Sodium is the primary cation (positive ion) in extracellular fluids in animals and humans. Sodium ions are necessary for regulation of blood and body fluids, transmission of nerve impulses, heart activity, and certain metabolic functions. Physiologically, it exists as an ion in the body. Sodium is needed by animals, which maintain high concentrations in their blood and extracellular fluids, but the ion is not needed by plants. The human requirement for sodium in the diet is less than 500 mg per day, which is typically less than a tenth as much as many diets "seasoned to taste. " Most people consume far more sodium than is physiologically needed. For certain people with salt-sensitive blood pressure, this extra intake may cause a negative effect on health.