Physiology: ventilation-perfusion

Question 1

Describe the roles ventilation and perfusion play in gas exchange

Answer 1

Ventilation: air to reach the alveoli
Perfusion: capillaries adjacent to alveoli having adequate blood

Enables gases to flow from air <-> alveoli <-> capillaries

Question 2

Do ventilation and perfusion vary throughout the lung?
Why are both ventilation and perfusion highest at the lowest points of the lung?
Where in the lung is the ventilation:perfusion ratio the highest?

Answer 2

Yes
Postural effects (orthostatic pressures)
Apex of the lung

Question 3

Is the optimal ventilation:perfusion ratio 1:1?
At what point (hint: rib number) does this occur?

Answer 3

Yes
3rd rib

Question 4

Impaired ventilation
- Describe in terms of ventilation perfusion ratio
- Describe what happens in terms of gas exchange

Answer 4

VA/Q < 1
Blood transits the alveolus without loading sufficient O2 or releasing sufficient CO2 (right to left shunt)

Question 5

Impaired perfusion
- Describe in terms of ventilation perfusion ratio
- Describe what happens in terms of gas exchange

Answer 5

VA/Q > 1
Not enough blood flow to load sufficient O2, or release sufficient CO2

Question 6

What happens to oxygen and CO2 levels as a result of severe ventilation perfusion mismatching?

Answer 6

Hypoxemia, hypercapnia

Question 7

3 main homeostatic mechanisms that enable ventilation perfusion coupling?

Answer 7

CO2 levels impact bronchiole dilation
O2 levels impact pulmonary arteriole dilation
CO2 and O2 levels impact drive to breathe

Question 8

CO2 levels impact bronchiole dilation
- What happens at high CO2 levels?
- What happens at low CO2 levels?
- Describe the regulation of bronchiole dilation at local and central levels.

Answer 8

High CO2 levels –> bronchiolar dilation –> more CO2 released

Low CO2 levels –> bronchiolar constriction –> less CO2 released

Local level: at the site of low/high CO2 levels
Central: sympathetic nervous system causes both lungs to undergo bronchiolar dilation/constriction

Question 9

O2 levels impact pulmonary arteriole dilation
- What happens when ventilation < perfusion

Answer 9

Decreased O2 levels in alveoli –> pulmonary arteriolar vasoconstriction
Reduced perfusion now matches reduced ventilation (locally). Blood flow redirected to areas of adequate perfusion

Question 10

O2 levels impact pulmonary arteriole dilation
- What happens when ventilation > perfusion

Answer 10

Increased O2 in alveoli

Increased O2 levels –> pulmonary arteriolar vasodilation

Increased perfusion now matches increased ventilation.

Question 11

Increased/decreased drive to breathe due to changes to O2 and CO2
- What happens with low O2 (hypoxia) and high CO2 (hypercapnia)
- What happens with high O2 and low CO2

Answer 11

Increased drive to breathe
Decreased drive to breathe