work of Breathing Flashcards
Lung Compliance is a fancy way of saying what?
‘lung stretchiness’
or more formally, the effort required to stretch the lungs during inspiration
degradation of elastin fibers leads to what?
degredation of elastin fibers (alpha1 anitrypsin deficiency) = lungs are TOO compliant
fibrosis of lung = lungs are not stretchy enough
what kind of curves are the expiration and inspiration curve in a ‘hysteresis’ (compliance curve)
Inspiration = sigmoidal
expiration = opposite of exponential?
how do you compute the ‘work’ of the compliance curve?
by finding the area of the compliance curve - a small amount of work is normal - indicates that it doesn’t require much work to inflate the lung, however a large amount of work indicates a diseased lung
during normal ‘quiet breathing’ what sort of work is being done?
in normal quiet breathing, inspiratory muscles do all the work as expiration is passive
what does the restrictive vs. obstructive lung disease look like on a Hysteresis graph?
in obstructive disease, while compliance may be unchanged, the work of breathing is increased by the elevated airway resistance
in restrictive disease, lung compliance is low and the elastic work of breathing is increased
what is the ‘elastic work’?
What is ‘non-elastic’ work?
elastic work = force to expand lugn against its elastic properties
non-elastic= airway resistance work = force to move air through airways
(elastic forces representing 70% of the total work)
how much of our total oxygen consumption is consumed by the lungs themselves?
2-5% - it’s very low and efficient - though it increases with exercise
the higher the tidal volume, the greater the… ?
the higher the tidal volume, the greater the elastic work
the higher the respiratory rate, the higher the… ?
the higher the flow and resistive (non-elastic work)
what is the optimum combination of tidal volume and rate to minimise work of the lungs?
rate 15/min
tidal volume 500ml
the higher the tidal volume the greater the elastic work
the higher the respiratory rate, the higher the flow and resistive (non-elastic) work
what makes up the elastic work?
tissue elastic forces = elastin fibers present in the airways and alveoli contribute to the behaviour of the lungs as an elastic body
surface tension force = develops at air-liquid interfaces and pressure inside alveoli is dtermined by surface tension and alveolar radius (Laplace’s law)
what is the alveolar surface tension?
- Thin liquid film lines each alveolus
- Air-water interface
- Water molecules more attracted to each other than air (surface tension)
–Resists expansion (water molecules oppose being pulled apart)
Reduces alveolar size
what would happen if alveoli are lined with water alone?
- If alveoli were lined with water alone, the surface tension would cause lungs to collapse. (becuase as we exhale the alveoli shrink and if there was also a tension force helping them collapse, it would be very difficult to reexpand them)
- Coupled with recoil force of elastin fibres.
- Would exceed opposing stretching force of transmural pressure.
- Would require exhaustive muscular pressure to breath.
what are the two factors that oppose the surface tension of water in the lungs?
1) pulmonary surfactant (secreted by type 2 alveolar cells) 90% lipids, 10% proteins - lowers surface tension, mixture of lipids and proteins, increases compliance, reduces lung tendency to recoil and collapse - note that surfactant is produced when you’re still a fetus
2) alveoli interdependence - when alveolus in a group of interconnected alveoli begin to collapse, the surrounding alveoli are stretched/recoil in response
if you have a smaller volume of alveoli- what is the pressure like?
small volume = high pressure
large volume = low pressure
therefore in the absense of any other factors, air would move from small alveoli into larger alveoli and we would be left with less and less alveoli
*this is also why surfactant is important = it stabilizes the small alveoli
what is Laplace’s Law?
small radius = high pressure
P= 2T/r
what does surfactant do to the surface tension ? How does it impact Laplace’s law?
surace tension is no longer a constant with surfactant = the surface tension becomes smaller as the alveoli get smaller because there is a larger concentration of surfactant = therefore the small and large alveoli equal out in pressure so there is no net movement of air between the two
how does the surfactant reduce the surface tension of the surfactant?
the lipid content of it avoids the water molecules - therefore it prevents the water molecules from collapsing due to surface tension - it’s like a wall between two magnets
what is respiratory distress syndrome of newborn?
- Developing foetal lungs do not normally synthesise surfactant until late in pregnacy.
- Therefore, premature infants may not have enough pulmonary surfactant and struggle to breathe.
- Treatment:
- Stimulated by corticosteroids given to mother prior to delivery of premature infant
- Oxygen through continuous positive airway pressure
- Survanta (surfactant)
- Acute cases may require mechanical ventilator
what is alveoli interdependence?
- When an alveolus in a group of interconnected alveoli begin to collapse, the surrounding alveoli are stretched.
- Neighbouring alveoli recoil in response to being stretched, pulling outwards on the collapsing alveoli and keeping it open.
what are the functions of surfactant?
- Lowers surface tension of fluid lining alveoli
- Increasing compliance and reducing work of breathing
- Preventing collapse at low lung volumes (atelectasis)
- Allows small and large alveoli to co-exist
- Influencing regional distribution of ventilation and relationship of ventilation to perfusion
- Contributes to defence mechanisms in the lung
Enhancing immune system activity (SPA & SPD)
what is the ‘resistance to air-flow’?
•Airway resistance is the impedance of air flow through the tracheobronchial tree as a result of friction of gas molecules
how does radius effect airway resistance?
- Hagen-Poiseuille Law
- Relates flow rate (volume transported per time unit) to airway radius and to distance the air is transported
- Applies to a scenario of laminar flow in a straight circular tube.
- Note the strong influence of the airway radius!!!
*the equation condition is that the trachea is a perfect cylindrical tube*
