Air is breathed in through the mouth and nose, travels down the trachea (wind pipe) into the bronchial tubes until it reaches the alveoli. At the alveoli, oxygen leaves the air you breathe and enters the blood stream and carbon dioxide moves from your blood into the lungs and leaves your body when you exhale.

The pleura is a thin, transparent sac that surrounds the lung. It is not considered a part of the lung.

The lung can be visualized as a leafy tree that has been cut off at the ground and turned upside down. The trunk represents the trachea. The branches represent the bronchial tubes as they repeatedly branch, becoming smaller and smaller.

The alveoli sprout from the very smallest bronchial branches (like leaves sprouting from the smallest branches of a tree). The alveoli are small clusters of air sacs that have the appearance of a cluster of grapes.

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