Respiratory system, computer artwork. The trachea (windpipe) can be seen at centre, splitting into two bronchi (one for each lung). Inside the lungs (purple) the bronchi further divide into many bronchioles. Each bronchiole becomes smaller, finally ending in alveoli (tiny air sacs, not seen). The lungs are the site of gaseous exchange; oxygen enters the blood and carbon dioxide leaves. Also seen is the thoracic diaphragm, a sheet of muscle that stretches across the bottom of the ribcage separating the thoracic (chest) cavity from the abdominal cavity. The diaphragm contracts and relaxes during inhalation and exhalation.