Alveolar -Blood Gas Exchange

Question 1

What is the formula for the partial pressure of gas in ambient air?

Answer 1

Pgas = Fgas x Patm

Patm: atmospheric pressure
Pgas: partial pressure of a gas
Fgas: concentration of a gas

Question 2

What is the formula for Partial pressure of gas in inspired air?

Answer 2

PIgas = Fgas (Patm - PH2O)

PIgas: partial pressure of inspired gas
PH2O: partial pressure of H2O vapor

Question 3

What is normal A-a gradient?

Answer 3

5-15 mmHg

Question 4

The A-a O2 gradient increases with age. What is the formula used to equate for age?

Answer 4

(age+4)/4

Question 5

What are 2 factors affecting alveolar PCO2?

Answer 5

metabolic rate and alveolar ventilation

PACO2 ∝ metabolic CO2 production/alveolar ventilation

Question 6

What is RQ?

Answer 6

respiratory quotient

is the ratio between CO2 production and O2 consumption at the cellular level

At steady state, RQ and RER are equal

Question 7

What is RER?

Answer 7

Respiratory exchange ratio is ratio of CO2 output and oxygen uptake occurring in the lung

CO2 produced (ml/min)/ O2 consumed mL/min
is normally 0.8

at steady state, RQ and RER are equal

Question 8

What is the alveolar air equation?

Answer 8

PAO2 = (Patm - 47)FiO2 - PACO2/RQ

Patm = atmospheric pressure, at sea level 760 mmHg
FiO2 = fractional concentration of O2, room air 0.21
PaCO2 = alveolar pressure of carbon dioxide, normally 40 mmHg

Question 9

What are 3 factors that can affect PAO2?

Answer 9

Patm = atmospheric pressure at sea level =760 mmHg
FiO2 = fractional concentration of O2; room air 0.21
PaCO2 = alveolar pressure of CO2, normally 40 mmHg

Question 10

What is FiO2?

Answer 10

fractional concentration of O2, room air 0.21

Question 11

What are structural features that affect the rate of diffusion?

Answer 11

A = surface area for exchange (i.e dec in emphysema, inc. in exercise)

T = thickness of the membranes between alveolar gas and capillary (i.e inc fibrosis and many other restrictive diseases)

Question 12

What is body T affect on PaCO2?

Answer 12

as body T decreases PACO2 decreases

as body T increases PACO2 increases

Question 13

What does a perfusion limited substance indicate?

Answer 13

if substance equilibrates between the capillary and interstitium, it is said to be in a perfusion-limited situation

Question 14

What is a diffusion limited substance?

Answer 14

if the substance does not equilibrate between the capillary and the interstitium it is said to be diffusion limited

Question 15

Is CO a diffusion or perfusion limited gas?

Answer 15

diffusion limited

Question 16

What is Ficks Law of diffusion?

Answer 16

Vgas = A/T x D x (P1-P2)

Vgas = alveolar blood gas transfer
A = surface area
T = thickness of the membranes between alveolar gas and capillary blood
D = diffusion constant is a measure of solubility mostly
P1-P2 gas parital pressure difference across alveolar membrane; Greater pressure difference the greater rate of diffusion

Question 17

What is the relationship of VCO to area and thickness ?

Answer 17

VCO ∝ A/T

Question 18

What is the formula for VCO?

Answer 18

A/T x D x PaCO