Lecture 1: Lung mechanics Flashcards
How much is the decrease in intrapleural pressure during inspiration?
Usually -2.5 -6 mmhg
At what point during breathing are pressures equal?
At the end of inspiration
What is the transpulmonary pressure?
The chest wall exerts a distending pressure on the pleural space, which is transmitted to the alveoli to increase its volume, lower its pressure, and generate airflow inwards
Which pressure is always positive?
Transpulmonary pressure
Which pressure is always negative?
Intra-pleural pressure
What happens to alveolar pressure during breathing?
Moves from slightly negative to slightly positive
What muscles are used during inspiration?
Diaphragm (75% change in volume)
External intercostals
- bucket-handle
Accessory muscles:
- scalene
- sternomastoids
What is compliance?
Static measure of lung “stretchiness”
Volume change per unit pressure change
What is the balance point (where the lung and chest are in equilibrium, after exhaling)?
Functional Residual Capacity (FRC)
What are the endpoints of breathing?
Total lung capacity (TLC) and residual volume (RV)
What happens to the compliance of the lung at higher volumes?
It decreases
Describe normal lung tissue in terms of compliance.
Compliance is just right.
Good compliance for low work of inhalation, and good retention of elasticity of alveolar units to allow effective exhalation.
Describe interstitial fibrosis in terms of compliance.
Compliance is decreased due to more stiff alveolar walls from scarring (called fibrosis).
Difficult to inflate.
Describe emphysema in terms of compliance.
Compliance is increased due to loss of alveolar interdependence.
Difficult to exhale.
What is surface tension?
Cohesive forces between molecules
Molecules on the surface have no atoms above them
Results in stronger attractive forces on nearest neighbours on the surface
Liquid surface area becomes as small as possible i.e. sphere
Tends to collapse the alveolus
Surface tension increases with
- Emphysema
- Age
What is the main component of pulmonary surfactant?
Dipalmitoyl phosphatidylcholine (DPPC)
How does surfactant reduce surface tension?
Hydrophilic and hydrophobic ends repel each other and interfere with liquid molecule attraction
Why is surfactant needed?
Increases lung compliance
- because surface forces are reduced
Promotes alveolar stability
Prevents alveolar collapse
- small alveoli are prevented from getting smaller
- large alveoli are prevented from getting bigger
Surface tension tends to suck fluid from capillaries into alveoli
- reduction of surface tension reduces hydrostatic pressure in tissue outside capillaries and keeps lungs dry
When are babies able to produce enough pulmonary surfactant?
Over 30 weeks
What does Laplaces law tell us?
Pressure in a spherical compartment is:
- Proportional to the tension
- Inversely proportional to the radius of that sphere
Surfactant causes surface tension (which contributes to alveolar wall tension, along with elastic recoil), to be lower in smaller spheres
What will occur when there are two alveoli, one bigger than the other, with no surfactant?
Both alveoli will have equal surface tension
The smaller will have higher pressure
The smaller is likely to collapse and be harder to inflate
What will occur when there are two alveoli, one bigger than the other, with surfactant?
The smaller will have less surface tension (more per surface area)
They will have equal pressure
The smaller will inflate at a faster rate than the bigger until they are equal in size
What is airway resistance?
Force needed to inflate lung is greater than elastic recoil
Airway resistance originates from friction between air and mucosa
Is the pressure difference between the alveoli and mouth, divided by the flow rate
Resistance = Pressure1-Pressure2 / Flow
What is laminar flow?
Smooth flow
Resistance generated is proportional to the radius (r4).
What is turbulent flow?
Irregular, chaotic, with eddie currents
Good for transferring heat (radiators try to enhance turbulent flow), but the resistance is high
What is predicted by Reynolds number?
When laminar flow converts to turbulent flow
As the airway generation increases, what happens to resistance?
It decreases
