breathing mechanics: dynamic forces Flashcards
what are dynamic mechanics
mechanical properties affecting flow of air into and out of lung as volume changes with time
resistance
flow
turbulence
what is poiseuilles law
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airway resistance in the trachea
1 airway
2.5cm^2 total corss-sectional area
100% airflow resistance factor
airway resistance in the 10th generation bronchi
1024 airways
13cm^2 total cross-sectional area
3% airflow resistance factor
airway resistance in the 17th generation bronchi
23360 airways
300cm^2 total cross-sectional area
0.9% airflow resistance factor
where does the most airway resistance occur
90% resistance to flow is in upper airway
what factors determine resistance
- lung volume and branching
- bronchiolar smooth muscle tone
- density and viscosity of gases
- affected by altitude
- important consideration in artificial environments
- laminal flow is a function of gas viscosity
- turbulent flow depends on gas density
what is bronchiolar smooth muscle
lines the upper airway
alterations in tone alter radius
major role to play in ventilation-perfusion matching
lung disease linked to resistance; e.g. asthma and COPD
how is bronchiolar smooth muscle radius decreased
by muscular contraction
parasympathetic activity
Ach neurotransmission
irritants - smoke, allergens
decreased alveolar pCO2
how is bronchiolar smooth muscle radius increased
by muscular relaxation
sympathetic activity
pulmonary stretch
NAdr (neural, weak)
Adr (β2, circulating, strong) - and salbutamol
what is the statistics of asthma
5.4 million sufferes in the UK
1.1 million are children
distinct genetic and dietary causes
how does asthma result in resistance
allergic inflammation of the airways
hyper-reactivity of airway smooth muscle contraction
bronchial smooth muscle thickening
what is the mechanism of action behind asthma and resistance
contraction of parabronchial smooth muscle raises upper airway resistance
covective gas flow in lungs much less efficient
gas flow slows to the pCO2 to rise
lung ceases to oxygenate Hb efficiently due to reversal of proper alveolar bohr and haldane effects
what is the asthmatic wheeze
flow resistance linked to turbulence
laminar flow = ΔP = flow rate x r
- ΔP is directly proportional to flow
complicated by dichotomous branching
reynolds number = ΔP is directly proportional to flow^2
how is airway resistance measured
by whole body plethysmography
R = (Pmouth - Palveolus)/flow
what is airway resistance during inhalation
AWR falls as gas flow and lung volume increases
as lungs expand airways are also physically widened
dynamic lowering of resistance during lung expansion
as the lung expands, the increasing tension in alveolar walls pulls conduction airways open
what is airway resistance during exhalation
the resistance of the upper airway helps to keep airway pressure high to maximise gas movement out of alveoli
at start of exhalation large amount of flow for little volume change
exhalation works against - smooth muscle contraction and upper airway resistance
what is the impact of the upper airway on airway resistance
aids gas clearance from alveoli
what is COPD
increased resistance to air flow
exhalation phase fails - much greater resistance to air flow during exhalation - breathing out is hard work
what is the ‘blue puffer’
- alveoli destruction
- reduced surface area
- reduced elasticity
patient oxygenation if poor
what is work of breathing
to overcome elastic and non elastic resistances
work = force x distance
= pressure x volume
what is the quiet breathing cycle
inspiration - energy input required to overcome
a) elastic component
b) surface tension
expiration - energy input required to overcome airway and tissue resistance
