GASEOUS DIFFUSION AND TRANSPORT II
Outline the effects on lung volumes of changes in stiffness of lungs
The elastic resistance of lungs comes from both the collagen and elastin in lung tissues and the surface tension in the air-fluid interface of alveoli.
The way that we prevent lungs form collapsing in on themselves:
Negative pressure in the interpleural space (except in parts of the airways in forced expiration)
Surfactant produced by type II pneumocytes
Lungs can either be too little or too compliant, and in both cases this is an issue.
Causes of lung stiffness could be tissue scarring in fibrosis, such as caused by asesptosis.
This lowers the lung function residual capacity FRC.
And causes of too high lung compliance can be emphysema, caused by tissue damage from smoking.
In the case of EMPHYSEMA, the floppy lungs have a reduced lung recoil, so this INCREASES FRC, causing what can be observed as barrel chest. (functional residual capacity.
The issues with smoking and Emphysema:
1. smoking will destroy the elastin in lung making them more compliant 2. the lungs can exert less of an inwards pressure on the chest wall 3. the chest volume is larger at rest, and the lungs are also larger at rest
4. there is less capacity for lungs to expand when needed
Typical values of lung compliance in adult male are 1.5 lkPa-1.
Explain the importance of surface tension forces at the alveolar air-fluid interface
Describe the production and role of surfactant
Describe simple tests of lung function; spirometry, FEV1/FVC and peak expiratory flow rate.
Spirometry: measures the volume of forcefully expired air vs time.
Importance? You can distinguish between Obstructive and Restrictive lung disease.
FEV1 – Forced expiratory volume in one second
FVC – Forced vital capacity
So, basically, if you have a small forced expiratory volume in one second relative to your forced vital capacity = you have trouble letting out air: Obstructive lung disease.
The numbers used for this are anything below 70-75%, if your FEV1/FVC is lower than that, it is classified as an Obstructive lung disease.
In this case: FEV1 is very low, FVC is low or normal, and FEV1/FVC is lower than 70-75%
If however, both your FEV1 and your FVC are low, with a normal FEV1/FVC this is classified as a Restrictive lung disease. This shows that you are not inspiring enough air (because you aren’t expiring enough either). There seems to be no issue with Obstruction because you are getting rid of the air normally in proportion to what is being inspired.
Looking at the maximum flow-volume loops, the COPD would show shifted to the left and with a concave side where the normal and Restrictive disorders are straight. This is because of the collapsed airways.
A note on lung compliance:
If you draw a static pressure-volume loop, you will observe that the expiration and inspiration curves are not exactly the same.
For the same transmural pressure, the lung volume is higher during expiration than inspiration. This is called hysteresis.
Compliance is maximal around normal tidal volumes, where small changes in pressure can cause big changes in lung volume, allowing for easy flow of air in and out of the lungs.