The lungs have to be characterized with at least two parameters:
Lungs size (“amount of parenchyma”)
It is the parameter that reflects anatomical size of the respiratory system. It is greater for tall persons, in average, and smaller for ones of smaller height.
The size of the lungs should not be mistaken for their volume. The volume, representing the amount of air in the lungs during respiration is a variable value. The size of the lungs is constant for a given patient and is only decreased with anatomical restriction (as lobe resection, for example). The size of the lungs (respiratory system) depends mainly on height (and gender), which is reflected in the relationship between spirometric indices and height.
It should be kept in mind that a greater respiratory system means:
- greater size of the lungs (Size);
- greater lumens of all bronchi, i.e. lower resistances: R, R(1/lumen) intermediate, R(1/lumen) peripheral;
- greater closing capacity CCC.
If we want to simulate a healthy person, these parameters should be changed simultaneously.
Cm (“specific compliance of the parenchyma”)
The parameter characterizing the elastic properties of the lung parenchyma. On the analogy of the term specific weight of a substance (weight of 1 cubic centimeter), that parameter can be called specific compliance (compliance of 1 cubic centimeter or 1 gram of the parenchyma). The lower is the compliance, the harder are the lungs to distend. The higher is the compliance, the greater is their limpness.
Similarly as e.g. the total weight of a substance is equal to the product of the amount of that substance and its specific weight, the total compliance of the lungs is equal to the product of “the amount of lung parenchyma” and “the specific compliance of the parenchyma”. For example:
- Resection of a part of the lungs decreases lung compliance since it reduces the amount of the parenchyma.
- Pulmonary tissue fibrosis reduces lung compliance since it decreases the specific compliance of the parenchyma.