Ventilation & Compliance 2 Flashcards
What creates surface tension on the alveolar surface membrane?
Surface tension occurs when there is an air-water interface and refers to the attraction between water molecules.
Seen as the alveoli are spherical, the resultant force/tension faces inward which makes the alveoli want to collapse.
What is the role of pulmonary surfactant?
Reduces surface tension on alveolar surface membrane thus reducing tendency for alveoli to collapse.
It reduces lungs tendency to recoil and makes breathing mechanics easier.
How does surfactant work?
It works by mixing with and dispersing between water molecules to reduce the forces of attraction between them which reduces overall surface tension.
What is the Law of LaPlace relationship?
Pressure = 2 x Surface Tension / radius
P = 2T/r
Pressure is greater in smaller alveoli compared to larger ones.
So how can alveoli of different size have the same pressures?
Surfactant in smaller alveoli can become more effective as they come closer together and are therefore more concentrated.
A larger surfactant effect reduces the overall membrane surface tension.
What is infant respiratory distress syndrome?
Condition which affects premature babies whereby they are not producing adequate amounts of surfactant. This means that immense efforts are required for them to breath to overcome the surface tension which is causing their lungs to recoil/collapse.
What is the definition of compliance?
Change in volume relative to change in pressure
i.e. how much does volume change for any given change in pressure.
What does compliance represent?
It represents the stretchability of the lungs (not elasticity)
What does a high compliance mean?
Large increase in lung volume for small increase in intra-pleural pressure
What does a low compliance mean?
What may cause low compliance?
Small increase in lung volume for large increase in intra-pleural pressure.
e.g. fibrosis
Describe how compliance will be affected in patients with emphysema.
Compliance is increased in obstructive lung disease like pulmonary emphysema, less in asthma and at a minor degree in chronic bronchitis.
In emphysema, the elastic recoil is decreased and the P-V curve is shifted up and left. This is due to the loss of elastic tissue as a result of alveolar wall destruction.
On a pressure-volume curve, what can be used to determine the compliance?
The gradient. The steeper the line, the more compliant.
Why is there low compliance at the start of inspiration curve?
Lungs have to overcome tissue inertia initially, once this has been achieved the lung volume will increase at a quicker rate with similar changes in intra-pleural pressure (i.e. higher compliance).
Lungs also have to overcome surface tension and elasticity.
It is similar to the initial struggle in blowing up a balloon, once this struggle has been overcome the balloon will inflate more easily.
Why is there low compliance at the start of expiration curve?
At the start of expiration, airway vessels are compressed and this creates resistance to air exiting the lungs and therefore initially lung volume does not decrease at a quick rate.
This explains why asthmatics struggle to get air out on expiration as the airways are being compressed as well as the smooth muscle surrounding the bronchioles constricting.
Why do patients with emphysema require extra work/effort during expiration?
Patients who suffer from emphysema lose the elasticity in their alveoli.
They must spend more energy in returning lungs to initial volume seen as they cannot rely on the elastic recoil of the alveoli alone.
What area of the lung is more compliant: base or apex?
Why is this the case?
The base of the lung is more compliant (i.e. large change in volume for small change in intra-pleural pressure).
This is because the base of the lung is hanging in the thorax, it is being compressed due to the forces of gravity and thus has capacity to stretch on inspiration.
The apex of lung is already being stretched by the weight of the remaining lung (again due to gravity), this reduces its capacity to stretch as much as the remaining lung and therefore low compliance.
What is the difference between obstructive and restrictive lung diseases?
Obstructive - obstruction of airflow, particularly on expiration, resistance to airflow
Restrictive - restriction of lung expansion, low compliance
What is fibrosis in the lungs?
The build up of fibrous tissue in the lungs around the alveoli
How does fibrosis affect compliance?
It causes it to decrease as it lowers stretch capacity.
