= Chest wall
The \b[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.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§22.3, The Process of Breathing: inspiration, pleural coupling and thoracic wall compliance]{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:
$$
\Delta P_{\rm total}=\Delta P_L+\Delta P_W=\frac{\Delta V}{C_L}+\frac{\Delta V}{C_W},\qquad
\frac1{C_{\rm total}}=\frac1{C_L}+\frac1{C_W}.
$$
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 $C_L=C_W=0.20\,\mathrm{L\,(cm\,H_2O)^{-1}}$, the combined slope is $0.10\,\mathrm{L\,(cm\,H_2O)^{-1}}$. 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.
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