3 - Lung Mechanics & Function Testing Flashcards
What has to happen for breathing to be successful?
The lungs and thorax must move (due to action of diaphragm etc) and gas exchange must take place.
What is pleural fluid?
A thin layer of fluid between the visceral (attached directly to the lungs) and parietal (attached to the thoracic cavity; both are mesoderm derivatives) layers.
What are the functions of pleural fluid?
Ensure the lungs fill the thoracic cavity and that they change volume as the thorax does.
What is a pneumothorax?
Where the integrity of the pleural seal is lost (i.e. a hole in the pleura) - the lungs will tend to collapse if this occurs.
Breathing is about the balance of forces. How do the lungs, thoracic cage and diaphragm affect the balance?
Lungs pull in and up, Thoracic cage pulls out, Diaphragm (passive stretch - contracts on inspiration) pulls down.
At resting expiratory level what is the disturbance in the balance of the forces?
There is no disturbance - forces are balanced. If disturbed - e.g. in the process of breathing the lungs will return to this state.
What structures are involved in quiet inspiration? Is it active or passive?
It is an active process. The Diaphragm (contraction) and Intercostals
What structures are involved in quiet expiration? Is it active or passive?
It is a passive process. Nothing involved - muscles involved in inspiration will relax.
When quiet breathing, what process expends the most energy? Can this change?
The stretching of the lungs. This may not be the case if the diaphragm struggles to move into the abdomen (e.g. pregnancy, obesity)
What structures are involved in forced expiration? Is it active or passive? How is inspiration that follows it affected?
It is an active process - must breathe out beyond resting expiratory level. This requires force which is exerted by abdominal muscles. Inspiration following a forced expiration will be passive up to the resting expiratory level.
What is lung compliance?
A measure of the stretchiness of the lungs. This can be calculated through (change in volume / change in pressue).
What is the lung specific compliance? Why is this important?
Due to the fact compliance will be different depending on the starting lung capacity.
i.e. (volume change / pressure change) / starting volume of lungs.
If the lungs have a high compliance what will their stretchiness be like?
Great. They will have a low resistance to stretching.
What are the two main contributing factors to lung elasticity?
Surface Tension,
Surfactant.
What is surface tension? How does lung elasticity affect it?
Surface tension is the interactions between molecules at the surface of a liquid. This makes the surface resist stretching.
If the lungs have high surface tension what will their stretchiness be like?
Reduced. A high surface tension will mean there is an increased resistance to stretching.
What are surfactants? How do they affect surface tension?
They are a mixture of detergents that reduce surface tension by disrupting the interactions between surface molecules.
Molecularly, what does surfactant consist of?
A mix of phospholipids and proteins. The hydrophilic ends of these molecules lies in the alveolar fluid with the hydrophobic ends projecting into the alveolar gas. These molecules therefore float on the surface of the lining fluid disrupting interaction between surface molecules.
In the lungs what is surfactant produced by?
Type 2 alveolar cells.
Does surfactant always affect surface tension the same way?
No it depends on how inflated the lungs are.
Deflated: reduces surface tension (therefore increasing compliance)
Fully inflated: does not affect surface tension
This means that breathing is easier initially and as the breaths get larger in volume, they become more difficult.
What is hysteresis?
The energy needed to stretch a film of surfactant.is not all recovered when the film recoils. Energy is lost - being at its greatest when tidal volume is maximal - little breaths are better.
What are bubbles?
They are formed when a film of fluid, surrounding gas, shrinks compressing gas until there is an equilibrium between tension and pressure.
What is Laplace’s Law and its relevance to bubbles?
Pressure = 2 x surface tension / radius
Big bubbles have a low pressure; small bubbles have a high pressure.
What is the ‘law of bubbles’? How does this apply to Laplace’s Law?
If bubbles are connected in series, air will flow from high pressure to low pressure, i.e. small bubbles to big bubbles. Therefore big bubbles eat small bubbles.