Final answer:
About 55% of blood is held in the systemic veins at rest, which act as blood reservoirs due to their high capacitance and ability to distend.
Step-by-step explanation:
At rest, about 55% of blood is held within the systemic veins that function as the body's blood reservoirs. Veins have high capacitance, allowing them to distend and hold large volumes of blood, thus acting as capacitance vessels. Systemic circulation is the part of the cardiovascular system that carries blood between the heart and body, and systemic veins play a crucial role by managing blood flow based on the body's demand, especially during activities like exercise or rest.