Oxygen Supply and Consumption in Tissues
To maintain a sufficient tissue oxygen tension for adequate aerobic metabolism mammals developed complex oxygen transport systems. Oxygen must diffuse from capillaries to intracellular compartments, where oxygen consumption takes place. The oxygen tension necessary to keep intra- and extramitochondrial oxygen consuming enzymatic processes saturated differs markedly. In addition a significant difference in the oxygen dependence of isolated mitochondria and in vivo oxidative phosphorylation exists. Cellular diffusion barriers, differences in oxygen solubility within the cytoplasm, mitochondrial clustering and differences in oxygen affinity of enzymatic processes may account for these observations. It is likely that in vivo several tissues already live at the margin of hypoxia. In addition extramitochondrial oxygen consumption may be limited at tissue oxygen tensions above values where substrate limitation of oxidative phosphorylation starts. Evidence suggests that inhibition of extramitochondrial oxidase reactions may result in significant organ dysfunction without changes in intracellular energy charge. Manipulation of systemic and organ oxygen consumption to gain “biological time” during episodes of decreased oxygen delivery or greatly enhanced energy needs is everyday clinical practice in intensive care units.
KeywordsOxygen Solubility Systemic Oxygen Biological Time Systemic Oxygen Delivery Oxygen Transport System
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