Vasodilator responses to acute blood loss.
There is new evidence from experiments in conscious animals that when acute blood loss exceeds about 30% of blood volume, reflex vasoconstriction is abruptly replaced by widespread vasodilatation, and there is a precipitous fall in arterial blood pressure. This vasodilatation is associated with a decline in sympathetic vasoconstrictor drive. It is likely that the signal which causes the switch from vasoconstriction to vasodilatation reaches the brain via afferent nerves from the heart. There is also circumstantial evidence that endogenous opiate mechanisms are involved in the translation of the cardiac afferent signal into failure of reflex sympathetic vasoconstrictor drive. These mechanisms may explain the 'vaso-vagal' reaction that can occur in man during or following acute, severe, blood loss.