The word 'respiration' is used in two conflicting senses: to mean the breathing in of air, but also a controlled biochemical process of 'burning' fuel.
Respiration is the process living things use to release chemical energy as they break down complex molecules in a series of controlled steps.
In 1780 Antoine Lavoisier and Pierre-Simon de Laplace published their memoir on heat, in which they concluded that respiration is a form of combustion.
Aerobic respiration involves the Krebs cycle, a series of carefully managed biochemical steps with standard products at each step, all controlled by enzymes.
Respiration can be both aerobic respiration, where oxygen is brought into the reactions and anaerobic respiration, where oxygen is not involved.
Of the two, aerobic respiration is more efficient than anaerobic respiration, because it produces more ATP from a given amount of starting material.
While both aerobic and anaerobic respiration produce adenosine triphosphate as the energy product, the two processes generate different end products.
Animals breathe in oxygen and breathe out carbon dioxide and that is a by-product of aerobic respiration. Water is also produced and absorbed.
Respiration is controlled by enzymes which are protein catalysts produced under the control of genes: if these fail to operate correctly, the organism dies.
Respiration has standard forms in all living things, with only minor differences, because the controlling enzymes have been conserved throughout evolution.
Most of the energy generated by both aerobic and anaerobic respiration is used to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP).
Animals use force to move and to live, and muscles are used to provide the forces needed to catch, swallow and digest food, to breathe and maintain blood flow.
Muscles require energy from respiration, and that means they require a good blood supply: exercise ensures the development of blood supply to muscles.