The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs. Its primary function is to provide oxygen to cells and remove carbon dioxide. External respiration is the exchange of gases in the lungs, while internal respiration is the exchange of gases at the cellular level. Air enters through the nose and mouth, and passes through multiple structures before reaching the alveoli in the lungs where oxygen and carbon dioxide are exchanged with blood.