There are many causes for hyponatremia. However, all causes of hyponatremia follow the same rule which is that they are caused by a process that results in an excess of water in relation of the amount of sodium in the plasma.
A. Intravascular Volume Depletion.
A person develops intravascular volume depletion, also known as “dehydration”, when there is a loss of fluid and sodium existing in the blood vessels. This happens whenever a person bleeds, has diarrhea, vomiting, does not eat or loses fluid through sweat. When any of these processes occurs, the body senses the lowered blood volume and a hormone called antidiuretic hormone (ADH) is secreted from the pituitary gland. This hormone acts at the level of the kidneys instructing them to decrease the urine production by preserving water. At the same time, low vascular volume increases thirst, prompting the person to drink more water. The combination of loss of sodium by losing blood or fluids, and the increase of water retention and intake cause the sodium concentration to decrease.
Marco A. Ramos MD