4. Ventilation-Perfusion Relationship Flashcards
What is the partial pressure of water vapor?
47 mm Hg
How does one calculate the inspired partial pressure of oxygen, PI(O2)?
(760 - 47) x (% of oxygen inhaled)
21% oxygen is normal
What is the respiratory quotient (R) with a mix of fuels? With glucose only?
.8 with a mix of fuels
1 with glucose only
How does one calculate the alveolar partial pressure of oxygen, PA(O2)?
PA(O2) = PI(O2) - Pa(CO2)/R
Also known as the alveolar gas equation.
What is the normal fraction of inspired oxygen (FiO2)?
21%
How does one calculate the A-a O2 gradient?
PA(O2) - Pa(O2)
What is a normal A-a O2 gradient?
< 20 mm Hg
How does the intrapleural pressure differ between the apex and base of the lung?
At the apex, there is less intrapleural fluid, so the pressure is more negative.
At the base, there is more fluid, so the pressure is less negative.
How does alveolar size differ between the apex and base of the lung?
At the apex, the alveoli are larger at rest due to more negative pressure.
At the base, the alveoli are smaller at rest due to less negative pressure.
What is the average ventilation/perfusion ratio?
V/Q
4 L/min) / (5 L/min
= .8
What kind of shunt does a very low V/Q ratio lead to?
Right to left shunt.
The blood leaves the lungs with the same O2/CO2 concentration, as if it never went to the lungs.
What happens to the lungs when the V/Q ratio is high?
No blood or little blood is coming to alveolus and the alveolar air becomes similar to the air outside the body. To compensate this decrease in blood flow, blood flow to other alveoli increases. This lowers the V/Q ratio in normal alveoli.
Overall Pa(O2) and Pa(CO2) decreases.
How does the V/Q ratio differ between the apex and base of the lung?
At apex, V/Q ratio is higher
At base, V/Q ratio is lower
