the function of the respiratory system is to transport air into the lungs and to diffuse oxygen into the bloodstream and carbon dioxide out it is separated into the upper and lower respiratory tracts air enters the upper respiratory tract through the mouth or nose and nasal cavity where it is filtered warmed and moistened next it passes into the pharynx and then into the larynx which is known as the voice box from here the air passes down into the trachea also known as the windpipe the inner membrane of the trachea is covered in tiny hair cells called cilia which catch particles of dust which are removed through coughing the trachea is surrounded by 15 to 20 C shaped rings of cartilage at the front and side which help protect it and keep it open the trachea divides into two smaller tubes called bronchioles once inside the lung the bronchi split several ways forming tertiary bronchi these continue to divide forming bronchioles which are narrow tubes less than one millimeter in diameter eventually air enters alveolar sacs and alveoli where gaseous exchange takes place