Gas Transport in Airways

Question 1

P_IO2 =

Answer 1

• partial pressure of O₂in inspired air
• P_IO2 = (P_B - 47 torr) x .21
  
  • P_B = barometric pressure
  • 47 torr = water vapor pressure @ airways
  • .21 = dry air is ~ 21% O₂

Question 2

P_AO2 =

Answer 2

• = partial pressure of O₂ in alveoli
• P_AO2 = P_IO2 - P_ACO2
  
  • “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)

Question 3

Alveolar gas equation that accounts for respiratory exchange ration

Answer 3

• R = (V_CO2)/(V_O2) = “respiratory exchange ratio”
  
  • amount of CO₂ generated per O₂ consumed
  • usually R = ~0.8
• P_AO2 = P_IO2 - P_ACO2/R + k
  
  • k = additional (negligable)

Question 4

Characteristics of atmospheric air

Answer 4

• composed of multiple gases:
  
  • Nitrogen (~79%)
  • Oxygen (21%)
  • CO₂ (~0%)
• constant barometric pressure (P_B)
  
  • P_B = P_N+ P_CO2 + P_O2

Question 5

Main characteristics of CO₂transport

Answer 5

• Diffusion of CO₂ is extremely fast ==> CO₂in pulmonary arteries and alveoli is in equilibrium
  
  • P_ACO2= P_aCO2​
• Efficiency of CO₂ transport depends on transport between alveoli and outside air (not on diffusion step)

Question 6

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

Answer 6

• decrease in V_a ==> increased P_ACO2 (decreased rate of removal of CO₂) and CO₂ accumulation ==> increase in P_aCO2
• increase in V_a ==> decreased P_ACO2 & P_aCO2

Question 7

P_ACO2 as a function of V_a

Answer 7

• @ equilibrium, amount of CO2 produced must be equal to the amount eliminated from the body, thus:
• V_CO2 = V_A x C_ACO2​
  • V_CO2 = quantity of CO₂ produced per minute by the body
  • C_ACO2 = concentration of alveolar CO₂

Question 8

C_ACO2 =

Answer 8

C_ACO2 = V_CO2 / V_A

Question 9

P_ACO2 = (in terms of V_A)

Answer 9

• P_ACO2 = (V_CO2 / V_A) x k
• k = constant according to units/conditions
• also, P_aCO2 = P_ACO2, so this equation can help determine arterial CO₂ pressure as well

Question 10

Hypo/hyperventilation definition

Answer 10

• 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 P_aCO2/increase in V_a
• hypoventilation = increase in P_aCO2/decrease in V_a

Question 11

Common causes of hyper/hypoventilation

Answer 11

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