3. Gas Exchange and Gas Transport Flashcards
what happens to PA02 and PACO2 during inspiration?
as inspired air mixes with alveolar air, PA02 will rise and PAC02 will fall
what happens to PA02 and PAC02 between inspirations?
PA02 will call and PACO2 will rise as gas is diffused to pulmonary cap blood
is PA02 and Pa02 the same at the lung alveoli region? PACO2 and PaCO2?
yes given complete equilibration
mixed arterial P02 is higher or lower than mean alveolar P02?
lower – due to shunting and VQ mismatch
what does alveolar hypoventilation cause?
hypoxemia: low arterial P02. alveolar and arterial P02 are both lower than normal; alveolar and arterial PC02 are both higher than normal.
hypoxemia with hypercapnia suggests that the problem is at least partly…?
hypoventilation
how might you achieve low hypoxemia with low PaC02?
high altitude: inspired PI02 is lower than sea level, but hypoxemia stimulates breathing which lowers PaC02.
what mechanisms cause reduced pulmonary diffusion?
thickening of the alveolar-capillary membrane, or reduced contact time between blood and alveolus
how does shunting cause hypoxemia?
mixed venous blood reaches arterial system without undergoing gas exchange.
will increasing the PI02 increase the hypoxemia in the setting of a shunt?
no, because the blood causing the hypoxemia is bypassing the lungs entirely
in situations of regional VQ mismatch, is hypoxia or hypercapnia usually more pronounced?
hypoxia: because hypercapnia can be fixed by hyperventilation, and also becasue of the slope of the C02 dissociation curve.
what is one assumption we make concerning PaCO2?
we assume it’s the same as PACO2 (ie that diffusion happens pretty completely)
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?
PAi02 = PI02-(1.2*PaC02)
PaCO2 is from blood gas
How can we calculate the alveolar-arterial PO2 difference (AaDO2??)
AaD02 = PAi02 (from above) - Pa02 (from blood gas)
if arterial P02 is normal, how can tissues experience hypoxia?
if 02 transport to tissues is inadequate
what is a good measure of the overall effectiveness of 02 transport by resp and circ systems?
oxygen level of mixed venous blood
how will a low P02 in mixed venous blood further depress arterial P02 levels?
by mixing with blood from shunted and VQ mismatch regions, basically adding low oxygenated blood to the arterial blood
what is a normal value for PI02 (assuming sea level)
150
what is a normal value for PA02?
100
mixed venous P02 usually starts at what value when it enters the gas exchange site?
40
how long will it normally take for CO2 to equilibrate in the gas exchange site?
0.25 sec
what is a problem with gas exchange, common to exercise and also thickening of the exchange zones?
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
why doesn’t PaC02 change as dramatically as Pa02 in a situation of shunting?
because the PCO2 of mixed venous blood is only slightly higher than that of arterial blood - when it intermixes, won’t affect value much
what does the AaD02 calculation tell us?
how well the lung is functioning as a gas exchanger
what is a normal A-a difference?
10ish
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?
02 content diff: 5 (from 20 to 15)
P02 difference: 60 (from 100 to 40)
