Not known. Assuming you mean high blood pressure, most cases are idiopathic (we don't know). It does run in families. There are occasional cases related to kidney disease, adrenal gland tumors, or narrowing in the aorta. If you mean a high red blood cell count, it can be a primary disease or secondary to other things such as lung disease.
Starts at gut first. High blood pressure can be normalized by taking orally monounsaturated fat 1.5 gm per day(oil change for the body) and beta glucans (from mushrooms)beta glucan relax the blood vessels. Take orally supplements made from shiitake, reishi or maitake mushrooms. If gut is irritated blood pressure will be high - leaky gut let's toxins into the body. If this persists need medication to help kidney.