ABSTRACT
The concept of homeostasis has its origins in nineteenth-century French physiology. Homeostasis is the central theme in physiology. There are a huge number of examples of homeostasis, including the maintenance of the correct levels of electrolytes and dissolved gases in body fluids, the maintenance of the optimum pH of body fluids, and so on. Obviously, then, homeostasis is a major theme running through animal physiology. To minimize this disruption to the homeostasis of body fluids, many animals drink at the same time as they eat. The fact that the external environment is changing means that the body fluids, e.g. blood plasma, hemolymph and extracellular fluid (ECF), which constitute the internal environment and which surround the cells of the animal are constantly being challenged. Animals must ensure that the intracellular fluid and ECF remain as stable as possible – gas concentrations must be held at the correct level, the pH and concentrations of solutes must be constant and so on.
