ABSTRACT
The constancy of the internal environment was brought to our attention by Claude Bernard in 1865. He proposed that the extracellular fluid (ECF) provided the medium in which cells are bathed. The solutes contained therein were regarded as being closely regulated with respect to their concentration so that vital functioning could be sustained. In simple terms, regulation of the cation sodium and the anion chloride is responsible for the regulation of ECF. Absorption and gastrointestinal handling of sodium is an efficient process, with very little sodium excreted in feces. Renal handling in normal kidneys is also efficient, with conservation of sodium by reabsorption generally proportional to physiological need. Important losses of sodium occur in sweat, but sweat gland handling is modified by both regular physical exercise that produces sweating and regular exposure to environmental heat and other conditions that activate the acclimatization process.
