The suggested mechanism of the role of calcium in blood pressure regulation is through the mediation of the vasoconstriction of vascular smooth muscle, alterations in extracellular binding of calcium, the interaction between serum calcium and other cations such as sodium, potassium and magnesium, renal vasoconstriction causing kidney dysfunction, and hyperactivity of the renin–angiotensin system caused by hyperparathyroidism [43,44]. Here, REN is linked to hyperparathyroidism.