In Galenic theory there were thus two kinds of blood, one moving in the liver and the veins, and the other in the heart, lungs, and arteries; blood of both types was the dominant humor, carrying the other humors through the body. Illness was caused by an imbalance in these humors, which was why the most common form of medical treatment was drawing blood, the only one of the humors for which the amount could be adjusted easily.
The transformation of blood into semen led many physicians to recommend that men limit the number of their ejaculations, and poets sometimes described sexual relations as little deaths – “oh, I die, I die” was a standard poetic conceit for orgasm.