External Respiration Definition
External respiration is the process of exchanging oxygen, carbon dioxide, and other blood solutes with the external environment. Respiration as a whole is the process of delivering oxygen to the cells to extract the energy from sugars in oxidative phosphorylation in the mitochondria. This process uses oxygen and produces carbon dioxide, as well as a number of other waste metabolites during the normal function of cells.
Aquatic animals tend to exchange these waste products through the gills during external respiration with the water in their environment. Terrestrial animals may rid the body of gaseous metabolites such as carbon dioxide and nitrogen through the lungs, but many others must be filtered out in the kidney and condensed into the urine.