Lec 32 Respiratory mechanics Flashcards
What are the two major forces to overcome to inhale
- elasticity: lungs and chest wall resist stretch, passively return to equilibrium after expanded or compressed
- resistance: viscosity of inhaled air, flow dependent airway resistance
What two proteins are responsible for the elastic properties of lungs?
- collagen
- elastin
What two forces oppose each other in lung to keep it in equilibrium
lung tissue elastic recoil = force inward wanting to collapse
chest wall = exerts force outward wanting to expand
What is compliance? its relationship to elasticity?
- measure of stiffness, an intrinsic property of lung or chest wall that depends on amount of elastic tissue
- measured as change in volume per unit change in pressure
- compliance is inversely correlated with elastance
What is the equation for compliance?
Compliance = ΔV/ΔP
How is airway pressure measured?
- in terms of atmosphere pressure [zero]
- above Patm is positive
- below Patm is negative
What is transmural pressure?
- pressure across the aloveolar wall
- transmural P = intra-alveolar P - extra-alveolar P
What is extra-alveolar pressure?
intrapleural pressure – the space between lungs and chest wall
How to measure lung compliance experimentally?
- excised lung placed in jar
- space outside lung mimics intrapleural pressure
- vacuum pump simulated changed in intrapleural pressure
- vol of lung measured by spirometer
- lung inflated by increasing negative pressure outside
- lung deflated by reducing negative pressure outside
- slope of P vs V is compliance
What is hysteresis? what causes it?
- difference in pressure required for inflation compared to deflation
- lung is more compliant during expiration than inspiration [high slope, bigger volume for same pressure]
- caused by surface tension at liquid-air interface
Is lung more compliant during expiration or inspiration?
expiration
What is law of laplace for alveoli?
P = 2T/r P = collapsing pressure on alveolus T = surface tension r = radius
Does a smaller alveolus or larger alveolus have higher collapsing pressure?
smaller alveolus
What is surfactant?
- reduces surface tension and thus collapsing pressure at air-liquid interface
- its an amphipathic phospholipid - one end hydrophilic and one end hydrophobic
- improves lung compliance
2 consequences of surfactant reducing surface tension
- stabilizes alveoli against collapse
- promotes compliance –> easier expansion/inspiration, less work to breath
What is the equation for respiratory system compliance
C respiratory system = C lung + C chest wall
How does compliance of respiratory system compare to that of chest wall and lung?
- chest wall and lung have equivalent compliance [same slope]
- combined compliance of respiratory system is less that of other structure alone
What is FRC?
- functional residual capacity
- the resting/equilibrium volume present in lung after normal breath exhaled
- FRC = ERV + RV
What is residual volume [RV]?
x
What is expiratory reserve volume [ERV]?
x
What is tidal volume [TV]?
x
What is inspiratory reserve volume [IRV]?
x
What is vital capacity [VC]
x
What is total lung capacity [TLC]?
x
Is normal intrapleural pressure negative or positive relative to atmosphere? what is it?
- negative relative to atm
- usually -3 mmHg
What does emphysema do to lung compliance? How?
- increase in lung compliance [higher slope]
- due to loss of elastic fibers
What does fibrosis do to lung compliance? how?
- decreases lung compliance [shallower slope]
- due to excessive collagen
What is the equation for flow from resistance?
Flow = ΔP / R
ΔP = pressure difference between mouth/nose and alveoli R = resistance = 8ηl/πr^4
What is poiseuille’s equation for resistance?
R = resistance = 8ηl/πr^4
What is the site of highest resistance?
medium sized bronchi
True or false: the smallest airways have the highest resistance
false: because even though really small airways so each will have individual high resistance they are arranged in parallel
What innervates airway smooth muscle?
- B2 sympathetic receptors: cause bronchial smooth muscle relaxation and bronchodilation
- M parasympathetic receptors: cause arrow narrowing and bronchospasm
What two things determine airway resistance
- lung volume
- viscosity of inspired air
What are two ways airway diameter can be altered? what consequences resistance?
- loss of surrounding lung parenchyma: loss of radial traction on airway –> airway narrowing, increased airway resistance. ex. emphysema
- pulmonary resistance increases radial traction, get larger airway diameter at given lung volume and decreased airway resistance
Why does lung volume determine airway resistance?
- airways are tethered by surrounding lung tissue
- airway diameter varies with lung volume
What is heliox? use?
- helium oxygen mixture, alters air viscosity
- used for asthmatics in distress
What is path of inspiration?
- diaphragm contracts
- abdominal contents pushed down
- ribs lift up and out
- get increased intra-thoracic V and thus lower intra-thoracic P causing air to rush in
What happens to intra-thoracic V and P to cause air to rush in during inspiration?
- higher intra-thoracic V
- lower intra-thoracic P
What is path of expiration?
- passive proccess
- everything reverses as diaphragm comes back, ribs move down and in
What helps inspiration in exercise?
- inspiration assisted by external intercostals and accessory muscles
What helps expiration in exercise and asthma?
- expiration assisted by internal intercostals and abdominals
What are the 2 muscles of active expiration?
- internal intercostal muscles
- abdominal muscles
what are the 2 major muscles of inspiration?
- external intercostal muscles
- diaphragm
what are the 2 accessory muscles of inspiration?
- sternocleidomastoid
- scalenus
What is alveolar pressure at rest?
alveolar p = atmospheric pressure = 0
what is intrapleural pressure at rest?
negative
-5 cm H2O
What is transmural pressure at rest?
trasnmural = intra-alveolar - intrapleural = 0 - -5 = +5 cm H2O
What is significance of positive transmural pressure at rest?
- positive transmural pressure means expansion
- ensures alveoli stay open
- if transmural P were negative, it would be a collapsing pressure
What is alveolar, intrapleural, and transmural pressures at rest?
alveolar: 0
intrapleural: -5
transmural: +5
What is alveolar, intrapleural, and transmural pressures in middle of inspiration?
alveolar: -1
intrapleural: -6.5
transmural: +5.5
What is alveolar, intrapleural, and transmural pressures in end of inspiration?
alveolar: 0
intrapleural: -8
transmural: +8
What is alveolar, intrapleural, and transmural pressures in middle of expiration?
alveolar: +1
intrapleural: -6.5
transmural: +7.5
What is alveolar, intrapleural, and transmural pressures at end of expiration?
End of respiration = rest state
alveolar: 0
intrapleural: -5
transmural: +5
What is significance of alveolar pressure = 0 at end of inspiration?
means you have no flow.
When is intrapleural pressure most negative?
at end of inspiration
What happens to pressures in forced expiration?
- get really high intra-alveolar pressure
- net positive transmural pressure
What happens in COPD in forced expiration?
- get not as high alveolar pressure in the part thats obstructed so get negative transmural pressure there
