Hormones, silly.
People should not confuse poetic ruminations that use body parts as similes and metaphors with real life organic functions, or expect that one can explain or justify the other.
Hope this helps!
hmm... i offer the solution of lust. ^_^
Hm... as for what I know the heart pump the blood around even to your cock... Your brain hold the desire and tell the heart to pump the blood out at a higher pace in cause of stress. Getting aroused is a stress function. So there you have it.
Want a diaper to fit that baby emote?