The respiratory system draws in fresh air from the environment, exchanges oxygen for waste gases in the lungs and expels the spent air back into the environment before taking another breath. The oxygen is then circulated through the body to fuel the metabolism of the cells.
External respiration begins with the intake of air by the lungs. In order to take a breath, a muscle at the bottom of the ribs called the diaphragm contracts and establishes a slight pressure gradient within the chest cavity. The negative pressure created draws air into the lungs through the trachea, which branches out into the lungs in a series of progressively smaller tubes called bronchi.
Gas exchange takes place inside tiny sacs in the lungs called pulmonary alveoli. These thin-walled sacs are heavily vascularized and permit the passage of oxygen and carbon dioxide through their tissues. Oxygen is then taken up by red blood cells, which contain a special protein called hemoglobin that bonds with the oxygen molecules. Red blood cells carry oxygen throughout the body and exchange it with cells for their waste, which is then carried back to the lungs and secreted through the alveoli and expelled with the next breath.