Let's say I go to the gym and lift some weights an hour. During this time my arms will grow due to the "pump" -- the extra blood rushing in to feed the muscles. For example, I've measured about 2-3 centimeters increase just in the diameter of the upper arm (bicep+tricep).

But where did this blood come from? Also, if my arms got bigger, since I'm the same weight that means some part of my body must have gotten smaller, right?