Respiratory: Physiology - Pulmonary blood flow and metabolism

Question 1

RA pressure

Answer 1

2mmHg

Question 2

RV systolic and diastolic pressures

Answer 2

25/0 mmHg

Question 3

Pulmonary artery pressure: systolic, diastolic and mean

Answer 3

25/8 mmHg
Mean 15mmHg

Question 4

Pressure at arterial and venous ends of alveolar capillary beds

Answer 4

Arterial: 12mmHg
Venous: 8mmHg

Question 5

LA pressure

Answer 5

5mmHg

Question 6

LV systolic and diastolic pressures

Answer 6

120/0 mmHg

Question 7

Systemic arterial pressure: systolic, diastolic, and mean

Answer 7

120/80 mmHg
Mean 100mmHg

Question 8

Pressure at arterial and venous ends of systemic capillary beds

Answer 8

Arterial: 30mmHg
Venous: 10mmHg

Question 9

What is the difference in structure between pulmonary and systemic arterial branches?

Answer 9

Pulmonary arterial branches have very little smooth muscle (low-pressure system)

Question 10

Describe the difference in pressure around alveolar, extra-alveolar, and large hilar vessels in the lung

Answer 10

Alveolar: transmural pressure is equal to difference between pressure within capillary and alveolar pressure (which normally equals atmospheric pressure)
Extra-alveolar: vessels pulled open by radial traction during inspiration and so pressure around them is lower than alveolar vessels
Large hilar vessels: subject to intra-pleural pressures

Question 11

Formula for vascular resistance

Answer 11

Vascular resistance = pressure difference / blood flow

Question 12

What fraction of systemic resistance is pulmonary vascular resistance?

Answer 12

~1/10th

Question 13

What two mechanisms reduce pulmonary vascular resistance in response to increased pulmonary arterial or venous pressure?

Answer 13

Recruitment and distension

Question 14

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

Answer 14

Reduced with large volumes due to traction of extra-alveolar vessels up to an extent: after this resistance is increased due to stretching of vessels

Reduced with small volumes due to alveolar collapse

Question 15

List 6 factors which induce smooth muscle contraction in the lung

Answer 15

5HT
Histamine
NA
Endothelin
TXA2
Alveolar hypoxia

Question 16

List 5 factors which induce smooth muscle relaxation in the lung

Answer 16

ACh
CCBs
NO
PDE
PGI2 (prostacyclin)

Question 17

Describe the Fick principle and give the equation

Answer 17

O2 consumption/min measured at mouth is equal to O2 taken up by blood in lungs/min

Q = VO2/(CaO2-CvO2)
Where Q = lung blood flow/min, VO2 = O2 consumption/min (from flow sensor), CaO2 = arterial O2 concentration (from radial artery puncture), CvO2 = mixed venous O2 concentration (from pulmonary artery catheter)

Question 18

Explain two methods of measuring pulmonary blood flow

Answer 18

1. Using Fick principle (measuring O2 consumption by lung and arterial-venous O2 difference)
2. Dilution techniques (indicator injected into venous circulation and arterial concentration measured)

Question 19

How can regional distribution of pulmonary blood flow be measured?

Answer 19

By injection of xenon-saline solution: xenon evolves into alveolar gas due to its low solubility in blood, then distribution of radioactivity can be measured by counters over the chest during breath-holding

Question 20

Describe the difference in distribution of pulmonary blood flow in upright vs supine position

Answer 20

Upright: decreasing blood flow from base to apex
Supine: increased apical blood flow while basal maintained, meaning more uniform distribution overall (although dependent posterior regions have increased blood flow compared with anterior)

Question 21

Describe the changes in distribution of pulmonary blood flow seen with exercise

Answer 21

Increased blood flow overall, less regional differences

Question 22

Describe the difference between zones 1, 2 and 3 of the lung

Answer 22

Zone 1: PA > Pa > Pv (therefore flow is obstructed; does not occur under normal conditions)
Zone 2: Pa > PA > Pv (flow determined by alveolar-arterial difference; recruitment occurs here)
Zone 3: Pa > Pv > PA (flow determined by arterial-venous difference; distension occurs here)

Question 23

What kinds of conditions may result in zone 1 description of pulmonary blood flow?

Answer 23

Decreased Pa (e.g. in shock)
Increased PA (e.g. PPV)

Question 24

What does zone 4 refer to as a description of pulmonary blood flow?

Answer 24

Decreased regional blood flow in under-inflated lung due to narrowing/collapse of extra-alveolar vessels (no traction to pull them open)

Question 25

What is the trigger for pulmonary hypoxic vasoconstriction? What is the impact of the CNS on this response?

Answer 25

Reduced PAO2 (NOT PaO2) Effect is not dependent on CNS

Question 26

Is pulmonary hypoxic vasoconstriction a linear response?

Answer 26

No: little change with PO2 >100mmHg, marked response with PO2 <70mmHg

Question 27

Vasodilatory factor important in regulating pulmonary vascular tone

Answer 27

NO

Question 28

Two vasoconstricting factors important in regulating pulmonary vascular tone

Answer 28

ET-1 TXA2

Question 29

Three clinical settings which would induce pulmonary vasoconstriction

Answer 29

1. Acidosis 2. Hypothermia 3. Increased sympathetic stimulation 4. Iron deficiency

Question 30

What changes occur at high altitudes with relation to the effect of pulmonary hypoxic vasoconstriction?

Answer 30

Globally reduced PO2 causes increased pulmonary artery pressure

Question 31

What is the role of pulmonary hypoxic vasoconstriction in the transition from placental to air breathing at birth?

Answer 31

First breath increases PO2 causing reversal of hypoxic vasoconstriction -> increased pulmonary blood flow

Question 32

Starling's equation

Answer 32

Net fluid out = K[(Pc - Pi) - σ(πc - πi)] Where K = filtration coefficient, Pc - Pi = capillary and interstitial hydrostatic pressure difference, σ = reflection coefficient (effectiveness of capillary wall in preventing protein passage), πc - πi = capillary and interstitial colloid pressure difference (in lung πc = 25-28, πi = 20)

Question 33

What is the normal lymph flow from pulmonary vessels?

Answer 33

~20ml/hr

Question 34

Describe the flow of lymph in the lung

Answer 34

Lymph tracks to perivascular and peribronchial space, then drains via lymphatics to hilar lymph nodes

Question 35

Describe the pattern of fluid accumulation in early vs late pulmonary oedema

Answer 35

Early: fluid collects as interstitial oedema (engorgement of perivascular/peribronchial spaces) Later stage: fluid in alveoli when interstitial lymph drainage rate is exceeded

Question 36

How is fluid in alveolar spaces removed on a cellular level?

Answer 36

Pumped out via Na+/K+ ATPase

Question 37

What vasoactive peptide is synthesised in the lung?

Answer 37

Angiotensin II is formed from angiotensin I via action of ACE in the lung

Question 38

List 5 metabolically active molecules removed from the circulation in the lung

Answer 38

Bradykinin 5HT NA PGE2 and PGFa2 Leukotrienes