all directly affect globin molecule and change affinity of haem for O2 and shifts curve left or right • Bohr effect

  10. CO2 Transport • Deoxygenated blood has high CO2 affinity • Oxygenation makes globin less able to combine with CO2 and H+ • Haldane effect • results in downwards and upwards shifts in CO2 dissociation curve • Blood contains 2.5 X more CO2 than O2

  11. CO2 Transport • Carried in 3 ways • Physical solution (5-6% total CO2, 10% in Venous) • Carbamino compounds (5-10%) • Bicarbonate (85-90%)

  12. CO2 Transport • H2O+CO2>H2CO3>H+ + HCO3- • slow in plasma but accelerated by carbonic anhydrase in RBC • RBC membrane impermeable to H+ so to maintain electroneutrality Chloride anion moves from plasma • Chloride shift • Deoxygenated Hb is less acid so will bind H+ and buffer cell

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