Coloured scanning electron micrograph (SEM) of lung tissue from a mammal showing alveoli and blood associated with the membrane area. The lungs are a pair of air-filled organs located on either side of the chest (thorax). The trachea (windpipe) conducts inhaled air into the lungs through its tubular branches, called bronchi. The bronchi then divide into smaller and smaller branches (bronchioles), finally becoming microscopic. The bronchioles eventually end in clusters of microscopic air sacs called alveoli. In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, that contains blood vessels and cells that help support the alveoli. Magnification: x260 when shortest axis printed at 25 millimetres.