That’s a good question. When you exercise the blood flow increases to your brain to keep it supplied with oxygen. This is controlled mainly by the heart which beats more often and with greater force. The brain can also regulate its’ own blood supply, and suring times of high metabolic demand (when the brain needs more energy) the blood vessels in the brain will contract and relax to direct the bood where it is needed most.
The upper vs lower body part of your question is interesting since when you are using your legs for instance, the muscle contractions squeeze the blood vessels in the muscles of your legs. This increases the amount of blood that gets back to the heart and therefore the strangth with which the heart beats. So although your legs are far away from your brain, they could be seen as having an indirect effect on the blood supply to the brain.