Respiratory System: Mechanics of Breathing Flashcards
Define airway resistance
Airway resistance = the force generated by gas molecules colliding with the airway surfaces, which opposes air flow.
What happens if airway resistance increases?
More resistance slows down flow. This can be overcome by increasing pressure gradient, but this has physical limitations, e.g. you can’t generate the force required to do so, or the airway might be completely obstructed.
If airflow is proportional to resistance, factors that then determine the level of resistance are…
..the cross sectional area of the airway lumen and the airflow pattern.
The Hagen-Poiseuille eqn shows that small decrease in radius will produce a large increase in resistance (and so dramatically decrease airflow).
What factors reduce the size of the airway lumen?
- Contraction of airway smooth muscle
- Oedema/swelling of the airway tissue
- Damage to the integrity of the airways structure (loss of patency)
- Excessive mucus secretion
These all reduce airway lumen size, increasing resistance and decreasing airflow
How does airflow pattern change airway resistance?
When airflow changes from a linear to a turbulent pattern, airway resistance increases.
Turbulence occurs in high velocities of airflow (e.g. during forced breathing manoeuvres) or in obstructed airways.
The vibration from turbulent airflow produces wheezing.
What is airway patency? What maintains it?
Airway patency (openness) also determines the degree of airway obstruction (‘loss of patency’ =obstruction) Elastic fibres within the airway wall and radial traction maintain patency.
Why is airway obstruction more noticeable during expiration?
As lungs expand during inspiration, the lung tissue and airways are stretched upon. During expiration, the lung tissue and airways are compressed. This explains why airway obstruction is more noticeable during expiration.
What else can reduce airway patency?
When intrapleural pa becomes positive (eg during forced expiration), collapsing force is exerted onto the airways, reducing patency.
In healthy individuals, the structural airway integrity prevents collapse, but is problematic in diseases w impaired airway structure (e.g. COPD)
Define lung compliance and trans pulmonary pressure
A change in transpulmonary pressure produces a change in lung volume. This relationship= lung compliance. It describes how easily the lungs can be distended.
Transpulmonary pa= Pa difference between the alveoli and intrapleural space. It determines the level of force to expand or compress the lungs.
What does it mean if you have higher or lower lung compliance?
Higher lung compliance = less elastic recoil which means less force required to inflate the lungs. ↑ volume change per pa change (↑gradient on volume-pressure curve)
Lower compliance = more elastic recoil, so more force required to inflate the lungs. ↓volume change per pa change (↓ gradient on volume-pressure curve)
Use this graph to explain the difference between static and dynamic compliance
The steeper the curve, the higher the lung compliance. This is bc a more compliant lung needs less force acting on it to expand in volume.
Static compliance is when the patient stops inspiration briefly so that airflow falls to 0. To determine dynamic compliance a patient breaths normally at tidal volume. Dynamic compliance=gradient of the line from the end of expiration to the end of inspiration.
Using a graph, explain the effect of fibrosis and emphysema on lung compliance
Emphysema= degradation of elastin fibres making the lung less stiff and more complaint (but reducing recoil). Pulmonary fibrosis= scarring and deposition of structural fibres such as collagen, making the lung stiff and less compliant.
Why do alveoli generate inward collapsing pressure?
Alveoli are lined with fluid to enable gas exchange (the gas molecules dissolve into water before diffusing.)
H bonds between water molecules create surface tension, exerting a collapsing force toward the centre.
The collapsing forces generates pressure. The amount in a specific bubble is described by the Law of Laplace (see image)
Unless sufficient force resists this collapsing pressure, the alveoli will collapse and fail to inflate.
Smaller radius generates greater pressure within alveoli. So what would happen if 2 different sized alveoli were connected via airways?
Seria pressure gradients between the alveoli, so the smaller alveoli would empty into larger ones. This would make lung inflation extremely difficult
This is resolved by pulmonary surfactant, a phospholipoprotein secreted by type II pneumocytes (alveolar cells).
What is pulmonary surfactant? How does it reduce surface tension and collapsing pressure?
Pulmonary surfactant is a phospholipoprotein secreted by type II pneumocytes (alveolar cells).
Surfactant molecules disrupt the attractive forces between water molecules, reducing surface tension and collapsing pressure.
Draw a flow chart demonstrating the pathophysiology of neonatal respiratory distress syndrome
How does compliance affect pressure-volume loops?
Compliance= the gradient of the overall line from end of expiration to end of inspiration.
How does airway resistance affect pressure volume loops?
Airway resistance= loop area/fatness. It increases w forced in/expiration and airway obstruction. It decreases w slow inspiration and no airway obstruction.
What is the alveolar gas eqn?
