 Chest wall

The chest wall is the bony and soft-tissue envelope around the thorax, including the rib cage, sternum, thoracic part of the vertebral column, surrounding muscles and overlying tissues. It protects thoracic organs and participates in the mechanical expansion of the lungs. The respiratory diaphragm forms the lower partition of the thorax and acts with the chest-wall muscles during breathing.

Movement is central to its function. Contraction of the respiratory diaphragm and inspiratory intercostal muscles enlarges the thoracic space. Coupling through the pleura changes the pressure surrounding the lungs, allowing lung volume to increase and air to enter. OpenStax distinguishes the inward elastic recoil of the lungs from the opposing chest-wall tendency and discusses the ability of the thoracic wall to expand under pressure.^[ref]

Chest-wall compliance and lung compliance describe different elastic contributions. For an original small-change, static model, let $C_L$ and $C_W$ be their volume-change/pressure-change slopes. At shared volume change $\Delta V$, the pressure increments add:These are incremental elastic pressures around a specified equilibrium; their baseline values need not both be positive. The calculation assumes no airflow during the static comparison and omits airway resistance and active muscle-force changes.

With stipulated $C_L=C_W=0.20\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$, the combined slope is $0.10\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$. A 0.50 L increase therefore requires a 5 cm water pressure increment in this model, with 2.5 cm water assigned to each elastic contribution. These are explanatory values, not clinical reference limits.

A pressure-volume experiment must specify whether it measures the entire respiratory system or separates lung and chest-wall contributions. Pleural pressure and transpulmonary pressure provide different pressure references. Observing chest movement alone does not identify every elastic property or determine airflow: motion, pressure and flow are complementary measurements.