Because penises don’t actually work that way, despite pop culture. The engorgement of blood vessels in an erection does not deny the rest of the body blood. Nor does an amputation of a limb cause a body to have an excess amount of blood.
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Erections work more on blood pressure and arousal increases heartbeat which in turn increases blood pressure. It's an auto response to tits and asses. Or abs and dicks if you're into that, too. If you have a problem maintaining an erection, talk to your doctor. They might be able to help.