The Mechanics Of Breathing

Pulmonary ventilation or breathing is the process by which gases are exchanged between the atmosphere and lung alveoli. The exchange of gases is facilitated by changes in pressure between the atmosphere and the lung alveoli.

Atmospheric pressure is measured at 760mmHg or 1 atm. For air to flow into the lungs, the internal air pressure must be decreased to below atmospheric pressure. This is achieved by increasing the lung volume as; the pressure of a gas in a closed container is inversely proportional to the volume of the container. This is expressed by Boyle's Law.

How is the Volume Of The Lungs Increased?

The volume of the lungs is increased during inspiration (The process of breathing in).

During inspiration;
  • The diaphragm contracts and flattens which increases the vertical dimension of the thoracic cavity.
  • The chest wall lifts up and out increasing the horizontal dimension of the thoracic cavity.
  • In turn the pleural cavity increases causing a drop in intrapleural pressure. Therefore the parietal pleura are pulled outwards by the expanding thoracic cavity.
  • The parietal pleura also pull the visceral pleura and the lungs out with them.

The net result is that the pressure in the lungs drops to 758mmHg, which is lower than the 760mmHg air pressure. As air flows from a region of high pressure to a region of low pressure, inspiration takes place. Air continues to flow into the lungs as long as the pressure difference exists. Once the pressure equalises the inspiration muscles relax and expiration can begin.


Expiration or breathing out also occurs due to changing pressure gradients. For expiration to occur the opposite conditions to inspiration must occur, i.e. the pressure in the lungs must be greater than the atmospheric pressure.
Expiration, unlike inspiration, is a passive process, i.e. no muscle contractions are involved. Expiration therefore occurs due to the elastic recoil of the chest wall and lungs, both of which spring back to their original shape after being stretched.

During expiration:

  • The diaphragm returns to its original dome shape therefore decreasing the vertical dimension of the thoracic cavity.
  • The chest wall drops down and in, therefore decreasing the horizontal dimension of the thoracic cavity.

These changes cause the lung volume to decrease, which in turn increases the pressure to 763mmHg. Therefore air once again flows down its concentration gradient and out of the lungs for as long as a pressure difference exists.
After expiration there is a short pause before the whole process starts again.

Tortora & Grabowski (2000) Principles Of Anatomy & Physiology. 9th Edn.New York. John Wiley & Sons Inc.