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Flashcards in Gas Transport in Airways Deck (11)
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1

PIO2 = 

  • partial pressure of Oin inspired air
  • PIO2 = (PB - 47 torr) x .21
    • PB = barometric pressure
    • 47 torr = water vapor pressure @ airways
    • .21 = dry air is ~ 21% O2

2

PAO2 =

  • = partial pressure of O2 in alveoli
  • PAO2 = PIO2 - PACO2
    • "alveolar gas equation"
    • CO2 is present in alveoli b/c it diffuses from blood --> must replace some component of inspired air
    • CO2 is exchanged for O2
    • this version of the equation assumes a 1:1 exchange of O2 and CO2 @ tissues (not always true)

3

Alveolar gas equation that accounts for respiratory exchange ration

  • R = (VCO2)/(VO2) = "respiratory exchange ratio"
    • amount of CO2 generated per O2 consumed
    • usually R = ~0.8
  • PAO2 = PIO2 - PACO2/R + k
    • k = additional (negligable)  

4

Characteristics of atmospheric air

  • composed of multiple gases: 
    • Nitrogen (~79%)
    • Oxygen (21%)
    • CO2 (~0%)
  • constant barometric pressure (PB)
    • PB = P+ PCO2 + PO2

5

Main characteristics of COtransport

  • Diffusion of CO2 is extremely fast ==> COin pulmonary arteries and alveoli is in equilibrium 
    • PACO2 = PaCO2
  • Efficiency of CO2 transport depends on transport between alveoli and outside air (not on diffusion step)

6

Impact of changes in alveolar ventilation (Va) on CO2 transort

  • decrease in Va ==> increased PACO2 (decreased rate of removal of CO2) and CO2 accumulation ==> increase in PaCO2 
  • increase in Va ==> decreased PACO2 & PaCO2

7

PACO2 as a function of Va


  • @ equilibrium, amount of CO2 produced must be equal to the amount eliminated from the body, thus:
     

  • VCO2 = VA x CACO2
    • VCO2 = quantity of CO2 produced per minute by the body
    • CACO2 = concentration of alveolar CO2

8

CACO2 = 

CACO2 = VCO2 / VA 

9

PACO2 = (in terms of VA)

  • PACO2 = (VCO2 / VA) x k
  • k = constant according to units/conditions
  • also, PaCO2 = PACO2, so this equation can help determine arterial CO2 pressure as well

10

Hypo/hyperventilation definition

  • alveolar ventilations abnormally low or high in relation to CO2 production
    • NOT in reference to frequency of breathing
    • rapid breathing rate ==> tachypnea
  • hyperventilation = decrease in PaCO2/increase in Va
  • hypoventilation = increase in PaCO2/decrease in Va

11

Common causes of hyper/hypoventilation

  • hyperventilation:
    • acute hypoxemia (low arterial O2)
    • metabolic acidosis
    • CNS stimulation
  • hypoventilation:
    • obstructive/restrictive disease
    • metabolic alkalosis
    • CNS depression