3. Gas Exchange and Gas Transport

Question 1

what happens to PA02 and PACO2 during inspiration?

Answer 1

as inspired air mixes with alveolar air, PA02 will rise and PAC02 will fall

Question 2

what happens to PA02 and PAC02 between inspirations?

Answer 2

PA02 will call and PACO2 will rise as gas is diffused to pulmonary cap blood

Question 3

is PA02 and Pa02 the same at the lung alveoli region? PACO2 and PaCO2?

Answer 3

yes given complete equilibration

Question 4

mixed arterial P02 is higher or lower than mean alveolar P02?

Answer 4

lower – due to shunting and VQ mismatch

Question 5

what does alveolar hypoventilation cause?

Answer 5

hypoxemia: low arterial P02. alveolar and arterial P02 are both lower than normal; alveolar and arterial PC02 are both higher than normal.

Question 6

hypoxemia with hypercapnia suggests that the problem is at least partly…?

Answer 6

hypoventilation

Question 7

how might you achieve low hypoxemia with low PaC02?

Answer 7

high altitude: inspired PI02 is lower than sea level, but hypoxemia stimulates breathing which lowers PaC02.

Question 8

what mechanisms cause reduced pulmonary diffusion?

Answer 8

thickening of the alveolar-capillary membrane, or reduced contact time between blood and alveolus

Question 9

how does shunting cause hypoxemia?

Answer 9

mixed venous blood reaches arterial system without undergoing gas exchange.

Question 10

will increasing the PI02 increase the hypoxemia in the setting of a shunt?

Answer 10

no, because the blood causing the hypoxemia is bypassing the lungs entirely

Question 11

in situations of regional VQ mismatch, is hypoxia or hypercapnia usually more pronounced?

Answer 11

hypoxia: because hypercapnia can be fixed by hyperventilation, and also becasue of the slope of the C02 dissociation curve.

Question 12

what is one assumption we make concerning PaCO2?

Answer 12

we assume it’s the same as PACO2 (ie that diffusion happens pretty completely)

Question 13

We are going to need the PAi02 (ideal amt of 02 in alveoli given inspired P02) in order to calculate the Aa difference. what is the formula for PAi02?

Answer 13

PAi02 = PI02-(1.2*PaC02)

PaCO2 is from blood gas

Question 14

How can we calculate the alveolar-arterial PO2 difference (AaDO2??)

Answer 14

AaD02 = PAi02 (from above) - Pa02 (from blood gas)

Question 15

if arterial P02 is normal, how can tissues experience hypoxia?

Answer 15

if 02 transport to tissues is inadequate

Question 16

what is a good measure of the overall effectiveness of 02 transport by resp and circ systems?

Answer 16

oxygen level of mixed venous blood

Question 17

how will a low P02 in mixed venous blood further depress arterial P02 levels?

Answer 17

by mixing with blood from shunted and VQ mismatch regions, basically adding low oxygenated blood to the arterial blood

Question 18

what is a normal value for PI02 (assuming sea level)

Answer 18

150

Question 19

what is a normal value for PA02?

Answer 19

100

Question 20

mixed venous P02 usually starts at what value when it enters the gas exchange site?

Answer 20

40

Question 21

how long will it normally take for CO2 to equilibrate in the gas exchange site?

Answer 21

0.25 sec

Question 22

what is a problem with gas exchange, common to exercise and also thickening of the exchange zones?

Answer 22

with exercise, less time to equilibrate. not a problem unless area is also thickened, in which case diffusion may take up to 0.75 sec to equilibrate. exercise raises the flow rate which is ok normally, but problematic with fibrosis

Question 23

why doesn’t PaC02 change as dramatically as Pa02 in a situation of shunting?

Answer 23

because the PCO2 of mixed venous blood is only slightly higher than that of arterial blood - when it intermixes, won’t affect value much

Question 24

what does the AaD02 calculation tell us?

Answer 24

how well the lung is functioning as a gas exchanger

Question 25

what is a normal A-a difference?

Answer 25

10ish

Question 26

think through the normal 02 dissociation curve. what is the normal 02 content difference between arterial and venous blood? P02 difference between arterial and venous blood?

Answer 26

02 content diff: 5 (from 20 to 15)

P02 difference: 60 (from 100 to 40)