A tiny branch of air tubes in the lungs.
Source: NCI Dictionary of Cancer Terms
Caption: Anatomy of the respiratory system, showing the trachea and both lungs and their lobes and airways. Lymph nodes and the diaphragm are also shown. Oxygen is inhaled into the lungs and passes through the thin membranes of the alveoli and into the bloodstream (see inset).Date last modified: 2007-05-31