Respiratory: Physiology - Pulmonary blood flow and metabolism Flashcards
RA pressure
2mmHg
RV systolic and diastolic pressures
25/0 mmHg
Pulmonary artery pressure: systolic, diastolic and mean
25/8 mmHg
Mean 15mmHg
Pressure at arterial and venous ends of alveolar capillary beds
Arterial: 12mmHg
Venous: 8mmHg
LA pressure
5mmHg
LV systolic and diastolic pressures
120/0 mmHg
Systemic arterial pressure: systolic, diastolic, and mean
120/80 mmHg
Mean 100mmHg
Pressure at arterial and venous ends of systemic capillary beds
Arterial: 30mmHg
Venous: 10mmHg
What is the difference in structure between pulmonary and systemic arterial branches?
Pulmonary arterial branches have very little smooth muscle (low-pressure system)
Describe the difference in pressure around alveolar, extra-alveolar, and large hilar vessels in the lung
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
Formula for vascular resistance
Vascular resistance = pressure difference / blood flow
What fraction of systemic resistance is pulmonary vascular resistance?
~1/10th
What two mechanisms reduce pulmonary vascular resistance in response to increased pulmonary arterial or venous pressure?
Recruitment and distension
What is the effect of lung volume on pulmonary vascular resistance?
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
List 6 factors which induce smooth muscle contraction in the lung
5HT
Histamine
NA
Endothelin
TXA2
Alveolar hypoxia
List 5 factors which induce smooth muscle relaxation in the lung
ACh
CCBs
NO
PDE
PGI2 (prostacyclin)
Describe the Fick principle and give the equation
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)
Explain two methods of measuring pulmonary blood flow
- Using Fick principle (measuring O2 consumption by lung and arterial-venous O2 difference)
- Dilution techniques (indicator injected into venous circulation and arterial concentration measured)
How can regional distribution of pulmonary blood flow be measured?
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
Describe the difference in distribution of pulmonary blood flow in upright vs supine position
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)
Describe the changes in distribution of pulmonary blood flow seen with exercise
Increased blood flow overall, less regional differences
Describe the difference between zones 1, 2 and 3 of the lung
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)
What kinds of conditions may result in zone 1 description of pulmonary blood flow?
Decreased Pa (e.g. in shock)
Increased PA (e.g. PPV)
What does zone 4 refer to as a description of pulmonary blood flow?
Decreased regional blood flow in under-inflated lung due to narrowing/collapse of extra-alveolar vessels (no traction to pull them open)
What is the trigger for pulmonary hypoxic vasoconstriction? What is the impact of the CNS on this response?
Reduced PAO2 (NOT PaO2)
Effect is not dependent on CNS
Is pulmonary hypoxic vasoconstriction a linear response?
No: little change with PO2 >100mmHg, marked response with PO2 <70mmHg
Vasodilatory factor important in regulating pulmonary vascular tone
NO
Two vasoconstricting factors important in regulating pulmonary vascular tone
ET-1
TXA2
Three clinical settings which would induce pulmonary vasoconstriction
- Acidosis
- Hypothermia
- Increased sympathetic stimulation
- Iron deficiency
What changes occur at high altitudes with relation to the effect of pulmonary hypoxic vasoconstriction?
Globally reduced PO2 causes increased pulmonary artery pressure
What is the role of pulmonary hypoxic vasoconstriction in the transition from placental to air breathing at birth?
First breath increases PO2 causing reversal of hypoxic vasoconstriction -> increased pulmonary blood flow
Starling’s equation
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)
What is the normal lymph flow from pulmonary vessels?
~20ml/hr
Describe the flow of lymph in the lung
Lymph tracks to perivascular and peribronchial space, then drains via lymphatics to hilar lymph nodes
Describe the pattern of fluid accumulation in early vs late pulmonary oedema
Early: fluid collects as interstitial oedema (engorgement of perivascular/peribronchial spaces)
Later stage: fluid in alveoli when interstitial lymph drainage rate is exceeded
How is fluid in alveolar spaces removed on a cellular level?
Pumped out via Na+/K+ ATPase
What vasoactive peptide is synthesised in the lung?
Angiotensin II is formed from angiotensin I via action of ACE in the lung
List 5 metabolically active molecules removed from the circulation in the lung
Bradykinin
5HT
NA
PGE2 and PGFa2
Leukotrienes
