If a person has lost a significant amount of blood but maintains normal blood pressure, which explains why perfusion may still be inadequate?

• A. Blood vessels are constricting causing greater peripheral resistance
• B. Cardiac output remains fully adequate
• C. Blood is redistributed to nonessential organs
• D. There is increased capillary permeability

Answer: (A)

Perfusion to tissues depends on the pressure that drives blood through the capillary beds and the resistance those vessels offer. When a significant blood loss occurs, the body activates sympathetic reflexes that constrict arterioles throughout the body, increasing peripheral resistance. This rise in resistance means that, even if the arterial pressure is kept normal to hold up blood pressure, the flow of blood to most tissues decreases because flow is basically the pressure gradient divided by resistance. In other words, higher resistance lowers tissue perfusion despite a normal or near-normal blood pressure. The body may shunt blood toward essential organs, but the net result is reduced perfusion to many tissues.