RAAS, or the renin-angiotensin-aldosterone system, increases blood volume and maintains normal concentrations of sodium and potassium. By enabling reabsorption of sodium and water in the kidneys, the circulating blood volume is increased. Sodium is the primary determinant of extracellular fluid osmolality, and water follows sodium. Potassium is excreted, because sodium and potassium have a reciprocal relationship in the kidneys. Aldosterone does not function in the body to increase potassium levels. Aldosterone does not function in the body to decrease sodium levels. Reabsorption of renal potassium would not result in increased blood volume.