Which physiologic response is activated first in response to acute blood loss?

• A. Parasympathetic nervous system
• B. Sympathetic nervous system
• C. Reticular activating system
• D. Spinal reflex arcs

Answer: (B)

In acute blood loss, the body’s first reaction is activation of the sympathetic nervous system driven by baroreceptors sensing the drop in arterial pressure. This quick response causes rapid tachycardia, increased cardiac contractility, and widespread vasoconstriction, especially in the skin and abdominal organs, to raise systemic vascular resistance and help preserve blood flow to vital organs. The parasympathetic system would counteract this protective effort by slowing the heart, so it isn’t the initial responder. The reticular activating system is about arousal and mental status, not the immediate cardiovascular adjustment, and spinal reflex arcs don’t drive the first compensatory change to hemorrhage. Hormonal systems (like vasopressin and RAAS) come into play later to support perfusion as blood loss continues.