blood pressure = (stroke volume x heart rate) x total peripheral resistance
But the equation also suggests two ways to compensate for a decreased stroke volume: you can increase the peripheral arteriolar resistance (i.e. vasoconstrict) and you can increase the heart rate. Both of these responses are mediated by the autonomic nervous system, and the result is that the brain's perfusion is utterly unaffected by your upright posture.
Of course, that only applies if your autonomic nervous system is alright. In the elderly, and in diabetics in particular, for instance, this reflex can be slow and ineffective, resulting in light-headedness and even fainting if the patient stands up too quickly.