A mineralocorticoid is a type of steroid hormone that the adrenal glands produce. Found at the top of the kidneys and the outer part of each gland, the adrenal glands, or the adrenal cortex, secrete a group of hormones known as corticosteroids, of which mineralocorticoids are one type. Mineralocorticoids are the steroids responsible for regulating the body's fluid balance and the concentration of minerals such as potassium and sodium. The main mineralocorticoid produced by the adrenal glands is called aldosterone and it acts to increase the level of sodium in the blood. Higher sodium levels mean more water is drawn into the blood, raising blood volume and blood pressure.
The principal mineralocorticoid, aldosterone, acts on the kidney, where it affects the movement of sodium, potassium and water. More sodium is reabsorbed into the blood, meaning less is removed from the body in urine. Along with the increase in sodium reabsorption, more water is taken up, raising the body's fluid levels and the volume of blood in the circulation. Another effect of aldosterone is that more potassium leaves the body in urine and less is reabsorbed by the kidneys. Aldosterone's mineralocorticoid activity also inhibits the loss of sodium from salivary and sweat glands, makes the taste buds more sensitive to salt and causes increased sodium to be absorbed inside the colon.
When sodium levels in the blood become low, or potassium levels rise, this stimulates the adrenal cortex to release more aldosterone. Potassium in particular affects mineralocorticoid secretion, with only a very small rise in blood potassium being enough to cause more aldosterone to be released. If blood pressure falls, a substance in the body known as angiotensin II also acts on the adrenal glands to increase aldosterone production.
Mineralocorticoids are necessary in order for a person to remain alive. A deficiency of aldosterone is potentially fatal, as it leads to low blood pressure and heart failure. Mineralocorticoid deficiencies may be caused by Addison's disease, or hypoadrenocorticism. In Addison's disease, the immune system attacks mineralocorticoid producing cells in the adrenal cortex as well as other cells which produce what are known as glucocorticoids, such as cortisol. Cortisol, like aldosterone, helps regulate blood pressure, but it also regulates blood sugar and affects the body's response to stress and infection.
Treatment of Addison's disease in essential to prevent serious illness and death. The condition is managed by giving medication to replace the missing mineralocorticoid and glucocorticoid hormones. With treatment, the outlook for people with Addison's can be quite positive with a predicted lifespan that is near average.