Mechanics of breathing Flashcards
What is the consequence of impaired airway function?
Insufficient ventilation
To reach gas exchange surfaces, what must air pass through?
A series of increasingly narrow and numerous airways
What is the rate of airflow dependent on?
Pressure gradient & level of airway resistance
What is Ohm’s law?
Airflow (V) = 𝚫 Pressure (P)/ Resistance (R)
⬆️ 𝚫P = ⬆️ Airflow
⬆️ R = ⬇️ Airflow
What is the Hagen-Poiseuille equation?
The Hagen-Poiseuille equation describes the relationship b/w resistance and the various properties of airways and airflow
When effective constants are removed, it demonstrates that resistance of a gas is inversely proportional to cross-sectional radius to the power of 4.
This means a small decrease in radius will produce a large increase in R.
What is airway resistance further increased by?
Turbulent airflow - This creates eddies, which prevents air from moving through in a laminar fashion
Turbulence occurs where high velocities of airflow are achieved, or if there is a sudden decrease in luminal area such as in obstructed airways.
The vibration generated by turbulent airflow is responsible for the wheezing sound produced in patients with obstructed airways.
What can cause airway obstruction and why do they cause obstruction?
- Loss of airway patency due to degradation of structure can cause airway obstruction. The open structure of airways is maintained by elastic fibres within wall of airway + radial traction. Pressure differentials b/w intrapleural space + airways can reduce airway patency during forced expirations. When intrapleural pressure becomes positive (forced expiration), collapsing force will be exerted onto airways and this can be problematic for those with diseases involving impaired airway structure (e.g. COPD)
- Contraction of airway smooth muscle
- Excessive mucus secretion
- Oedema/Swelling of the airway tissue
All of these reduce the size of the airway lumen, increasing airway resistance and decreasing airflow
Describe lung compliance
Lung compliance quantifies the relationship between the level of expansive force applied to the lung and the resulting change in lung volume
Lung compliance - The change in lung volume produced by a particular change in transpulmonary pressure
Compliance = 𝚫Volume/𝚫Pressure
Higher lung compliance = Less elastic recoil = Less force required to inflate = ⬆️Volume change per pressure change- This will create a steeper gradient on a V-P curve, where compliance is the gradient.
The opposite is true for lower lung compliance
What happens during inspiration in terms of pressure?
Increasing levels of negative intrapleural pressure generated as lung volume increases due to elastic properties of lung tissue.
What factors and diseases affect lung compliance?
Number of diseases that affect recoil or structure of lungs + chest wall, leading to altered lung compliance:
- Scoliosis - Affects chest wall mechanics, decreases lung compliance
- Muscular dystrophy - Affects chest wall mechanics, decreases lung compliance
- Obesity - Affects chest wall mechanics, decreases lung compliance
- NRDS - Affects alveolar surface tension, decreases lung compliance
- Fibrosis - Affects elastin fibres, decreases lung compliance
- COPD - Affects elastin fibres, increases lung compliance
How is lung compliance affected by disease? (Specifically emphysema and pulmonary fibrosis)
Emphysema (COPD) involves degradation of the elastin fibres, makes the lungs less stiff, more compliant (but reducing recoil), therefore the lungs can expand more but it also means there’s no elasticity for the lungs to compress and therefore expiration is difficult
Pulmonary fibrosis - Scarring + deposition of structural fibres such as collagen making the lung stiff and less compliant, lungs cannot expand enough
What do air-liquid interfaces (e.g. alveoli) generate and how?
Surface tension, which resists inflation.
Alveoli are lined with fluid to enable gas exchange (the gas molecules dissolve into water before diffusing. The water molecules have a tendency to form H bonds with each other, and this is what generates the surface tension, as they produce force on each other. However, the water molecules on the top of the water layer don’t want to react with air, and so they generate a net force downwards (due to the fact that the H bonds it forms with molecules on its left and right will cancel out), and this makes the water layer get thinner, and a collapsing force is generated towards the centre. As the layer gets thinner, it creates surface tension with the air, and as the tension increases it ‘pulls’ on the alveoli, this is the collapsing force.
The Water-air interface formed b/w the lining fluid and alveolar airspace creates a ‘bubble’
Within the bubble formed by the water-air interface, surface tension arises due to H-bonds between the water molecules, exerting a collapsing force toward the centre of the bubble, hence the alveoli collapses.
What does the law of Laplace describe?
The pressure generated by the surface tension within a bubble.
The collapsing forces generates local pressure. The amount of pressure generated within a specific bubble is described the Law of Laplace:
P = 2T/r (T= surface tension)
If T remains constant, then P ∝ 1/r
Where is more pressure to collapse generated?
More pressure to collapse is generated within a smaller bubble than a larger bubble
What would happen if 2 bubbles of different radius were connected to each other (e.g. different size alveoli connected via airways)?
Pressure gradients would be created between different sized alveoli, resulting in smaller alveoli emptying into larger ones
