The Physiology and Pathophysiology of Gas Exchange

Abstract

The cells of the body require a continuous exchange of energy to subserve their functions. This is supplied in almost all cases by the high, free energy of hydrolysis of the terminal phosphate bond(s) of adenosine triphosphate (ATP). In most cells ATP is present only in low concentrations—approximately 5mM. This concentration is maintained relatively constant, despite large variations of the rate of energy exchange, through mechanisms that produce ATP at rates commensurate with its utilization within the cell. An increased ATP production rate can be mediated from three sources:

• 1 Breakdown of creatine phosphate in the creatine kinase reaction (i.e., creatine phosphate may therefore be considered to be a labile store of high-energy phosphate).

• 2 Oxidative phosphorylation in which hydrogen atoms derived from ingested substrates are processed through the mitochondrial electron transport chain as protons and electrons (Fig. 10.1), with the terminal oxidation at cytochrome oxidase being provided by molecular oxygen.

• 3 The production of ATP through anaerobic glycolysis in the cell cytoplasm.