Gas transport in airways

Question 1

Transport of O2 through airways into alveoli depends on what?

Answer 1

ventilation

Question 2

What are PIO2 and PAO2

Answer 2

PIO2: partial pressure of O2 in inspired air at the point at which it has just entered the airways. PAO2: partial pressure of O2 in alveoli. These limit the amount of O2 that can enter the blood

Question 3

Calculate the partial pressure of oxygen in inspired air PIO2.

Answer 3

PIO2 = (PB – 47 Torr) × 0.21. 47 Torr represents the partial pressure of water vapor, PB is barometric pressure(760Torr at sea level, or 630 at denver), and 0.21 is the percentage of O2 in the air.

Question 4

Why is alveolar oxygen pressure lower than inspired air oxygen pressure?

Answer 4

Inspired air only has O2 and N2, whereas alveolar air has O2, N2 AND CO2. The overall barometric pressure can not change, so the pressure of oxygen decreases in alveolus as the pressure of CO2 increases

Question 5

Calculat alveolar oxygen PAO2 when there is 1:1 exchange of CO2 for O2

Answer 5

PAO2 = PIO2 - PACO2. Where PIO2 is inspiratory oxygen pressure, PACO2 is alveolar CO2 pressure

Question 6

Define the respiratory exchange ratio and describe why values can vary.

Answer 6

ratio of amount of CO2 generated (VCO2) per amount of O2 consumed (VO2). R varies for different metabolites (carbs vs fats)

Question 7

Calculate alveolar PAO2 given known values of PACO2, barometric pressure, and respiratory exchange ratio.

Answer 7

PAO2= PIO2 - (PACO2/R) + k. Where R (respiratory exchange ratio)= VCO2/ VO2 and k is negligible

Question 8

Which alveolar gas equation is used when patient is breathing 100% O2?

Answer 8

The equation for R=1 (PAO2 = PIO2 - PACO2) b/c th only gas present to make up for CO2 deficit is oxygen

Question 9

What is rate-limiting for CO2 removal.

Answer 9

CO2 transport from alveoli to outside air depends on alveolar ventilation. If ventilation decreases, alveolar CO2 pressure goes up followed by arterial CO2 goes up.

Question 10

compare arterial to alveolar CO2 pressure

Answer 10

PACO2 (alveolar) = PaCO2 (arterial). Diffusion is very fast, so CO2 is at near perfect equilibrium in alveoli and pulmonary capillaries.

Question 11

Calculate arterial PaCO2 and alveolar PACO2 given known values of alveolar ventilation and CO2 production.

Answer 11

PACO2 = (VCO2/ VA) x k. Where VCO2 is volume of CO2 produced in one minute, VA is alveolar ventilation, k is constant. This also calculates PaCO2 which is nearly equivalent to PACO2.

Question 12

Define hypoventilation, hyperventilation

Answer 12

Hypoventilation: Alveolar ventilation is low in relation to CO2 production and elimination rate. PaCO2 increases and Va decreases. Hyperventilation: alveolar ventilation is high in relation to CO2 production and elimination rate. PaCO2 is low and Va is high.

Question 13

What is normal PaCO2 at sea level and at Denver

Answer 13

sea level: 35-45Torr. Denver: 30-40Torr.

Question 14

What is normal PaO2

Answer 14

90-100 Torr (sea level), 80-85 Torr (Denver)

Question 15

What is hyperpnea

Answer 15

Increased ventilation during moderate exercise

Question 16

What causes hyperventilation

Answer 16

acute hypoxemia (low arterial oxygen), metabolic acidosis, or CNS stimulation

Question 17

What causes hypoventilation

Answer 17

obstructive/restrictive disease, metabolic alkalosis, or CNS

depression.

Question 18

Blood CO2 is directly or indirectly regulated by alveolar ventilation?

Answer 18

Directly

Question 19

Blood O2 is directly or indirectly regulated by alveolar ventilation?

Answer 19

Indirectly - via effects on alveolar CO2