pulmonary circulation

Question 1

Where does bronchial circulation emerge from and where is it drained?

Answer 1

Bronchial circulation comes out of the thoracic aorta and drains into the veins within the pulmonary circulation

Question 2

How do the arteries in the pulmonary circulation differ to normal arteries?

Answer 2

Pulmonary circulation is under lower pressure so the arteries have a greater lumen: wall thickness ratio. This makes them more distensible and compliant. Still elastic to convert pulsatile pressure to continuous flow. The circuit is shorter so the resistance in circuit lower as resistance is proportional to length.

Question 3

Which gases are exchanged in pulmonary circulation?

Answer 3

Carbon dioxide, oxygen, nitric oxide and carbon monoxide are examples of exchanged gases.

Question 4

What are the functions of the pulmonary circulation?

Answer 4

1- gas exchange
2- metabolism of vasoactive substances
3- filtration of blood

Question 5

What are the metabolic roles of the pulmonary circulation?

Answer 5

• ACE is within the walls of the pulmonary endothelium
• It converts angiotensin I to angiotensin II
• ACE also degrades bradykinin (which works antagonistically with angiotensin II)

Question 6

What is the filtration role of the pulmonary circulation?

Answer 6

The pulmonary circulation filters before the systemic arteries. Small emboli (venous thrombus, fatty plaques and air) could get trapped and then eliminated in the pulmonary circulation.
Air bubbles can be compacted and moved back out into the airspaces. Fatty plaques and thrombi can be enzymatically degraded. A large embolus could get stuck in a bigger artery which would decrease the amount of local perfusion and could potentially lead to sudden death

Question 7

What is the difference between embolus and embolism?

Answer 7

Embolus is the mass within the circulation that is capable of causing obstruction

Embolism is the obstruction of a major artery

Question 8

Why is the pulmonary circulation important for filtration?

Answer 8

Prevents things from getting stuck in the brain or heart and causing stroke or death

Question 9

What is a pulmonary shunt?

Answer 9

Bypassing the respiratory exchange surface

Question 10

Give examples of pulmonary shunts and explain them

Answer 10

1) Bronchial circulation - it comes out of the thoracic aorta and perfuses the airways and then it returns to the pulmonary veins and goes back to the left side of the heart. This means that it goes through the left side of the heart twice before returning to the right side - it bypasses the respiratory exchange surface.
2) foetal circulation (foramen ovale and ductus arteriosus) - The foramen ovale is a hole between the right and left atria - it creates a low pressure alternative for blood flow. Foetal blood gets oxygen from the mother via the placenta so the blood will come to the right side of the heart and it will follow the path of least resistance. The blood would much more favourably go through the foramen ovale or ductus arteriosus to get back to the systemic circulation - there is no need to go to the lungs because the oxygen is coming from the placenta.
3) Congenital heart defect - atrial septal defect, patent foramen ovale and ventricular septal defect. In ASD, mixed venous blood moves from the right atrium to the left atrium. VSD is more a congenital defect rather than mal-correction after birth.

Question 11

What is the % of cardiac output that goes to the bronchial circulation?

Answer 11

1

Question 12

How does an increase in cardiac output affect pulmonary vascular resistance and how is this different to what would be expected otherwise?

Answer 12

An increase in cardiac output would lead to an increase in MAP in the pulmonary circulation. This will increase the hydrostatic pressure and push more fluid into the interstitial space, causing pulmonary oedema, reducing pulmonary function.

However, the pulmonary circulation is a low resistance high capacity circuit . There can be an increase in cardiac output with a small increase in MAP. This is because the pulmonary arteries are distensible. There is also increased perfusion to the hypoperfused capillary beds. Minimal changes in MAP means no pulmonary oedema.

Question 13

Is pulmonary perfusion equal at rest and why?

Answer 13

No
Basal capillary beds are much more perfused at rest because blood leaving the heart wants to go with gravity and follow the path of least resistance.

Question 14

How is the difference in perfusion of the apex and the base affected by vascular recruitment?

Answer 14

The difference between the apex and the base is still present in increased vascular recruitment but to a lesser extent.
(Vascular recruitment is the increased use of the vascular beds which were not being used because there wasn’t enough pressure to access them)

Question 15

How does ventilation affect pulmonary vascular resistance?

Answer 15

Inspiration compresses alveolar vessels as the alveoli expand. Expiration compresses extra-alveolar vessels as the chest gets smaller meaning that there is more pressure on them, making them constrict .
Influencing these vessels will lead to changes in the resistance of the pulmonary circulation.
Resistance will increase when you’re at the extremities of your vital capacity.

Question 16

What are the effects of hypoxia on pulmonary vascular resistance?

Answer 16

Pulmonary vascular response to hypoxia is vasoconstriction (compared to vasodilation in systemic circulation). Systemic circulation dilates because a low oxygen environment means that we are underperfusing the tissues distal to the vessel so blood flow must increase to meet demands.

Question 17

When is vasoconstriction in response to hypoxia detrimental?

Answer 17

COPD - reduced alveolar ventilation and air trapping. This means the vessels will vasoconstrict so increased resistance in pulmonary circuit. This can lead to pulmonary hypertension, right ventricular hypertrophy and congestive heart failure.

Question 18

How is pulmonary fluid balance maintained?

Answer 18

The plasma + interstitium, and hydrostatic + oncotic pressures are involved in pulmonary fluid balance.
Plasma hydrostatic pressure is greater at the arterial end and lower at the venous end. The interstitial hydrostatic pressure is pretty much zero in healthy individuals. Plasma oncotic pressure is a pulling force. As soon as deposition of fluid exceeds the capacity of the lymphatics to drain fluid - fluid accumulates and you get oedema.

Question 19

In healthy individuals, how much fluid is mopped up by lymphatics?

Answer 19

1 mm Hg

Question 20

What are the causes of pulmonary oedema?

Answer 20

• Mitral valve stenosis/heart failure: causes increase in hydrostatic pressure
• Hypoproteinaemia/protein losing nephropathies/liver cirrhosis etc: plasma oncotic pressure is reduced
• Infection/pulmonary endothelial damage: if proteins and WBCs accumulate in the insterstitium then there is going to be greater interstitial oncotic pressure so more pulled out into the interstitium
• Cancer: if lymphatics cannot drain the fluid then the fluid expelled can build up over time

Question 21

What is the difference between oncotic and colloid pressure?

Answer 21

oncotic and colloid pressure are the same

Question 22

How does cardiac output, volume of blood, mean arterial pressure, presure gradient, resistance, velocity and compliance differ between the pulmonary and systemic circulation?

Answer 22

• Cardiac output on both sides is around 5 L/min
• More volume within the systemic circulation because there are more vessels
• MAP in the pulmonary circuit is 15% that of the systemic circuit - this is because we aren’t pumping as far, so less pressure is needed
• The pressure gradient is much greater in the systemic (92) compared to pulmonary (9)
• The resistance against the pulmonary circulation is 10% that of the systemic
• The velocity of the flow is also going to be greater in the systemic compared to the pulmonary (due to the much greater pressure gradient)
• Compliance is higher in the pulmonary circulation (the arteries are more distensible) so they can easily handle an increase in cardiac output

Question 23

How do the pressure within the circulatory system change?

Answer 23

Systemic circulation is under high pressure compared to pulmonary.
Blood will flow down the pressure gradient and will return to the right atrium at very low pressure. There is a pressure gradient between the right ventricle and the left atrium which allows the blood flow to take place. (The pressure measured at the thoracic aorta is 93 not 120 because of distension and recoil of which buffers the pressure changes between systole and diastole).

Question 24

Why is vasoconstriction of pulmonary vessels in response to hypoxia clever?

Answer 24

Perfusing a non-ventilated alveolus is wasted perfusion as the blood will not get oxygenated.

Question 25

When is vasoconstriction in response to hypoxia useful?

Answer 25

During foetal development. Blood flows down the path of least resistance. High-resistance pulmonary circuit means increased flow through shunts. First breath increases alveolar PO2 and dilates pulmonary vessels.