The respiratory system is responsible for supplying oxygen to the ~blood~ and expelling waste gases, of which carbon dioxide is the primary constituent, from the body. The upper structures of the respiratory system are combined with the sensory organs of smell and |taste| (in the |nasal cavity| and the |mouth|) and the digestive system (from the oral cavity to the |pharynx|). At the |pharynx|, the specialized respiratory organs diverge into the airway. The |larynx|, or voicebox, is located at the head of the |trachea|, or windpipe. The |trachea| extends down to the bronchii which branch off at the trachial bifurcation to enter the |hilus| of the left or right |lung|. The |lungs| contain the narrower passageways, or |bronchioles|, which carry ~air~ to the functional unit of the |lungs|, the |alveoli|. There, in the thousands of tiny alveolar chambers, oxygen is transferred through the membrane of the alveolar walls to the |blood cells| in the |capillaries| within. Likewise, waste gases diffuse out of the |blood cells| into the ~air~ in the |alveoli|, to be expelled upon exhalation. The |Diaphragm|, a large, thin muscle below the |lungs|, and the intercostal and abdominal muscles are responsible for contracting and expanding the thoracic cavity to effect respiration. The |ribs| serve as a structural support for the whole thoracic arrangement, and pleural membranes help provide lubrication for the respiratory organs so that they are not chafed during respiration.
