How is air drawn into the lungs?
By expanding the volume of the thoracic cavity
What is work done during breathing doing?
Moving the structures of the lungs and thorax to overcome the resistance to flow of air through the airways
What is the pleural space?
The space bewteen the lungs and thoracic wall
What is the pleural space normally filled with?
A few millimetres of fluid
What is the purpose of the fluid in the pleural space?
The surface tension of which forms a pleural seal holding the outer surface of the lungs to the inner surface of the thoracic wall
What is the result of the pleural fluid holding the lungs to the thoracic wall?
The volume of the lungs changes with the volume of the thoracic cage
What happens if the integrity of the pleural seal is broken?
The lungs will tend to collapse
What happens in a pneumothorax?
Air gets in between the two layers of the pleura, fluid surface tension is lost and the lungs collapse
What is meant by lung compliance?
The 'stretchiness' of the lungs
What is lung compliance defined as?
The volume change per unit pressure change
What does high compliance mean?
Lungs are easy to stretch
How is compliance measured?
By measuring the change in lung volume for a given pressure
What does a greater lung volume mean for compliance?
What is it more usual to calculate than compliance?
Why is it more usual to measure specific compliance?
Becasue, even with the constant elasticity of lung structures, compliance will also depend on the starting volume from which it is measured
How is specific compliance calculated?
Volume change per unit pressure change / Starting volume of lungs
Draw a diagram illustrating the compliance for;
- Elastic lungs
- Normal lungs
- Stiff lungs
What do the elastic properties of the lungs arise from?
- Elastic tissue in the lungs
- Surface tension forces of the fluid lining the alveoli
What is meant by surface tension?
The interactions between molecules at the surface of a liquid
What is the effect of surface tension on stretchiness?
It makes the surface resistant to stretching
What does a higher surface tension mean for compliance?
The higher the surface tension, the harder the lungs are to stretch and therefore the lower the compliance
What happens to the surface tension of the lungs at low lung volumes?
It is much lower than expected
Why is the surface tension of the lungs much lower than expected at low lung volumes?
Due to the disruption of interactions between surface molecules by surfactant
What produces surfactant in the lungs?
Type 2 alveolar cells
What is surfactant?
A complex mixture of phospholipid and proteins, with detergent properties
Where does the hydrophilic end of surfactant molecules lie?
In the alveolar fluid
Where does the hydrophobic end of surfactant molecules lie?
Projects into alveolar gas
What is the result of the position of the hydrophilic and hydrophobic ends of the surfactant molecule?
They float on the surface of the lining fluid, disrupting interaction between surface molecules
When does surfactant reduce surface tension?
When the lungs are deflated, but not when fully inflated
What is the result of surfactant only reducing surface tension when the lungs are deflated?
Little breaths are easy, and big breaths are hard, and it takes less force to expand small alveoli than it does large ones