Ventilation Perfusion Ratio

Question 1

What are the main differences between the systemic and pulmonary vasculature?

Answer 1

1) In the pulmonary vasculature both arteries and veins are compliant, while in the systemic circulation only the veins are compliant

2) They systemic vasculature resistance is extensively controlled by the neural system while the pulmonary has limited neural control on the vascular resistance (mainly through local control mechanisms)

3) The systemic vasculature is dilated by hypoxia, while the pulmonary vasculature is constricted by hypoxia

4) Unlike the systemic circulation, the pulmonary circulation is a low-pressure and low-resistance system

• Blood flow (Q) is the same in both (Q = change in P/R)

Question 2

Describe the relationship between the MAP and Resistance of the pulmonary circulation and systemic circulation

Answer 2

• CO is equal and blood flow
• MAP of the pulmonary circulation is 1/10 of that of the systemic circulation and so is the resistance, even the pressure

Question 3

What happens to the pulmonary resistance if the CO to the pulmonary artery increases?

Answer 3

The pulmonary pressure will increase and so the pulmonary resistance will decrease

Question 4

What are the mechanisms that DECREASES the pulmonary vascular resistance?

Answer 4

1) Capillary recruitment

• Normally not all capillaries are perfused, when you recruit more capillaries that were previously closed, you will decrease the resistance

2) Capillary distention

• Dilating the vessel (increasing their compliance)

Question 5

What are the collective effects of increasing the recruitment and distention of the capillaries?

Answer 5

1) Decrease the blood velocity, maintaining adequate time for gas exchange

2) Increase the capillary surface area, increasing the gas exchange

3) Decreasing the capillary hydrostatic pressure decreasing the risk of pulmonary edema

4) Decreases the load on the right heart

Question 6

Describe the difference between the intra-alveolar and extra-alveolar vessels

Answer 6

• Intra-alveolar pressure courses in and around the alveoli, they are subjected to alveolar pressure, that varies with the respiratory cycle
• Extra-alveolar vessels are subjected to a more negative pressure “intrapleural pressure”, the Intrapleural pressure turns positive during expiration
• This indicates that the extra-alveolar vessels are more distended that the intra-alveolar vessels

Question 7

What is the effect of the lung volume on the pulmonary vascular resistance?

Answer 7

• They are affected by the pleural pressure:

1) High lung volume

• Higher volume = more negative pleural pressure = dilated extra-alveolar vessels (due to their distention as they are distended due to the lower pleural pressure) and increased alveolar pressure = constricted alveolar vessels (greater effect compared to the extra-alveolar dilation and thus it will increase the pulmonary vascular resistance)

2) Low lung volume

• The pulmonary resistance will increase due to the constriction of the extrapulmonary vessels, even though the alveolar vessels will dilate, the effect of the constriction of the extra-alveolar vessels is greater and thus a total increase in pulmonary resistance will result

Question 8

When is the pulmonary resistance at its highest and when is it at its lowest?

Answer 8

1) Highest at:

• TLC
• RV

2) Lowest at the FRC (RV + ERV (Expiratory Reserve Volume))

Question 9

What is the alveolar vessels and extra-alveolar vessels?

Answer 9

• Alveolar vessels are the capillaries that surrounds the alveoli
• The extra-alveolar vessels are the arteries and veins

Question 10

What are the agents that reduced the vascular resistance through dilation?

Answer 10

1) Increases alveolar oxygen levels

2) Decreases alveolar CO2 levels

3) Increased pH

4) Histamine and H2 agonists

5) PGI2

6) Bradykinin

7) Theophylline

8) Ach, Nitric Oxide

Question 11

What are the agents that affects the pulmonary vascular resistance through constriction?

Answer 11

1) Decreased alveolar oxygen levels

2) Increased alveolar carbon dioxide levels

3) Decreased pH

4) Some Histamine, H1 agonist

5) a-receptor agonist

6) Angiotensin-II

Question 12

How can alveolar hypoxia cause the contraction of nearby vascular smooth muscles?

Answer 12

1) Hypoxia induces depolarization of the smooth muscle cells

2) Depolarization will open the voltage-gated Ca2+ channels

3) Calcium will enter into the cell and cause contraction

Question 13

What will happen to the pulmonary vascular resistance if the pulmonary vascular pressure increases?

Answer 13

It will decrease

• Increase CO = Increase pulmonary pressure = decrease pulmonary resistance

Question 14

What are the two unique features of the pulmonary vasculature?

Answer 14

1) Much more affected by physical forces as it is dispensable, low-pressure circulation

2) It is constricted during hypoxia, which diverts the blood flow towards ventilated alveoli

Question 15

What is the effect of alveolar hypoxia on pulmonary arteries?

Answer 15

1) Regional hypoxia

• Diverts blood to the ventilated area
• Minimal effect on the pulmonary arterial pressure

2) Generalized hypoxia (precapillary constriction occurs throughout the lungs)

• In high altitudes and chronic diseases (asthma, emphysema)
• Increases the pulmonary arterial pressure, resulting in pulmonary HTN and thus right heart hypertrophy
• This mechanism plays a crucial role in fetal circulation (1st breath of the baby > inc po2 > eliminates the hypoxic condition > BVs dilate > normal circulation starts )

Question 16

How is the fluid exchanges across the pulmonary capillaries?

Answer 16

• In addition to the hydrostatic pressure and the colloid osmotic pressure there is:

1) Alveolar surface tension: Pulls the fluid towards the interstitial space and alveoli, which might lead to edema if it is very high due to low surfactant

2) Alveolar pressure: The higher the more fluid is pushed into the interstitial space and the capillaries

Question 17

What are the causes of pulmonary edema (other than colloid and hydrostatic factors)?

Answer 17

1) Increased alveolar surface tension due to low surfactant

2) Decreased alveolar pressure

3) reduced lymph drainage

Question 18

Describe the distribution of blood flow in the lungs

Answer 18

• Apex:

1) Zone-1:

• Lowest. the driving force is the difference between alveolar pressure and arterial pressure (here PA is contributing a lot and if it increased there will be constriction of BV)
• High alveolar pressure > capillaries tend to constrict and collapse at apex

2) Zone-2:

• Medium, the driving force is the difference between the arterial pressure and the alveolar pressure (due to the gravitational effect on the hydrostatic pressure, vein pressure is still the lowest)

3) Zone 3:

• Highest, The driving force here is the difference between the arterial pressure and the pulmonary vein pressure
• Base
• Intrapleural pressure is more negative at the apex, due to the weight of the lungs

Question 19

How can zone-1 become a dead space?

Answer 19

As a result of the gravitational effect, arterial pressure (Pa) at the apex of the lung may be lower than alveolar pressure (PA), which is approximately equal to atmospheric pressure.
If Pa is lower than PA, the pulmonary capillaries will be compressed by the higher alveolar pressure outside of them.
This compression will cause the capillaries to close, reducing regional blood flow.

Normally, in zone 1, arterial pressure is just high enough to prevent this closure, and zone 1 is perfused, albeit at a low flow rate.
However, if arterial pressure is decreased (e.g., due to hemorrhage) or if alveolar pressure is increased (e.g., by positive pressure breathing), then PA will be greater than Pa and the blood vessels will be compressed and will close. Under these conditions, zone 1 will be ventilated but not perfused. There can be no gas exchange if there is no perfusion, and zone 1 will become part of the physiologic dead space.

Question 20

Describe the difference between the ventilation and perfusion between the apex of the lungs and the base

Answer 20

1) Apex

• Intrapleural pressure is more negative = Greater transmural pressure gradient = alveoli gets larger = less ventilation
• Lower Pa = decreased capillary distention and recruitment = higher resistance = low blood flow

2) Base

• Intrapleural pressure is less negative = Smaller transmural pressure gradient = alveoli gets smaller = more ventilation
• Greater Pa = increased capillary distention and recruitment = lower resistance = higher blood flow

Question 21

What is meant by ventilation-perfusion ratio (VA/Q)?

Answer 21

The ventilation-perfusion ratio (VA/Q), it compares the alveolar ventilation to the blood flow in the lung regions, the normal value is 0.8 (This value means that alveolar ventilation (L/min) is 80% of the value for pulmonary blood flow (L/min))

• The matching of the alveolar ventilation to the perfusion will determine the alveolar and arterial blood gas values

Question 22

What is the effect of gravity on the blood flow and alveolar ventilation in the base and apex of the lungs?

Answer 22

1) At the apex: There is less ventilation and perfusion but the ventilation is more, and so the blood that reaches the alveoli is oxygenated well, and there is a lot of CO2 excreted

• As a result the ventilation/perfusion ratio is highest at the apex of the lung

2) At the base: The perfusion is more than the ventilation, lots of blood but not lots of air, as a result the ventilation perfusion ratio is very low

Question 23

In which zone is the blood glow the highest?

Answer 23

Highest in zone 3 and lowest in zone 1

Question 24

In which zone is the alveolar ventilation the lowest?

Answer 24

In zone 1, while it is highest in zone 3

Question 25

Where is the partial pressure of arterial oxygen the highest?

Answer 25

it is highest in zone-1 and lowest in zone-3

Question 26

Where is the ventilation perfusion ratio the highest?

Answer 26

In zone-1 (3), while it is the lowest in zone-3 (0.6)

Question 27

What is the effect of VA/Q ration on the PO2 & PCO2?

Answer 27

The higher the VA/Q the higher the PO2 and the lower the PCO2

Question 28

What are the consequences of a defective VA/Q?

Answer 28

1) Dead space (when there is maximum ventilation but no perfusion) “air is wasted”

• Constricted vessels
• Hypercapnia with no hypoxemia

2) Pulmonary shunt (when there is a maximum blood flow but no ventilation) “basically the same deoxygenated blood is returned (no gas-exchange), blood will go to the systemic circulation directly AKA shunt”, hyperventilation will only keep the CO2 levels within normal range, it wont increase the oxygen levels

• Constricted alveoli

Question 29

What can be a cause of a dead space (maximum ventilation but no perfusion)?

Answer 29

Pulmonary emboli (in which blood flow to a portion of the lung (or even the entire lung) is occluded)

Question 30

What can be the cause of a pulmonary shunt?

Answer 30

1) They are illustrated by airway obstruction, no gas-exchange occur (zero ventilation, while their is perfusion “PaO2 is 40 mm Hg, and PaCO2 is 46 mm Hg”)

• There will be a hypoxemia and hyperventilation wont solve it (it will only keep CO2 at normal levels)

Question 31

Describe the relationship between hyper and hypoventilation with the PO2 and PCO2

Answer 31

1) Hyperventilation is when there is more ventilation compared to perfusion (CO2 is blown off and PO2 is increased in both the alveoli and arterial blood)

2) During hypoventilation, when ventilation is relatively low compared to perfusion, which results in a decreased PO2 and Increased PCO2

Question 32

Which of the following statements regarding zone 1 of the lung is correct?

1) Pressure in the alveoli is less than the pressure in the arteries

2) It has the least amount of blood flowing through it
Pressure in the veins is greater than in the arteries

3) Low alveolar pressure induces opening of the arterioles

4) It has the highest amount of blood flowing through it

Answer 32

2