The respiratory system functions to transport gases to and from the circulatory system. It includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli. Oxygen is absorbed into the bloodstream in the alveoli and carried to cells while carbon dioxide is absorbed by red blood cells from tissues and released in the lungs. Breathing is controlled by the medulla oblongata and diaphragm to regulate lung volume and gas exchange.