Gas Exchange, O2 and CO2 Transport

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Gas Exchange, O2 and CO2 Transport

• Fiona Gilmour
• SHO
• 20/05/04

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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

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• 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

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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
• PAO2 decreases if PIO2 decreases
• PACO2 decreases in hyperventilation and increases
  in hypoventilation

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Ficks 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)

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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

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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

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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

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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

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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

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CO2 Transport

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

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CO2 Transport

• H2OCO2gtH2CO3gtH 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