respiratory mechanics Flashcards
mechanics: explain the mechanical relationship between the chest wall, pleural membranes and the lung; distinguish the mechanical forces involved in tidal and maximal ventilation, and fluid- and air-filled lungs
chest wall relationship with lung
chest wall has tendency to spring outwards while lung recoils inwards
when are forces in equilibrium
end-tidal respiration (neutral position of intact chest)
inspiration: relationship with muscle effort and chest recoil vs lung recoil
inspiratory muscle effort + chest recoil out > lung recoil in
expiration: relationship with muscle effort and lung recoil vs chest recoil
expiratory muscle effort + lung recoil in > chest recoil out
what pleura are lungs surrounded by
visceral pleura
what pleura is chest wall covered in
parietal pleura
what is in between the two pleuras
pleural cavity and fixed-volume fluid, with a double-folded layer to allow both surfaces to work together. meaning 2 pressure gradients (pleura and alveoli, which affects size of alveoli, and alveoli and atmosphere, which affects direction and rate of flow)
mechanics of ventilation: inspiration
diaphragm contracts to flatten, while ribs move upwards and outwards → parietal pleura pulled away from visceral → wider space so lower intrapleural pressure → pressure gradient between alveoli and plerual space widens, so alveoli inflate → intralveolar pressure drops → air flow into alveoli from atmosphere
Palv, Patm and Ppl at rest (cmH2O)
0, 0, -5
P[TT; intrapleural space - transthoracic atmospheric], P[TP; intra-alveolar - intrapleural] and P[RS; intra-alveolar - respiratory atmosphere] at rest (cmH2O)
-5, 5, 0
pressure/volume relationships (sigmoid-shaped graph)
at no external pressure from intercostals and diaphragm, small change in pressure causes large change in volume; at extremes, a large change in pressure required to effect a change in volume
forced exhalation pleural pressure
increase in pleural pressure to -2 cmH2O, due to inward muscle force being larger than outward recoil force
forced inhalation pleural pressure
increase in negative pressure (becomes more negative) to -8 cmH2O, due to outward muscle force being larger than inward recoil force, leading to pulling apart of the pleura
effect of restrictive respiratory disease on pressure/volume relationships (sigmoid-shaped graph)
squashed reduced vital capacity so lower on graph; stretched with shallower curve because more effort to move air in; chest wall:lung interface less compliant
effect of obstructive respiratory disease on pressure/volume relationships (sigmoid-shaped graph)
operates at higher volumes so higher on graph; smaller vital capacity as tissue is more compliant, so squashed with steeper curve
mechanical forces involved in tidal breathing
75% due to diaphragm contraction and 25% external intercostals with passive recoil
mechanical forces involved in maximal ventilation
accessory muscles and internal intercostals affect much larger change in volume, and hence gas exchange
mechanical forces involved in fluid-filled lungs (surfactant)
fluid-water interface increases compliance (surface tension), so fluid-filled lungs expand under greater pressure
mechanical forces involved in air-filled lungs
lack of fluid-water interface decreases compliance (no surface tension), so much larger change in pressure needed to inflate lungs
mechanics of ventilation: expiration
diaphragm relaxes, while ribs move downwards and inwards → parietal pleura rejoins visceral → narrower space so higher intrapleural pressure → pressure gradient between alveoli and plerual space narrows, so alveoli deflate → intralveolar pressure increases → air flow out of alveoli into atmosphere
