Pulmonary Circulation

Question 1

How do pulmonary arteries differ from systemic arteries?

Answer 1

Pulmonary arteries are thinner and have a larger diameter than the systemic arteries

Question 2

Why does gravity have an effect on lung perfusion?

Answer 2

Because pulmonary arterial pressure is so low

Question 3

Outline the three zones of lung perfusion in the upright state

Answer 3

Apex - intermittent flow (only flows during systole)
Centres - pulsatile flow (flow greater in systole than diastole)
Bases - continuous blood flow

Question 4

Describe the blood flow to the apices of the upright lung

Answer 4

No blood flow during expiration as the blood vessels here have very thin walls, so the arterioles and venules collapse during expiration when the alveolar pressure is greater than the arterial pressure

Question 5

Describe the blood flow to the centre of the upright lung

Answer 5

The flow is pulsatile; flow increases during inspiration and decreases during expiration and average flow increases the further down the lung you go

Question 6

How does average blood flow change the further down the lung you go?

Answer 6

Increases the lower you go down the lung

Question 7

Describe the blood flow to the base of the upright lung

Answer 7

Flow occurs continuously as pulmonary arterial and venous pressures always exceeds alveolar pressure

Question 8

What is the ventilation-perfusion ratio?

Answer 8

The ratio of the amount of air reaching the alveoli per minute to the amount of blood reaching the alveoli per minute

Question 9

What is the ideal ventilation-perfusion ratio?

Answer 9

Approximately 1.0

Question 10

How can the distribution of blood flow be measured?

Answer 10

Using radioactive xenon; injected into venous blood and evolves into alveolar gas from the pulmonary capillaries, and therefore radiation counters measure the amount of xenon passing through each lung zone

Question 11

How can you measure lung compliance?

Answer 11

Compliance = change in lung volume/change in pleural pressure of the lungs

Question 12

How does the compliance of the bronchioles change throughout the lungs?

Answer 12

Bronchioles are stretchier/more compliant at the base of the lung and are stiffer at the apex

Question 13

How may the apex of the lung be characterised?

Answer 13

More ventilation and less blood flow

Question 14

How may the base of the lung be characterised?

Answer 14

Less ventilation and more blood volume

Question 15

How does gas exchange differ between the apex and base of the lung?

Answer 15

Apex has high pO2 and low pCO2 (good gaseous exchange but low blood flow) whereas the base has low pO2 and high pCO2 (less efficient gas exchange but greater blood flow)

Question 16

How may an airway obstruction affect the V/Q ratio?

Answer 16

V/Q ratio will be lower than normal as V will be smaller but will be divided by the same Q

Question 17

How may a blood flow obstruction affect to V/Q ratio?

Answer 17

V/Q ration will increase as V will stay the same but Q will decrease, so the number will increase

Question 18

What is the body’s response to pulmonary hypoxia?

Answer 18

Causes local vasoconstriction in the lung area affected –> diverts blood away from the poorly ventilated, hypoxic lung regions towards more perfused regions.

Question 19

How does pulmonary artery resistance change during exercise?

Answer 19

Resistance decreases

Question 20

How do the pulmonary arteries change during exercise?

Answer 20

Pulmonary arteries and arterioles have thin walls –> become distended with increases CO at start of exercise –> stretching –> generates reflex relaxation of arterial smooth muscle –> vessels relax and enlarge.

Question 21

What happens if the pulmonary vascular resistance doesn’t drop when a neonate first breathes air?

Answer 21

Enters respiratory distress –> hypoxic. Small percentage of NO added to inspired gas in the distressed baby and works to make PVR drop to normal