When substantial blood loss occurs, which response is least likely?

• A. Increased heart rate
• B. Constricted arterioles
• C. Increased renin release
• D. Decreased heart rate

Answer: (D)

When substantial blood loss occurs, the body activates sympathetic reflexes to maintain blood pressure and perfusion. Baroreceptors detect the drop in arterial pressure and increase sympathetic outflow while decreasing parasympathetic activity. This leads to a faster heart rate and stronger heart contractions to keep cardiac output up. Simultaneously, sympathetic arteriolar constriction raises systemic vascular resistance, helping to stabilize blood pressure. The kidneys respond by increasing renin release, which starts the renin–angiotensin–aldosterone system, producing angiotensin II to constrict vessels and aldosterone to retain water and sodium, supporting blood volume. A decreased heart rate would oppose these compensatory effects and is therefore unlikely in this scenario.