The respiratory system allows us to breathe by taking in oxygen and removing carbon dioxide. Air enters through the nose or mouth and travels through the trachea into bronchi and bronchial tubes, which divide into tiny alveoli in the lungs. In the alveoli, oxygen passes into blood and carbon dioxide transfers from blood into the alveoli to be exhaled. Inspired air contains more oxygen and less carbon dioxide than expired air. This complex process happens quickly with each breath.