Approximately what share of the blood volume sits in the systemic veins at any moment?

1. Veins have large lumens and relatively thin walls, so they distend readily at low pressure. 2. That high capacitance lets the systemic veins hold about 64 percent of blood volume at any given time. 3. This is why veins are described as capacitance vessels and as blood reservoirs. 4. Figures near a quarter or over four fifths both miss the reservoir role that veins actually play. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.1 Structure and Function of Blood Vessels_