How does blood pressure get elevated in polycythemia rubra vera?

Blood pressure elevation in polycythemia rubra vera

Increased number of circulating erythrocytes increase the blood viscosity and clog the renal circulation causing renal ischemia, which triggers the secretion of renin. Renin in turn increases the secretion of angiotensin, a potent vasoconstrictor which increases blood pressure. Angiotensin increases secretion of aldosterone which causes salt and water retention, another reason for increased blood pressure. Blood pressure elevation in polycythemia rubra vera is thus due to the effects of angiotensin and aldosterone.