If severe bleeding occurs and is untreated, what happens to blood pressure and blood flow?

• A. The blood pressure increases
• B. Blood flow decreases
• C. Blood pressure and blood flow decrease
• D. No change

Answer: (C)

Severe, untreated bleeding causes a drop in circulating blood volume (hypovolemia). With less blood returning to the heart, the preload and thus the stroke volume fall, so cardiac output declines even if the heart rate increases a bit. This leads to a fall in arterial blood pressure. Lower pressure means less driving force to push blood through the vessels, so overall blood flow to tissues decreases. The body may try to compensate with vasoconstriction and faster heart rate, but with ongoing blood loss these mechanisms can’t maintain pressure and perfusion, so both blood pressure and blood flow drop.