The organs of respiration consist of two lungs, one on either side of the chest and the air passages that lead to them. The lungs are located in the upper thoracic cavity of the chest, one on each side of the median line, and are separated from each other by the heart, the greater blood vessels and the larger air tubes. The lungs are spongy, porous, and their tissues are very elastic, containing innumerable air-sacs or alveoli. Each lung consists of an apex and a base. The base is directed towards the diaphragm, the muscular septum, the dividing wall between chest and the abdomen.