4 V/q matching pulm circ Flashcards
definition of ventilation? perfusion?
V = rate at which gas enters or leaves the lung. P = process of delivery of blood to a capillary bed
ventilation perfusion ratio definition? abbreviation?
V/Q ratio: measurement used to assess efficiency and adequacy of matching of these two variables in the lung
why is V/Q matching important
ideally, oxygen provided via ventilation would be just enough to saturate the blood fully = ideal V/Q ratio would be 1
in typical adult, 1 L blood has how much oxygen? how much in air? what is V/Q ratio?
200 mL/oxygen in blood, 210 mL in dry air –> but with humidified air the ideal V/Q ratio would be 1 (actually in reality is 0.8 ish)
is ventilation uniform throughout lung? why?
not uniformly distributed: imbalance because intrapleural pressures different in different regions. most blood flow goes to lower lung, so bases heaver than apices = lung pull downward, intrapleural pressure at top is more negative than at bottom (stretched open more at top)
apical alveoli vs. at base: size?
bigger aka more fully expanded at the top, since there is more negative pleural pressure
how does intrapleural pressure change from top to bottom? w/ breathing?
top most negative, then gets less negative at bottom. breathe in = 3 cm H2O more negative
where is ventilation greatest? because ventilation is proportional to?
ventilation higher at base of the lung. v = proportional to change in volume of alveoli (alveoli at bottom can expand more with less pressure change but at top are already expanded)
is perfusion uniform throughout lung? why?
no: pulmonary arterial pressure increases from apex to base –> from top to hilum, 11 mmHg more, then at bottom 11 mmHg more than at hilum (column of blood 15 cm high exerts hydrostatic pressure of 11 mmHg)
pulmonary venous pressure is also dependent on?
vertical height: so lowest at top, then gets more and more at bottom
perfusion equation
Q = delta P / R –> amount of flow, aka perfusion, = pressure gradient divided by resistance
describe flow from top to bottom of lung using equation
top no flow, because pressure favoring collapse > pulm arterial/inflow pressure –> arterioles collapse. going down, pulm. arterial pressure goes up and overcomes collapsing pressure = get flow. And you get more and more flow going down the lungs.
in order for blood to flow through any vessel, what must be greater than what?
inflowing pressure > pressure favoring vessel collapse
in normal lung, how much “collapsing” pressure is there?
there is a surrounding pressure of ~4 mmHg exerted on the small precapillary vessels (probably from elasticity of arteriolar smooth muscle)
why is there a small drop in perfusion at the very bottom of the lung
venous + arterial pressures so high that fluid leaks out into interstitial = increased resistance to blood flow (recall Q = delta P /R)
