Which vessels hold about two-thirds of the circulating blood flow under normal conditions?

Which vessels hold about two-thirds of the circulating blood flow under normal conditions?

• A. Veins
• B. Arteries
• C. Capillaries
• D. Lymphatics

Answer: (A)

Veins are the body’s major reservoir for circulating blood. They are capacitance vessels with high compliance, so they can store large volumes with only small increases in pressure. At rest, roughly two-thirds of the total blood volume sits in the venous system, especially in the systemic veins. This large reservoir makes it possible to rapidly increase venous return to the heart and maintain arterial flow when needed, such as during activity or stress. Arteries hold a smaller share of blood, capillaries contain relatively little blood due to their exchange role, and lymphatics carry lymph rather than blood, so they don’t contribute significantly to circulating blood volume.

Veins are the body’s major reservoir for circulating blood. They are capacitance vessels with high compliance, so they can store large volumes with only small increases in pressure. At rest, roughly two-thirds of the total blood volume sits in the venous system, especially in the systemic veins. This large reservoir makes it possible to rapidly increase venous return to the heart and maintain arterial flow when needed, such as during activity or stress. Arteries hold a smaller share of blood, capillaries contain relatively little blood due to their exchange role, and lymphatics carry lymph rather than blood, so they don’t contribute significantly to circulating blood volume.