<P> Homeostasis can be defined as the stable condition of an organism and of its internal environment; or as the maintenance or regulation of the stable condition, or its equilibrium; or simply as the balance of bodily functions . The stable condition is the condition of optimal functioning for the organism, and is dependent on many variables, such as body temperature and fluid balance, being kept within certain pre-set limits . Other variables include the pH of extracellular fluid, the concentrations of sodium, potassium and calcium ions, as well as that of the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity . Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life . </P> <P> Homeostasis is brought about by a natural resistance to change in the optimal conditions, and equilibrium is maintained by many regulatory mechanisms . All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control centre, and an effector . The receptor is the sensing component that monitors and responds to changes in the environment, either external or internal . Receptors include thermoreceptors, and mechanoreceptors . Control centres include the respiratory centre, and the renin - angiotensin system . An effector is the target acted on, to bring about the change back to the normal state . At the cellular level, receptors include nuclear receptors that bring about changes in gene expression through up - regulation or down - regulation, and act in negative feedback mechanisms . An example of this is in the control of bile acids in the liver . </P>

What physiological process will cause the calcium ion concentration to rise back to a normal level
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