Final answer:
Veins serve as blood reservoirs due to their capacitance, storing a large volume of blood at low pressure. They are distensible and can accommodate approximately 64 percent of the blood volume at any given time.
Step-by-step explanation:
Veins serve as blood reservoirs in addition to their main function of returning blood to the heart. Systemic veins can store approximately 64 percent of the blood volume at any given time due to their high capacitance.
They are able to accommodate a large volume of blood at low pressure because of their large lumens and thin walls, making them distensible. This makes veins capacitance vessels, allowing them to store a significant amount of blood.