The human respiratory system allows for gas exchange between the blood and air through breathing. Air enters through the nasal cavity and is warmed and moistened before passing into the trachea and branching bronchi and bronchioles that lead to millions of tiny alveoli in the lungs. The alveoli are adapted for efficient gas exchange with very thin walls, a large surface area, and moist interiors surrounded by blood capillaries. Breathing is driven by the contraction and relaxation of the diaphragm and intercostal muscles, inhaling when they contract to decrease thoracic pressure and exhale when they relax.