Organisation and control

The physiology of the body divides itself into different levels of functional organisation. Natural boundaries define three major levels, those of metabolism, cellular function and organ systems. The control mechanisms at each level form an essential part of the physiology, and direct function towards the ultimate goal of homeostasis. Homeostasis can be defined as maintenance of the composition and properties of extracellular fluid.

Metabolism and metabolic control

Metabolism is a global term encapsulating the mass of biochemical pathways that form the chemical machine providing energy and materials for the maintenance of life. These pathways are controlled at a biochemical level by various factors that determine the rate of metabolic reactions, including:

• Chemical parameters affecting reaction rates, e.g. concentration of reactants, temperature, activation energy requirements, presence of catalysts

• Enzyme concentration and activity. Factors affecting the rate of enzyme-mediated reactions include substrate concentration, presence of cofactors or coenzymes, and activation or inhibition by reaction products (see allosteric and covalent modulation, below)

• The law of mass action and feedback control

At a systemic level metabolism is controlled largely by the endocrine hormones. These substances produce broad physiological changes in the body by exerting multiple effects on cell biochemistry. Some examples of the cellular metabolic effects of hormones are given in Figure PG1.

Cell function and control

Cell function involves both intra- and extracellular processes.

Intracellular functions

Intracellular functions include:

• Maintaining the internal milieu

• Reproducing DNA

• Production of RNA

• Repairing cell structures

• Synthesis of substances for export

• Metabolism of imported substances

• Production of chemical energy

• Cell motility