The human respiratory system takes in oxygen and expels carbon dioxide using the lungs and other organs. The lungs carry out the exchange of gases as we breathe. Red blood cells collect oxygen from the lungs and distribute it throughout the body. During inhalation, the diaphragm and intercostal muscles contract and expand the rib cage and lungs, increasing their volume and allowing more air to enter. Exhalation is the movement of air out of the bronchial tubes and lungs to the external environment during breathing.