2.3 - mechanics of breathing Flashcards
what is compliance?
stretchiness of the lungs
what is compliance defined as?
the volume of change per unit pressure change (emphasis on pressure change)
when is compliance increased?
during emphysema - alveoli air sacs permanentely stretched
how is compliance decreased?
during lung fibrosis - loss of elastin in alveoli wall so alveoli can’t stretch
even with constant elasticity of lung structures, what will compliance also depend on?
the starting volume from which it is measure:
specific compliance = volume change per unit pressure change / starting volume of lung
where do the elastic properties of the lungs arise from?
- elastic tissues in the lungs
2. surface TENSION forces of the FLUID lining the alveoli
where is surface tension found?
the airways and alveoli of the lungs are lined with a film of fluid which is increased in area as the lungs expand
this increase in area is opposed by ST of the lining fluid (VdW forces)
what does an area of a gas-liquid interface always tend to?
a minimum
what does the alveolar lining fluid contain?
a surfactant whose effect is to reduce tension forces
what is surfactant?
a complex mixture of phospholipid & proteins, with detergent properties
where do the hydrophilic and hydrophobic ends of the surfactants lie?
hydrophilic ends in alveoli fluid (facing fluid)
hydrophobic towards gas (away from liquids - phobic)
what does the orientation of the surfactants result in?
surfactants float on the surface of the lining fluid, interspersed between the fluid molecules, disrupting interaction between surface molecules (reduce VdW) and so reducing surface tension
what does surface tension of the alveolar fluid vary with?
the surface area of the alveolus
what happens as the alveolus expand?
its surface area increases & the surfactant molecules are spread further apart, making them less efficient in reducing surface tension
as alveolus expands, surface tension of fluid lining it increases
what happens as an alveolus shrinks?
the surfactant molecules come closer together increasing their conc on surface - therefore more efficient in reducing surface tension
effect of surfactant is to reduce surface tension forces greatly as area of the alveolus decreases
what is the force required to expand small alveoli?
less than force required to expand large ones
due to less surface area as surfactant as more closely dispersed
what is the property of surfactant also to serve?
to stabilise the lungs, by prevent small alveoli collapsing into big ones
which law are the alveoli determined by?
law of Laplace:
pressure = (2 x ST) / radius of alveolus
(P=2T/r)
alveoli - like interconnected series of bubbles
what happens if T is constant to smaller alveoli?
smaller radius, so higher pressure
higher pressure will empty into larger alveoli with lower pressure , so smaller alveoli will collapse
alveoli vary in size, so if surface tension is constant, what would happen?
the alveoli would collapse to form a huge air filled space ‘bullae’
what would happen if a bullae formed?
hughly reduce surface are for gas exchange
why do bullae not form in lungs?
as alveolus expands, increasing radius
surfactant molecules are spread further apart, making them less efficient, thereby increasing the surface tension ‘T’, so..
as alveolus expands, r & T increases
alveolus shrinks, r & T decreases
what does surfactant mean in relation to bullae and lung alveoli?
different sized alveoli can have the same pressure within them
stabilises lungs by preventing small alveoli from collapsing into big ones
what are the 3 functions of the surfactant?
- increase lung compliance by reducing surface tension
- stabilises the lungs, preventing small alveoli collapsing into big ones
- prevents the surface tension in alveoli creating a suction force tending to cause transudation fluid (pulling fluids from) pulmonary capillaries
what is respiratory distress syndrome?
lung surfactant is absent from alveoli of a foetus younger than about 30 weeks (sometimes absent from full term babies) if born premature, lack surfactant
so ST of alveolar sacs is high - leading to increase tendency of alveoli collapse
what are typical signs of respiratory distress?
cyanosis, grunting, intercostal & subcostal recession
what is the treatment for RDS?
surfactant replacement via endotracheal tube, and supportive treatment with oxygen and assisted ventilation
what else happens aside from work done against the elastic nature of the lungs?
energy must be expended to force air through the airways
what is resistance of airway to flow determined by?
Poiseuille’s Law (when flow is laminar) - true of most airways of lungs
resistance = pressure / rate of flow
(radius^4 increase, resistance decrease)
when does resistance of a single tube sharply increases?
with falling radius (terminal bronchioles etc.)
however, combined resistance of small airways is normally low because they are connected in parallel over a branching structure
where is total resistance to flow higher?
total resistance to flow in downstream branches is less than upstream branch
most of resistance to breathing therefore resides in upper respiratory tract, except when small airways are compressed during forced expiration (resistance increases significantly)
what is asthma and COPD characterised by?
airways narrowing
what happens in asthma?
inflammatory mediators (histamine?) released as a result of hypersensitivity reaction, cause contraction of bronchial smooth muscles and inflammatory swelling of the mucosa –> leading to small airways
what happens in COPD?
the host response to inhaled cigarette smoke and other toxic substances cause chronic inflammation and oxidative injury
how are the airways narrowed in COPD?
- excessive mucus in lumen
2. breakdown of elastin leading to destruction of alveolar walls, causing loss of ‘radial traction’
what is radial traction?
the outward tugging action of the alveolar walls on the small bronchioles which holds them open
why is there excessive mucus in lumen in COPD?
from increased mucus production and reduced clearing of mucus due to ciliary dysfunction
overall, what is work done against in lungs?
work is done against the elastic recoil of lungs and thorax, greatest part of work being against more or less equal to:
- elastic properties of lung tissues
- surface tension forces in alveoli
how much effect does resistance to flow through airways have?
of little significance to total work load of breathing in healthy subjects, through it can often be affected by disease
at rest, the work of breathing consumes how much oxygen consumption?
only 0.1% of total O2, so efficient
