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47 Cards in this Set
- Front
- Back
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where does pulmonary circulation branch off from the airways & how?
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at the level of terminal bronchioles, splits up to supply capillary bed in alveolar walls so densely it is a sheet
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describe the pulmonary venous return
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oxygenated blood collected by capillary bed by interlobular pulmonary veins & returned to the L atrium
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how does pulmonary vessel pressure compare to systemic & what does this mean for vessel structure
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very low, about 1/10th
very pulsatile walls of arteries can be very thin and not much smooth muscle (can be mistaken for veins) |
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why can the pulmonary circulation have such low pressure? (2)
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doesn't need work to direct blood anywhere (except the top of the lung or if there's localised alveolar hypoxia)
always needs to accept the whole cardiac output |
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how does pulmonary capillary pressure compare to arterial & venous?
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about halfway between
more symmetrical pressure distro along capillary vs systemic (where most of pressure drop is upstream of capillary) |
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what pressure surrounds pulmonary vessels? what implications does this have?
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uniquely virtually surrounded by gas
pulm capillaries effectively at alveolar pressure ie close to atmospheric pressure collapse when surrounding pressure exceeds inside pressure |
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what is transmural pressure?
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the difference between inside and outside pressures
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how do extra-alveolar vessels behave differently from intra-alveolar vessels?
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extra alveolar are pulled open by traction of lung parenchma (increase when lung expans)
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how are hilar vessels different to those inside the lung?
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they are affected by intrapleural pressures as are outside lung substance
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what is the formula to approximate pulmonary vascular resistance?
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input pressure - output pressure
__________ blood flow |
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how pulmonary vascular resistance compare to that of the systemic circulation
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about 1/10th, same ratio as pressure
ie as low as is compatible with distributing blood flow |
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what is normal pulm vascular resistance & how is it computed?
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1.7mmHg/liter/min
15 (pulm map) - 5 (LA pressure)/6 (blood flow) |
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what is the main relationship of pulmonary resistance vs pressure
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as pressure increases (eg exercise), resistance falls to keep pressure the same
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what are the two main mechanisms for lowering pulmonary resistance?
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recruitment
distention |
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what is recruitment? when does it happen?
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use of vessels that would normally not conduct blood flow
(chief mechanism for decreased resistance when arterial pressure increased) |
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what is distention? when does it happen?
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distention of individual capillary segments
occurs at relatively high pressures |
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how does resistance in extra alveolar vessels relate to lung function?
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at large lung volumes, low resistance (stretched)
at low volumes, high resistance as smooth muscle and elastic tissue resist distention |
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what is the critical opening pressure
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if the lung is collapsed, pulmonary arterial pressure must increase above downstream pressure before there is any flow
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how does increasing alveolar pressure affect capillary resistance?
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increashing alveolar pressure eg deep inspiration squashes capillaries -> increases resistance
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what drugs influence smooth muscle contraction and hence resistance in the lung?
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increased contraction -> increased resistance
serotonin, histaine, Nad relaxed smooth muscle ACh isoproterenol |
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how does alveolar hypoxia relate to resistance?
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small pulm arteries constricted -> increased resistance
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what is the fick principle for measuring pulmonary blood flow? how do we use it to measure pulm blood flow in a living subject?
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O2 consumption per minute =
vol blood passing through lungs/min x (conc O2 in blood exiting - conc O2 in blood entering lungs) can be arranged to calculate flow calculate O2 consumption per minute in spirometer; mixed venous blood in pulm artery catheter & radial art line for that leaving |
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how equal is blood flow in the human lung?
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considerable inequalities
most dependent parts get most flow |
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what causes differences in lung perfusion?
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will have a 30cm hydrostatic pressure drop from top to bottom of 30cm lung which is a big difference in a low pressure system
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what decreases the differences in lung zone perfusion?
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exercise increases flow to all zones, decreases regional differences
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what is zone 1 of the perfused lung?
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only occurs in severely low arterial pressure (eg haemorrhage) or high alveolar pressure (eg PPV)
PA > Pa > Pv as arterial less than alveolar pressure, the capillaries are squashed & there is no flow |
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what is alveolar dead space?
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a ventilated but not perfused lung as seen in zone 1
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what is zone 2 of the perfused lung?
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Pa > PA > Pv
hydrostatic effect keeps pulmonary arterial pressure above alveolar pressure but as alveolar pressure exceeds venous pressure, the normal relationship of arterial venous flow determining difference does not exist |
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what is the starling resistor?
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named after rubber tubes which collapse if chamber pressure exceeds downstream pressure
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what happens to capillary recruitment in zone 2?
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increases
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what is zone 3 of lung perfusion?
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Pa > Pv > PA
flow determined as usual |
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how does blood flow increase in zone 3?
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chiefly by distention
internal pressure increases down the zone while alveolar pressure is constant |
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what is zone 4 of lung perfusion?
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low lung volumes
extraalveolar vessel resistance becomes important -> base of lung gets less blood flow as less expanded |
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what is hypoxic pulmonary vasoconstriction?
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contraction of smooth muscle in arteriole walls localised to areas of low alveolar PO2 independent of central nervous or respiratory connections
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when is hypoxic pulmonary vasoconstriction important? (3)
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is released at birth (present in utero) to transition from placental to air breathing
directs blood flow away from diseased lung in adult generalised @ high altitude |
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why is water balance important in the lung?
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critical to keep fluid out of alveoli or gas exchange would be impaired
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what relationship does water balance in the lung have to starling's law?
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obeys it - capillary hydrostatic pressure pushes water out & colloid osmotic pressure pulls it in
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what happens when fluid leaves the pulmonary capillary at low pressures?
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enters the interstitial space -> perivascular/peribronchial spaces & travels to hilar lymph nodes via numerous lymphatics in perivascular space
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what happens to fluid balance in late pulmonary oedema?
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fluid crosses the alveolar epithelium into the alveoli and is actively pumped out by a Na/ATP pump
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what is early pulmonary oedema in terms of fluid balance?
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engorgement of perivascular/peribronchial spaces = interstitial oedema
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what are the non gas exchange functions of pulmonary blood flow? (2)
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- reservoir for blood when supine
- blood filtration (small thrombi, WBCs) |
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what are the general metabolic functions of the lung? (6)
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1. gas exchange
2. metabolism of blood borne substances 3. blood clotting 4. immunoglobulin secretion (IgA) 5. synthetic 6. carbohydrate metabolism |
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why is the lung good for metabolising blood borne substances? (2)
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only non heart organ that receives the whole circulation & contains a substantial proportion of the body's vascular endothelium
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what is the prototype lung metabolising function?
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angiotensin I converted to angiotensin II via AVE
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what are some other modifying metabolic functions of the lung?
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removes serotonin/NAd
metabolites of arachidonic acid almost completely removed (prostaglandins E2 & F2 alpha; leukotrienes) |
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how is the lung involved in blood clotting?
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mast cells in interstitium contain herpain
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what are the synthetic functions of the lung? (2)
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phopholipids in surfactant
proteins (collagen & elastin) for lung |
