Among the following, which contains the greatest portion of circulating blood at rest?

• A. Arterial system
• B. Venous system
• C. Capillary beds
• D. Pulmonary circulation

Answer: (B)

The venous system holds the greatest portion of circulating blood at rest because veins are highly compliant vessels that can store large volumes with only small changes in pressure. This capacitance makes them a reservoir that the body can mobilize to maintain venous return and cardiac output as needed. About six to seven tenths of the total blood volume resides in the systemic venous circulation. In contrast, the arterial system carries blood at higher pressure and with less capacity, so it contains a smaller portion. Capillary beds hold a relatively small amount since their role is exchange rather than storage, and the pulmonary circulation also contains blood but in a smaller share than the systemic veins.