Ventilation-Perfusion Relationships

Question 1

What are the maximum and minimum partial pressures of oxygen in the alveolus that are possible at sea level?

Answer 1

Max = 150 mm Hg
Min = 40 mm Hg

Question 2

What would have to happen to get the arterial oxygen pressure to match that of the alveoli?

Answer 2

The ventilation to perfusion ratio would have to increase

Question 3

What would the ventilation to perfusion ratio be if the pressure of oxygen in the arteries were 40 mm Hg?

Answer 3

Zero; this is a reflection of perfusion to a physiologic dead space

Question 4

What are the two main reasons why ventilation and perfusion never perfectly match?

Answer 4

1) Ventilation is not homogenous

2) Perfusion is not homogenous

Question 5

Where in the lung is blood flow the greatest?

Answer 5

The base

Question 6

What are the West zones?

Answer 6

Even though the alveolar pressure is largely homogenous in the lungs at inspiration, the blood vessels at various portions have various radii, so the resistance and pressure of those vessels varies.

Question 7

What is the state of lung tissue in West zone 1?

Answer 7

Blood flow tends to be collapsed and perfusion is minimal. The alveolar pressure is greater than that of the vessel

Question 8

What is the state of lung tissue in West zone 2?

Answer 8

There is partial perfusion in the blood vessels and they will be in various states of open/shut. As a general rule, the arterial pressure is higher than that of the alveolar pressure, but the alveolar pressure is higher than the venous pressure

Question 9

What is the state of the lung tissue in West zone 3?

Answer 9

There is high perfusion in the capillaries—arterial and venous pressure tend to be higher than the pressure of the alveolus.

Question 10

Where is the transpulmonary pressure highest and why?

Answer 10

In the apex-gravity pulls the lung downward and makes the pleural pressure more negative.

Question 11

Where are alveoli best ventilated and why?

Answer 11

At the based of the lung because they have higher compliance

Question 12

What are the consequences of having a low V/Q ratio?

Answer 12

A low arterial oxygen pressure

Question 13

What happens to the arterial oxygen pressure when blood from an alveolus with good V/Q ratios is mixed with blood from an alveolus with a bad V/Q ratio?

Answer 13

The overall partial pressure of oxygen will reflect the more poor V/Q ratio,

Question 14

How does a lung mitigate poor V/Q ratios?

Answer 14

In hypoxic conditions, the lung will initiate vasoconstriction of the poorly ventilated alveoli in favor of those with good ventilation

Question 15

What is a consequence of having a high V/Q ratio?

Answer 15

hypercapnea

Question 16

What is the origin of the bronchial arteries and what is their typical pressure?

Answer 16

The bronchial arteries arise from the aorta and typically exhibit systemic pressure

Question 17

Why would pulmonary arterial pressure need to be low?

Answer 17

Low pressures permit high rate of flow to be maintained and also minimize the amount of fluid that can escape into the pneumatic tissue

Question 18

How is pulmonary vascular resistance calculated?

Answer 18

80*(Mean pulmonary arterial pressure-Pulmonary capillary wedge pressure)/Cardiac Output

Question 19

How does the PVR change in exercise?

Answer 19

It drops to accommodate greater flow

Question 20

What are the two mechanisms by which the lung can decrease the PVR?

Answer 20

It can encourage distension in capillaries that are already opened or it can recruit previously collapsed vessels (especially in the apexes)

Question 21

What happens to alveolar blood vessels on inspiration?

Answer 21

Due to their large compliance, they tend to collapse, especially when the lung volume increases

Question 22

How does the lung compensate for the restriction of alveolar blood vessels?

Answer 22

Blood vessels elsewhere within the respiratory network dilate to keep overall resistance and pressure normal

Question 23

At what point is the resistance within the pulmonary network the lowest?

Answer 23

At the functional reserve capacity

Question 24

How might the lungs control their own vasoconstrictive forces in hypoxia? (i.e. the mechanism)

Answer 24

Low O2 seems to affect smooth muscle cells to close their K channels, and depolarization occurs permitting Ca++ to enter the cell and trigger smooth muscle contractions

Question 25

What mechanisms have control over hypoxic vasoconstriction?

Answer 25

Local regulators like NO and prostacyclins (dilators) and endothelin-1 and angiotensin II (constrictors). The nervous system has no major impact on these mechanisms

Question 26

What is shunting? How can it be gauged clinically?

Answer 26

A shunt occurs when blood travels from the right heart to the left without encountering gas exchange at an alveolus. It cannot be alleviated by giving the patient oxygen clinically