What is the cause of my high blood pressure?

My comment is twofold. First, I am wondering if I have low blood volume. I cannot donate blood due to passing out at about the 1/2 pint range. I am not nervous about giving blood but each of the several times I've tried to donate, I get different degrees of almost passing out with the last try passing out cold. However, I have high blood pressure without medication. Just read an article that says low blood volume could be a cause of hypotension. I am wondering if high blood pressure could also be a cause of low blood volume. Also, I am wondering how I can find out what is the cause of my high blood pressure. Is there a test to figure this out?