Lecture 3 - Mechanics of breathing 2 Flashcards Preview

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What is functional residual capacity

It is the volume of air present in the lungs after passive expiration. opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles. usually around 2500 ml.


What is compliance

how distensible the lung is


What effects inflation/ distensibility of the lung

tissue elastic forces (collagen and elastin) and surface tension. increase in either would make it harder for the lungs to inflate


What is surface tension

forces in alveoli that try to collapse the bubble


What is elastence

property of


Relationship between introthoracic pressure and lung volume during tidal breathing

during inspiration, the intrathoracic pressure becomes more negative as lung volume increases
during expiration, intrathoracic pressure increases (becomes less negative) and volume decreases.


What would happen in a perfect elastic system

there would be a linearity in the curve


What causes the hysteresis loop? what does it define

the elastic resistance of the tissues - collagen, elastin etc
it defines the work required to actually breathe



takes energy from outside to increase energy of the machine.
difference between heat produced when you expand elastic band and the heat released when you contract it is hysteresis.
as you breath out passive process, as you breathe in active process.
difference in heat is hysteresis.


Why does hysteresis occur?

because of the elastic nature of the tissues


What is compliance

the elasticity of the lungs. Compliance is defined as the change in volume/the change in pressure


How do you measure compliance?

it is dynamic not static.
measure spirometry for volume.
oesophageal balloon for pressure. place it in intrathoracic compartment. these values a relative (changes in pressure)


What happens in a lung compliance curve during expiration

during expiration
alveolar pressure is 0 and pleural pressure is -3.
there is a negative pressure that holds your lungs out and stops them from collapsing
recoil pressure is +3


What happens in a lung compliance curve at the end of expiration

as lungs start expanding outwards, the negative pressure in the lungs start increasing and alveolar pressure becomes -5. recoil pressure( pressuring opposing it) is +5. not much increase in volume


What happens in a lung compliance curve at peak inspiration

as you take a deep breath in, the recoil pressure of the lungs increases by 30. takes you up to total lung capacity. chest wall moves out, diaphragm moves out, intrapleural pressure goes to about -30.


How is the compliance curve formed

the links between the various recoil pressures between expiration and inspiration forms the curve. from high to low compliance


High compliance is?

for a little change in pressure you get quite a lot of change in volume


Low compliance

Massive amounts of change in pressure for a change in lung volume


Chest wall compliance

has its own compliance
wants to spring outwards.
pressure exerted by the chest wall


after maximal expiration

chest wall has a recoil pressure of -30. glottis is closed, muscles are relaxed


as you start to breathe in

small change in volume, open glottis, muscles still relaxed
large change in pressure
-30 to -5 when only 25% of expansion has occurred


during maximal inspiration,

large change in volume, close glottis, relax muscles
however small change in pressure from -5 to -3. high compliance lung. lung finding it harder to expand requires more pressure.


chest wall curve and lung compliance curve

superimpose on eacother


How do you determine FRC from a compliance curve

FRC is the relaxation point of the resp system when chest wall and lung pressures are equal but opposite (eg. -5 and +5) There is a neutral point on the curve that indicates it. defines end of quiet breathing.
lungs are in equilibrium.


During anesthesia, patients lungs collapse to what point?

so that they are in equilibrium, at FRC neutral point.


Which diseases reduce compliance

Pulmonary fibrosis
elastic tissue is replaced with fibrous tissue - more rigidity.
needs a lot of increase in pressure for a little change in volume.


What is kyphoscoliosis

deformed chest wall. difficult to increase and decrease the lung. decreases lung compliance.


circumferential thoracic burn

skin when burnt loses elasticity and effects compliance as it makes it difficult to breathe.


Diseases that increase compliance

destroys actual structure of lung from inside. left with air spaces and pockets and so there is reduced elasticity. thoracic outward forces take over, massive lung expansion.
lost all alveoli so become hypoxic.


ventilation and blood at different parts of the lung

role of gravity.
top of lungs - lots of ventilation
not much blood. middle of lung, some ventilation and some blood. at the bottom, lots of blood (due to gravity), little ventilation