ABSTRACT
The steady-state oxygen concentration in tissues is determined by the balance between oxygen supply and demand. Oxygen is supplied from the blood, mainly in a form that is bound to haemoglobin in red blood cells and is consumed by cells primarily through the process of oxidative respiration in the mitochondria. In this process, mitochondria use oxygen as the terminal electron acceptor in a cascade of reactions collectively termed the electron transport chain. Here, nutrients are oxidized to produce the cellular energy currency ATP (adenosine triphosphate). Respiration plays a very important role in energy production by extracting the maximum amount of energy from cellular nutrients. For example, 1 molecule of glucose produces 36 molecules of ATP under conditions where oxygen is present but only 2 molecules of ATP when oxygen is absent (anaerobic glycolysis). The consumption of oxygen in this process gives rise to a limited ability of oxygen to diffuse through unvascularised tissues. Estimates of the oxygen diffusion distance range from 75 to 200 microns depending on the actual respiration rate (oxygen consumption rate) of the tissue/cells in question.
