We all have an upside down tree in our chests. The trunk resembles the main bronchus (the windpipe), the branches are the large and small airways, and the bright green leaves are the little air sacs (alveoli) where oxygen and carbon dioxide exchange takes place.
These are healthy lungs….
In some forms of COPD, such as Emphysema, the trunk, the branches, and the leaves become stiff and brittle, and the tree slowly withers away.
With severe asthma, the trunk and branches can become stiff and scarred, but the leaves often remain green. The tree struggles, but it manages to survive for long time.
This is my tree. My branches are narrowed and scarred, but my O2 sats are usually normal because my leaves are still intact and functioning.