There is also a condition called "orthostatic hypertension" in which the blood pressure rises significantly upon standing. Dr. Bell described a patient who suffered from this due to hypovolemia (low blood volume). It can also be caused by blood pooling in the legs (often seen in the elderly) as well as by "vascular adrenergic hypersensitivity" which, I think, increases vasoconstricition (and thus BP) on standing.
I think, in hypovolemia in particular, the increase in blood pressure on standing may still not be sufficient to completely compensate for the low blood volume. So, even though you'd have high blood pressure, you're ability to stand would be impaired by low blood flow to the brain, as it is in orthostatic hypotention.