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The lung are the organs responsible for carrying out breathing and gaseous exchange in most air-breathing mammals. They are located either side of the heart and are enclosed by the rib cage which provides protection as well as a minor role in the breathing process. In human lungs, the trachea divides into two bronchi that enter into the base of the lungs. The bronchi continually subdivide into the bronchioles which end in hollow cavities known as the alveoli. Alveoli are surrounded by blood vessels which allows gaseous exchange to occur. Gaseous exchange takes place by diffusion with oxygen moving from a high concentration in the lungs to a low concentration in the blood, and carbon dioxide moving into an area of low concentration from a high concentration in the blood. There are on average 300 million alveoli in the adult respiratory system [1].

Around each alveolus is the interstitium, which is a thin layer of cells and blood vessels which provides support for the alveoli. The blood vessels surrounding the alveoli are what gas exchange occurs through.


Breathing (ventilation) is the process by which air enters and is removed from the lungs. Breathing is mainly carried out by the diaphragm, a sheet of skeletal muscle at the bottom of the rib cage. Contraction of the diaphragm increases the volume of the lungs, thereby decreasing pressure inside them and allowing air to enter. Upon relaxtion of the diaphragm the lungs are compressed and the pressure inside them increases, causing air to leave. 
The air within the lungs can also be forced out in active expiration due to the relaxing of the diaphragm and intercostal muscles and the contracting of the abdominal muscles.


  1. Gray's Anatomy 38th ed. 1995 Churchill Livingstone. p1670
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