Lung mechanics Flashcards
What are the stages of resting inspiration?
- Diaphragm contracts, which ‘compresses’ the abdominal cavity and ‘decompresses’ the thoracic cavity
- Intrapleural pressure decreases from -5 cmH2O to -8 cmH2O
- The lung expands to prevent further decreases in Intrapleural pressure
- Alveolar pressure decreases (Boyle’s Law) and air flows in until alveolar pressure returns to 0 cm H2O
- At end inspiration, Ppl is lower than before inspiration, and Palv is the same as before inspiration
What are the stages of resting expiration?
- Diaphragm relaxes, and the elastic recoil of lung increases Palv
- Air flows out of lungs down the pressure gradient
What direction of airflow to negative and positive transmural pressure cause?
Negative transmural pressure will cause air to flow into the lung - inspiration
Positive transmural pressure will cause air to flow out of the lungs - expiration
What is the composition of lung tissue?
high proportion of collagen and elastin
What is compliance?
Compliance = ΔV / ΔP
propensity for lungs and the chest wall to stretch and distort out of shape
change in volume per unit of pressure
most applicable to changing lung volume away from FRC
What is elastance?
Elastance = ΔP / ΔV
tendency of the lungs to recoil to their original volume after removal of inspiratory forces
change in pressure per unit change in volume
greater change in pressure per unit of volume reflects a higher degree of elastance
most applicable to changing lung volume towards FRC
What are the volume pressure curves for lungs?
Trans respiratory pressure
Sigmoid shape
Greater pressure required near full capacity
How do volume pressure curves change for obstructive and restrictive lung disease?
Restrictive:
Down because VC less
Chest wall/lung interface less compliant
Obstructive:
Up because VC more
Steeper because tissue more compliant
see notes
What is surface tension?
tendency for water molecules to attract each other
important when discussing an air-water interface
amount of surface tension exerted is proportional to the volume of the alveolus
exerted surface tension is proportional to lung volume
reduction in surface tension is proportional to the concentration of surfactant at the fluid-air interface.
Major contributor to elastance
What is pulmonary surfactant?
- Secreted by type 2 pneumocytes into alveolar spaces
- composed of polar phospholipids (80%), neutral lipids (10%) and protein (10%)
How does pulmonary surfactant work?
- polar phospholipids migrate to the surface and arrange themselves with their hydrophilic heads in the fluid, and the hydrophobic tails protruding into the air
- molecules interrupt the attraction between water molecules and reduce (but not eliminate) the surface tension to make the alveolus more compliant
What are the function of pulmonary surfactant?
- Prevents collapse of small alveoli - limits pulmonary oedema
- Increase compliance by reducing surface tension
- Reduces work of breathing
see notes Law of Lapace
How does surfactant regulate alevolar size?
- larger alveoli have a larger air-fluid interface so the surfactant is less concentrated so requires a greater pressure to increase its volume relative to a smaller alveolus
- because of the positive relationship between alveolar volume and surface tension, the increasing surface tension in ‘more compliant’ alveoli will hinder their expansion, allowing smaller, less compliant alveoli to increase in size
- Helps lung tissue behave as 1 unit
What is the effect of fluid filled lungs on compliance?
Increased compliance
Due to reduced surface tension
What is interdependence?
Each alveolus doesn’t have own wall
Shared with adjacent alveoli
Forces acting on one directly affect others
