To the biochemist the living organism is a complex, highly organized, heterogeneous system of compounds displaying a high degree of chemical activity. Despite this activity, it is striking that the chemical composition of the several tissues of the normal adult animal remains fairly constant. It is today recognized that in many instances this constancy of composition is not attributable to thermodynamic equilibrium or to kinetic stability. In general, in the living animal the organic reactions that comprise intermediary metabolism take place far from the equilibrium point, and the molecules of substrate, activated in most cases by enzymes, have far less stability than do the same compounds in pure form in bottles on the shelf. Thus glucose, undergoing no measurable decay at 37 C in the pure state, will, in the presence of animal tissues, rapidly disappear.
The constancy of composition of body fluids and tissues with respect to many organic