What happens when blood volume is lost from the body?

• A. The arteries contract to increase the blood pressure.
• B. The heart speeds up only after changes in breathing.
• C. Blood pressure rises due to increased heart contraction.
• D. The veins return blood more slowly.

Answer: (A)

When blood volume is lost, the body rapidly activates a reflex to keep vital organs perfused. Baroreceptors detect the drop in arterial pressure and trigger the sympathetic nervous system to constrict arterial smooth muscle. This narrowing of the arteries raises systemic vascular resistance, which helps push blood pressure back up even though there is less blood in the system. The heart may beat faster and with stronger contractions as part of the same reflex, but the key step is the arterial constriction that raises pressure quickly to maintain perfusion.

That’s why the statement about arteries contracting to increase blood pressure is the best fit. The other ideas don’t capture the main immediate mechanism: breathing changes aren’t what start this response, increased heart contraction alone doesn’t explain the rise in pressure when volume is down, and venous return isn’t simply slowed—the body also constricts veins to help maintain preload.