Illustration of the respiratory system: -nasopharynx -larynx and trachea -lungs, the left in cross-section showing bronchial tubes, bronchiolus and alveolus. The lungs and heart are the organs of the thorax surrounded at the top by the upper mediastinal orifice, the ribs at the side and the diaphragm below. The brain sends information to the diaphragm through the phrenic nerve, intercostal muscles and intercostal nerves to enlarge the rib cage so that the lungs can fill with air when breathing in and then to lessen the space in the rib cage when air is breathed out of the lungs.