<P> Some potent neurotoxins, such as batrachotoxin, increase the sodium ion permeability of the cell membranes in nerves and muscles, causing a massive and irreversible depolarization of the membranes, with potentially fatal consequences . However, drugs with smaller effects on sodium ion motion in nerves may have diverse pharmacological effects which range from anti-depressant to anti-seizure actions . </P> <P> Whenever there is an increase in sodium concentration in the blood, the kidney releases most of it in order that there will be enough water for use of the body . But when there is a decrease in its concentration, there is more release of water to store more sodium which the body needs dearly . This process is known as osmoregulation . </P> <P> Sodium is the primary cation (positive ion) in extracellular fluids in animals and humans . These fluids, such as blood plasma and extracellular fluids in other tissues, bathe cells and carry out transport functions for nutrients and wastes . Sodium is also the principal cation in seawater, although the concentration there is about 3.8 times what it is normally in extracellular body fluids . </P> <P> Although the system for maintaining optimal salt and water balance in the body is a complex one, one of the primary ways in which the human body keeps track of loss of body water is that osmoreceptors in the hypothalamus sense a balance of sodium and water concentration in extracellular fluids . Relative loss of body water will cause sodium concentration to rise higher than normal, a condition known as hypernatremia . This ordinarily results in thirst . Conversely, an excess of body water caused by drinking will result in too little sodium in the blood (hyponatremia), a condition which is again sensed by the hypothalamus, causing a decrease in vasopressin hormone secretion from the anterior pituitary, and a consequent loss of water in the urine, which acts to restore blood sodium concentrations to normal . </P>

Where are sodium ions found in the body