Polycythemia is an increase in red blood cells circulating in the blood stream. This increases the viscosity of the blood which can cause high blood pressure due to the increase need to pump thicker blood through the circulatory system. In addition, pylycythemia can also cause clogging of the vessels that feed the kidneys which can trigger the release of renin. Increased secretion of renin increases the secretion of angiotensin, a very potent vasoconstritor, which raises blood pressure. Angiotensin increases the secretion of aldosterone, which causes salt and water retention. The increased retention of water and salt increases blood pressure.