Venom is a sort of replacement for blood in a vampires body. It is thicker than blood, and is the only known liquid to reside in a vampires body. A controversial other one could be sperm, because Edward could father Renesmee. The venom is known to be very strong, since it can burn away blood and veins, and dissolve contact lenses. When a human is bitten, the venom eventually enters the bloodstream, slowly and painfully making it's way to the heart, where a transformation takes place.