The right and left lungs, (Fig. 7) are sitting in the chest cavity and are enclosed by the ribs, diaphragm, chest and back muscles. The fist-sized heart is located between the lungs. The right lung has three lobes : upper, middle and lower. The upper part of the lung is called its apex : the lower part its base. Lungs are the organs of breathing. The tubes carrying the air into the lungs by inhaling through nostrils, divide and subdivide a great number of times, finally reaching some 250 million thin-walled alveoli (grape-like bunches of minute air sacs) in which the air comes into very close contact with the blood over a very large area. Oxygen can readily diffuse from the air to replenish that of the venous blood flowing into the lungs ; at the same time carbon dioxide can diffuse out into the alveoli to be exhaled. The lungs are worked for breathing purpose by the diaphragm below them and the intercostals muscles that lie between the ribs.