The respiratory system serves to exchange gases between the blood and the external environment, bringing oxygen into the bloodstream for transportation to the tissues, and taking carbon dioxide from the tissues to the lungs to be expelled into the external environment (Respiratory, 2005). The lungs consist mostly of passageways, alveoli (air sacs) and elastic tissue. The bronchial tree, the system of passageways in the lungs, begins with the right and left primary bronchi which branch from the end of the trachea (Applegate, 2000, 311). The right primary bronchus is shorter, wider and more vertical than the left primary bronchus.
After the primary bronchi enter the lungs, they branch several times into smaller passageways called secondary bronchi to form the bronchial tree (Applegate, 2000, 311). These secondary bronchi then branch again into even smaller passageways, the tertiary bronchi. The right side has three secondary and 10 tertiary bronchi, while the left side has two secondary and eight tertiary bronchi. There are further branching passageways extending from the tertiary bronchi which are known as bronchioles. Tertiary bronchi branch yet again into the respiratory bronchioles, which lead into the alveolar ducts (Applegate, 2000, 311).
The alveolar ducts terminate in clusters of minute air sacs known as alveoli, the functional units of the lungs where gaseous exchange takes place (The Human, 2005). Alveoli have very thin walls, with only a single l
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