Respiration is the metabolic process by which living organisms produce needed energy by the controlled oxidation of nutrients, such as carbohydrates.

In the enzyme-controlled process, sequenced reactions slowly release the energy (essentially, the ability or capacity of an organism to perform work, an activity or function) in the nutrients. In most cases, oxygen, either molecular or that bound in organic compounds, is utilized, and carbon dioxide (CO₂), water (H ₂O) and energy are produced. In the case of the sugar, glucose, the following formula typifies the reaction:

• C₆H₁₂O₆ + 6O₂ ’ 6CO₂ = 6H ₂O + Energy-dissipated Heat (The process is often discussed as two separate processes):
• External respiration, such as breathing and the pumping water over gills, is a simultaneous mass-and-heat-transfer process whereby oxygen is moved into the organism, and carbon dioxide is respired. This involves the gas transfer of molecular oxygen into a cell and CO₂ by-product out...