breathing mechanics: static forces Flashcards
what are static mechanics
mechanical properties of the lung that influence gas flow but which are independent of volume change
elasticity
compliance
surface tension
what is the pleural sac
the pleural sac links elastic forces in the chest wall and lung
what is elasticity
elastic forces link pleural pressure to alveolar pressure
elastin in alveoli acts as an inward ‘collapsing’ force - elastic recoil
opposing elastic recoil of ribs acts as an outward expanding force
balance of forces results in a sub-atmospheric intrapleural pressure
this also shows the effect of gravity and posture
what happens to elasticity during quiet breathing
intrapleural pressure is always sub-atmospheric (0)
elastic retraction enables alveolar pressure to go above atmospheric
what is compliance
relationship between the change in volume and the change in pressure
compliance = Δvolume / Δ pressure
static compliance is measured under conditions of no gas flow
what does a high compliance lung mean
lungs with high compliance is easily distended
low elasticity
what does a low compliance lung mean
lung with low compliance is difficult to distent
high elasticity
what is normal compliance
c = 0.5l/2.5cmH2O = 0.2l/cmH2O
what is compliance with emphysema and COPD
c = 1.0l/2.5cmH2O = 0.4l/cmH2O
what is compliance with fibrosis/ idiopathic pulmonary fibrosis
c = 0.25l/2.5cmH20 = 0.1l/cmH2O
how is pulmonary compliance/elastic recoil produced
- elastin connective fibres
- alveolar surface tension
what is surface tension
molecules at the surface experience fewer favourable interactions than those in the bulk because the surface molecules can only interact with other molecules in 2-dimensions whereas those in bulk solution interact in 3-dimensionsw
what is the surface tension in an alveolus
resits stretch (greater surface tension = less compliance)
tends to become smaller
tends to recoil after stretch - contributes to elastic recoil pressure
what is the effect of surface tension on lung compliance
inflating with liquid overcomes recoil effect by dissipating surface tension
lung requires greater trans-pulmonary pressure to inflate with air in order to overcome surface tension and elastic recoil effect
what is the effect on exhalation of surface tension and hysteresis
less energy expended on exhalation due to elastic and surface tension recoil
breathing costs more energy on one loop of the ventilation cycle than the other
this relationship shows hysteresis, revealed as a lag phase due to the surface tension effect during inhalation
what is the effect on inhalation of surface tension and hysteresis
surface tension can be strong
input of energy needed to overcome surface tension (intercostals/diaphragm)
less energy required to inflate lung after this
what is the law of laplace
small diameter bubbles have higher surface tension than large diameter ones
variation in alveolar size/volume would cause small alveoli to collapse into larger ones
pressure in buble = 2x surface tension / bubble radius
P = 2T/r
what is pulmonary surfactant
stabilises alveolar structure by reducing surface tension
- decreases density of water molecules at air-water interface
- the hydrophobic tail pulls the surfactant molecule upward
what is pulmonary surfactant composed of
dipalmitoyl phosphatidyl choline (DPPC) packaged around surfactant proteins (A-D)
where is pulmonary surfactant secreted
sectreted by type II alveolar epithelial cells to reduce surface tension in alveoli
what is the mechanism of action of pulmonary surfactant
- prevents collapse of alveoli during lung expanstion and contraction
- reduces pressure required to inflate lungs
- effect on surface tension varies with alveolar surface area - as radius falls, surfactant molecules crowd together, surface tension is reduced, smaller alveolus stabilised
* alveolus also stabilised by mechanical interactions between neighbouring alveoli to prevent alveolar collapse
