Splanchnic venoconstriction occurs further enhancing emptying. Humoral mechanisms have onset delays on the order of minutes following orthostasis and therefore are relatively unimportant for the immediate response to postural change. Over longer time courses, however, humoral mechanisms are highly effective means for altering blood volume and sympathetic tone. Epinephrine may have an instrumental role in mechanisms of vasovagal syncope since an epinephrine surge invariably accompanies the onset of syncope. A corollary is that dogs don t faint.
Canines are, on the average, far more physiologically viable circulatory specimens than man.