External respiration is the process by which oxygen is exchanged for carbon dioxide in the lungs. This occurs in the alveoli, tiny air sacs where oxygen from inhaled air passes into the blood, while carbon dioxide, a waste product from metabolism, moves from the blood into the alveoli to be exhaled. This exchange is crucial for maintaining the body's oxygen levels and removing carbon dioxide, which can be harmful in high concentrations. The efficiency of external respiration can be influenced by factors such as lung health, altitude, and exercise, all of which can affect how well oxygen is absorbed and carbon dioxide is expelled.