It's a completely reasonable question. Blood pressure is a function of many variables, including vascular resistance, compliance, heart function, and --yes-- blood volume. In fact, one of the treatments for blood pressure, though rarely a first line, is to use a diuretic to reduce fluid volume. As other posters have commented, the body will compensate this to some degree, and it is not a very lasting or well-controlled effect, so the doctor is correct that bloodletting is not the standard of care for hypertension. But a mental experiment on the extreme margins can help address the question intuitively as well: suppose you removed all the blood, or else suppose you 100x-ed the blood supply. Would pressure fall and rise respectively? Of course it would.