Ventilation - Anatomy & Physiology

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Ventilation is the movement of air into and out of the lungs. Air moves from high pressure to low pressure with the rate of flow being proportional to the pressure difference. As environmental pressure is generally constant, the respiratory system must change its internal pressure to maintain air flow. The lungs are not fixed to the thoracic wall, but follow the changing volume of the thoracic cavity thanks to the pleura.


During Inspiration, the pressure within the pleural cavity drops to less than the pressure of the external environment. This is achieved by increasing the lung volume, brought about by the inspiratory muscles. The diaphragm contracts, causing the dome to flatten and move caudally. The external intercostal muscles contract, causing the ribs to be pulled outwards cranially.


During expiration, the pressure within the pleural cavity increases to greater than that of the external environment, in order to expel air from the lungs. This is achieved by decreasing the lung volume, and is brought about by relaxation of inspiratory muscles. The dome of the diaphragm is pushed back by the intrathoracic pressure and the ribs recoil back to their original position.

Species Differences

Avian ventilation is different to mammalian ventilation, requiring different muscles.


Budras, K.D., McCarthy, P.H., Fricke, W. and Richter, R. (2002) Anatomy of the Dog. 4th ed. Hannover: Schlutersche GmbH & Co. KG, Verlag und Druckerei.

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