Vasodialation in the arteries allowing blood to flow in, coupled with compression of the veins, preventing the blood from flowing back out. In the presence of mechanical stimulation, this is an entirely autonomic (parasympathetic) process, generally independent of conscious control.
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Vasoconstriction in the veins leading out of the erectile tissue prevents blood and other fluid from escaping, resulting in an erection.
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Stimulation to your peepee