When blood volume is lost, the body's response is mediated through the baroreceptor reflex, which controls:

• A. Heart rate
• B. Venous return
• C. Cardiac output and systemic vascular resistance
• D. Blood viscosity

Answer: (C)

The baroreceptor reflex maintains arterial pressure by adjusting two main outputs when blood pressure falls: cardiac output and systemic vascular resistance. When blood volume drops, mean arterial pressure decreases, and baroreceptors in the carotid sinus and aortic arch sense this change. They reduce their firing, prompting the brainstem to increase sympathetic activity and reduce parasympathetic activity. This drives the heart to beat faster and harder, raising cardiac output, while also constricting many systemic arteries to raise systemic vascular resistance. The combined effect helps restore blood pressure toward normal. Blood viscosity isn’t rapidly altered by this reflex, and although venous return can be influenced indirectly, the immediate, primary controls are on cardiac output and systemic vascular resistance.