Pulmonary ventilation (breathing) is the process by which air is exchanged between the
alveoli in the
lungs and the external environment. Breathing has two stages:
inspiration (breathing in) and
expiration (breathing out).
Pulmonary ventilation is the first stage in
respiration.
Before inspiration
The
external intercostal muscles are relaxed and the
diaphragm is relaxed. The ribcage is not expanded.
Inspiration (breathing in)
During
inspiration the
external intercostal muscles contract, elevating the ribcage upwards and outwards and increasing the size of the
thoracic cavity. At the same time the
diaphragm contracts and flattens, further increasing the size of the
thoracic cavity. The
lungs expand, following the shape change in the
thoracic cavity. The sudden shape change causes an area of
low pressure inside the
lungs, and air rushes in to fill the newly available space.
Expiration (breathing out)
During
expiration the
external intercostal muscles and
diaphragm relax. The
tissues of the ribcage are elastic and return the
thoracic cavity to its original, smaller shape. The ribcage falls back down and inwards and the
diaphragm relaxes into a domed shape. The air
pressure increases inside the
lungs and air is forced out of the body.
During
active expiration, which is a forceful expelling of the air from the
lungs (such as during coughing), the
internal intercostal muscles contract and the
abdominal muscles contract, quickly decreasing the size of the
thoracic cavity and forcing air out of the
lungs.
Figure 12.2.1 - Diaphragmatic breathing
By John Pierce (Own work) [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-3.0(www.creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Respiratory volumes and capacities
The volume of air the
lungs can hold under different circumstances is an indicator of overall lung health. These volumes are the respiratory volumes and capacities.
Tidal volume (VT)
Inspiratory reserve volume
(IRV)
- The maximum amount of air that can be breathed in during a deep inspiration.
Expiratory reserve volume
(ERV)
Residual volume
(RV)
- The leftover volume of "dead" air that is left over in the lungs after a forceful expiration
Total lung capacity
- The maximum capacity of the lungs including the residual volume.
Vital capacity
(VC)
- The maximum capacity of the lungs minus the residual volume.
Functional residual capacity (FRC)
- The leftover volume of air after passive expiration
Figure 12.2.2 - Modified from Lung Volume By Vihsadas at en.wikipedia (Transferred from en.wikipedia) [Public domain], from Wikimedia Commons
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1 - Anatomy and Physiology Basics