When I lift my feet up they go numb and don't get enough blood I suppose. Why isn't the same thing happening for my head in a normal upright posture?
This is a very good question. The answer: because your head was meant to be above your body!
Your body has very specific mechanisms for maintaining a constant blood flow in the cerebral circulation despite shifts in blood pressure, either due to changing blood pressure in the rest of the circulation or due to a different 'local' pressure because of position . This is termed autoregulation.
The blood pressure that the brain 'sees' is called 'cerebral perfusion pressure' (CPP). Technically CPP is the difference between intra-arterial pressure and the pressure in the veins, but venous pressure is very low (2-5 mm Hg), so we can estimate it as the arterial blood pressure (here, a weighted average of systolic and diastolic pressures). In a normal person accustomed to normal blood pressures, the body can maintain a constant blood flow of ~50 mL per 100 g of brain tissue per minute with a CPP range of ~60 to 160 mmHg. That’s a big range!
The mechanisms of autoregulation are incompletely understood. Most likely reductions in CPP stimulate the release of substances that cause vasodilation (candidates include H+, K+, O2, adenosine), thereby increasing flow. On the other side, high pressures stimulate constriction of the myocites in cerebral vessels, reducing flow.
The end result is that your brain ‘sees’ a relatively constant pressure regardless of what position you’re in or other factors that may change blood pressure. That having been said, if blood pressure fluctuates outside the range for which auto regulation can accommodate, position does indeed matter for blood pressure. If a patient is markedly hypotensive, for instance, it is traditional to tilt the bed so that their head is below their body.1 At the extremes, this manipulation can indeed affect blood flow to the brain.
1. This is termed the Trendelenberg position, although recent evidence indicates that this is not a good idea for hypotensive shock due to more complex cardiovascular considerations.
All of this material is summarized nicely in this publicly available textbook:
Cipolla MJ. The Cerebral Circulation. San Rafael (CA): Morgan & Claypool Life Sciences; 2009. Chapter 5, Control of Cerebral Blood Flow.