What is the scientific term for the muscle pump?

And what can you do to get a better, longer muscle pump? Of course you're supposed to do high repetitions, but if I do TOO many reps then I won't get a pump. Is there a certain "window" of repetitions/weight that I should do? Also, what should I eat to get a good pump? And why can't I get a pump in my pecs or my shoulders, but get a huge pump in my forearms (and sometimes my biceps). Is it because they are smaller muscle groups? Thanks in advance