Influences on Pulmonary Vascular Resistance

Question 1

3 features of pulmonary vessels

Answer 1

1. Little vascular SM
2. Low intravascular P
3. Increased distensibility and compressibility

Question 2

What is vessel diameter influenced by (5)

Answer 2

EXTRAVASCULAR FORCES

1. Gravity
2. Body position
3. Lung volume
4. Alveolar/IP pressure
5. Intravascular P

Question 3

Effect of BP on PVR

Answer 3

Increase in BP decreases resistance in pulmonary system

Question 4

what are the 2 mechanisms by which an increase in BP decreases R in pulmonary system

Answer 4

1. Recruitment - open previously closed vessels
2. Distension - increase in calibre of vessels

Question 5

Effect of hypoxia on PVR

note: vasodilation in systemic circulation

Answer 5

VASOCONSTRICTION @ level of pre-capillary arteriole

Improves matching of blood flow with ventilation

Blood will be diverted to alveoli that are better ventilated

Limits blood flow to lungs of foetus (gas exchange across placenta)

Question 6

High altitude and hypoxia

Answer 6

Increased R to flow in pulmonary circuit

Increased P in pulmonary artery

Better perfusion of lung apex

Question 7

Mechanism of action of hypoxic pulmonary vasoconstriction

Answer 7

Response occurs locally

Does NOT require innervation

Mediators have not been identified

weak response due to limited pulmonary vasculature muscle

Question 8

Disadvantages of hypoxic pulmonary vasoconstriction

Answer 8

1. Causes increased PVR
2. Increased afterload in right ventricle
3. increased risk of RV hypertrophy - risk of R sided heart failure
4. Increased tone of SM in pulmonary arterioles does not reverse hypertrophy of SM, which does not reverse pulmonary hypertension due to narrowing of pulmonary artery

Question 9

How does hypoxia induce vasoconstriction

Answer 9

O2 sensitive K+ channels in SM surround BVs within alveolar walls

PO2 decreases, closing K+ channels

Triggers SM cell depolarisation

COntraction

Vasconstriction of pulmonary vessels

Blood flow to poorly ventilated lung regions is decreased

(opposite response in systemic circulation)

Question 10

Where is PVR lowest

Answer 10

Near FRC

Question 11

What are alveolar vessels surrounded by

Answer 11

Gas, therefore can collapse/distend

• P_alv > P_cap, capillaries collapse
• P_alv < P_cap, capillaries remain open

Question 12

How are the extra-alveolar vessels (arteries and veins) tethered to lung

Answer 12

Connective tissue

Question 13

How are extra-alveolar vessels opened

Answer 13

Pulled open by lateral traction - INSPIRATION

exposed to -ve IP pressure

Keeps large PVs open

Question 14

Effects of a decrease in lung vol

Answer 14

ABOVE FRC

PVs outside alveoli are distended

decreased R

Septal capillaries outside alveolar walls are compressed = dominant effect, so overall INCREASED R

Question 15

Effects of a decreased lung volume

Answer 15

BELOW FRC

no longer stretched by surrounding lung tissue

increased R

decreased alveolar size

Alveolar capillaries are less compressed

decrease in R of capillaries

Overall, increased R

COLLAPSE OF EXTRA-ALVEOLAR VESSELS = dominant effect

(better blood flow at bottom of lungs)

Question 16

Difference in blood flow from base to apex

Answer 16

Almost constant, although low values at apex

Question 17

Where is the higher P in lungs

Answer 17

Due to gravity, P is greates at base

Question 18

What is the pressure difference in lungs

Answer 18

23 mmHg