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The functional residual capacity is made up of expiratory reserve volume (ERV) plus the residual volume (RV) - i.e. the volume of air remaining in the lungs after passive expiration.

Compliance of the lungs is said to affect FRC. For example, if compliance of the lung increases (e.g. in emphysema), there is less elastic recoil and less air is passively expired, resulting in increased FRC. Could one explain this as increased compliance (where compliance = ∆V/∆P) meaning that the same intrapleural pressure at end of passive expiration results in a greater lung volume (FRC) at that time? This seems to explain instantaneous volumes rather than the actual expiration of less air.

How does this translate to compliance of the chest wall? As recoil of the chest wall tends to be outwards, would reduced compliance mean that there is greater tendency to recoil outwards, making intrapleural pressure more negative and reducing expiration of air, leaving greater FRC?

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    $\begingroup$ How is recoil of the chest outwards? Doesn't the chest tend to recoil inwards? Expiration is passive due to recoil of the chest inwards. $\endgroup$ – Willk Feb 11 '20 at 17:41
  • $\begingroup$ Recoil of the lungs (alveoli) is inwards, but chest outwards, I thought? As this is what sets up an equilibrium that allows the intrapleural pressure to be negative relative to atmosphere? $\endgroup$ – pincushion44 Feb 11 '20 at 18:38
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As far as lung is concerned both inspiration and expiration are passive. Lung just responds to intrathoracic pressure by accommodating more air during inspiration and expelling air during expiration. The active processes in the lung as far as air flow is concerned are limited to the bronchial tree smooth muscles which can increase or decrease the resistance to air flow. The compliance of the lung parenchyma is a function of its anatomical composition.

During inspiration the thoracic volume is actively increased by:

  1. Outward expansion of ribs by the intercostal muscles
  2. Downward expansion of diaphragm

This increase in thoracic volume is refected in all intra thoracic organs:

  1. Venous return increases due to the negative pull
  2. Negative pleural pressure causes air to be pulled into the lungs

Normal expiration on the other hand is passive due to relaxation of the above mentioned muscles. Relaxation will cause a decrease in the thoracic volume which will push the excess air and blood out.

An increase in the compliance of the thoracic cavity would mean weakening of the fascia and bones of the thoracic cage. In other words, the structures that the muscles attach to will become weak. This will cause muscle action to cause excessive expansion of the thoracic cage during inspiration and during expiration due to weaker pull by the attached structures, even residual tone of the muscle may cause distortion of anatomy preventing adequate reduction in the thoracic volume causing increased residual volume. Thusb reducing the volume of the air expired.

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