DISORDERS OF WATER, ELECTROLYTES AND URATEBALANCES
Homeostasis is the maintenance of a stable internal environment within the body. This stability is necessary for optimum functioning of proteins, particularly enzymes, cells, tissues, organs and systems. Many substances have to be maintained at appropriate concentrations, including water, electrolytes, such as Na+, K+, Ca2+, Mg2+ and Pi, and the acid–base components H+ and HCO3–.
In addition, waste products, such as urea and urate, must be kept below toxic levels. Normally biochemical and physiological mechanisms regulate and control the concentrations of all these components and, in general, homeostatic controls involve negative feedback mechanisms. A receptor detects unacceptable levels of a particular substance under homeostatic control and sends a signal to a regulatory center that initiates a response that corrects the imbalance and returns conditions to a physiologically acceptablestate (Figure 8.1). Once normality returns, the receptor is no longer stimulated and the center ceases to respond. Disorders of homeostasis can occur, often as a result of failures in the control mechanisms or because of damage to the regulatory center by external agents.
Within the body, the kidneys regulate water, electrolyte concentrations, including acid–base balance , and excrete nitrogenous wastes, for example urate. Disorders of water, electrolyte and acid–base homeostasis and urate excretion account for a large number of investigations that are carried out in hospital laboratories.