APA Dictionary of Psychology

maintenance of the concentration of sodium ions in blood plasma within normal limits. Sodium ions are the principal cations of plasma and extracellular fluid and play a crucial role in fluid and electrolyte balance. Falling levels of plasma sodium trigger release of aldosterone from the adrenal cortex. This hormone stimulates reabsorption of sodium ions by the kidney tubules, thereby restoring plasma sodium levels. Severe sodium deficiency can be fatal, whereas excessive plasma sodium levels can result in hypertension.