In humans, the function of external respiration is provided by both the respiratory system and the circulatory system. Atmospheric air enters the lungs from the nasopharynx through the larynx and the tracheobronchial tree enters the pulmonary alveoli. Normal breath is associated with the activity of the muscles of the diaphragm and the external intercostal muscles. During the inhalation, the diaphragm goes down while the ribs. During the exhalation, the diaphragm rises while the ribs move down. The human lungs have a large gas exchange surface and cannot be filled with air without external force. Cockroaches breath atmospheric air and receive oxygen through spiracles, conducting air to the trachea, and from there to the cells. Oxygen molecules penetrate inside the cells by diffusion of the thinnest trachea - tracheoles. The tracheal system branches many times while the narrowest tracheoles deliver air directly to each cell. As cockroaches are small, their tracheoles can supply each cell with the oxygen by simple diffusion.