pk 4 - Alveolar Gas Equation & Diffusion

Question 1

Normally only a small fraction of air is replenished within the respiratory zone with each breath. How much is this?

Question 2

The alveolar partial pressure of O2 and CO2 are essentially stable at ….kPa and ….kPa respectively and these set the partial pressures found in ………….. …………..

Answer 2

13 and 5

arterial blood

Question 3

The alveolar partial pressures of CO2 is related to …………….. ………. and …………. related to alveolar ventilation. Consequently, if VA is varied in proportion to VCO2, PACO2 and hence PaCO2 can be maintained ……………

Answer 3

metabolic rate (VCO2)
inversely 
constant

Question 4

What are the venous gas tensions in the pulmonary capillaries?

Answer 4

CO2 = 6
O2 = 5

Question 5

The alveolar partial pressures of CO2 is related to …………….. ………. and …………. related to alveolar ventilation. Consequently, if VA is varied in proportion to VCO2, PACO2 and hence PaCO2 can be maintained ……………

Answer 5

metabolic rate (VCO2)
inversely 
constant

Question 6

What determines alveolar CO2?
What equation can be produced from this?
Which constant is used?

Answer 6

the relationship between metabolism and ventilation
alveolar CO2 = 115x (VCO2/ VA)
115kPa

Question 7

What shape graph defines the relationship between alveolar CO2 and alveolar ventilation at any particular metabolic rate?

Answer 7

metabolic hyperbola

Question 8

What is the definition of hyperventilation?

Answer 8

ventilating more than our metabolic needs

Question 9

The more you hyperventilation the more PCO2 will move towards …… , (the level of CO2 in inspired air)

Answer 9

0

Question 10

If metabolic rate is increased, how does the metabolic hyperbola change?

Answer 10

It shifts upwards and right, in order to maintain alveolar PCO2 at 5

Question 11

Arterial CO2 is a clinical measure of ………… …………. adequacy

Answer 11

alveolar ventilation

Question 12

What determines alveolar O2?

What has to be taken into account?

Answer 12

metabolism and ventilation

the fact that inspired O2 is not zero

Question 13

What equation do you get if you combine the equations for alveolar O2 and CO2?

Answer 13

the alveolar gas equation

PAO2 = PIO2 - (PACO2/R)

Question 14

When doing calculations with the alveolar gas equations, how do you calculate PiO2?

Answer 14

Needs to be of dry air at 37C

Need to remove water vapour pressure

Question 15

How can the alveolar gas equation be used to see if there is a respiratory impairment?

Answer 15

calculate alveolar oxygen, using arterial CO2
compare to arterial O2
if there is more than 1kPa difference then there is an impairment

Question 16

Gas movement within the ling depend crucially upon …….. within both liquids and gases and so is dependent upon …………….. …………, …………. and …………, Co2 is …..x more diffusive than O2, making ……….., rather than ……….., the first sign of diffusion limitation

Answer 16

diffusion
concentration gradients
molecular weight
solubility
20
hypoxia
hypercapnia

Question 17

What three layers make up the alveolar-capillary membrane?

Answer 17

alveolar epithelium, basal lamina, capillary endothelium

Question 18

In gas, grahams law states that bigger molecules diffuse ……… than smaller ones.
Info about CO2 and O2

Answer 18

slower

CO2 is bigger so diffuse 0.85 as fast as O2

Question 19

In liquids, henry’s law says that a more soluble gas maintains a …….. concentration difference and diffuses …….

Answer 19

higher
easier
CO2 is 23x more soluble than O2 and so diffuses 23x as fast

Question 20

What information do we get if we combine Henry’s Law and Graham’s Law?

Answer 20

0.85 x 23 = 20

CO2 diffuses 20 times more easily than O2

Question 21

-

Answer 21

solubility

chemical combination

Question 22

Which two gases are used to show the extremes of how gases move across a membrane?

Answer 22

N2O which is perfusion limited, low solubility and no chemical combination, the partial pressures with equilibrate very quickly, the only way to get more N2O into the body at 13kPa is to increase perfusion
CO which is diffusion limited, it is highly soluble and has a high chemical concentration. At an alveolar PCO of 13kPa, partial pressures of CO with not equilibrate and CO is therefore diffusion limited

Question 23

Is O2 perfusion or diffusion limited?

Answer 23

perfusion limited

Question 24

What does a high diffusion reserve for O2 allow us to do? How?

Answer 24

2/3 of transit time in which transit has occurred. This means we can increase the speed of blood through the pulmonary capillaries and decrease the transit time and still reach equilibration. This means we can get more oxygen into the blood and can therefore exercise

Question 25

What can decrease diffusion reserve in a healthy person? Why?

Answer 25

Their oxygen is becoming more diffusion limited - failure to achieve equilibration of PO2. become hypoxic during exercise –> breathlessness. E.g. in pulmonary oedema, thickened membrane, altitude

Question 26

What factors affect pulmonary diffusing capacity?

Answer 26

diffusibility of gas
area
thickness
pressure gradient

Question 27

What can be measured using low conc CO? Why? Whats the equation?

Answer 27

diffusing capacity of the lung
Dl = V/(P1-P2).
CO combines with Hb so P2 is effectively zero.
DL = V/PACO

Question 28

What 3 things increase diffusing capacity?

Answer 28

body size, exercise (efficiency), lying down (blood distribution)

Question 29

Pulmonary Diffusing Capacity is reduced by:
Reducing effective surface area e.g. ………
Increasing diffusion path length e.g. ……….

Answer 29

loss of lung tissue, airway obstruction, capillary obstruction, V/Q mismatch
thickened alveolar-capillary membrane, accumulation of fluid, increasing intracapillary distance (anaemia)

Question 30

How does a patient with diffusion limitation typically present?

Answer 30

hypoxiac with cyanosis that is aggravated by exercise. increased alveolar ventilation and decreased PaCO2
Normal ventilatory capacity