The venous system as the body's blood reservoir
At any given moment, the majority of circulating blood, often estimated at around two thirds, sits in the veins rather than the arteries. Veins are thin-walled and highly distensible, so they hold a large volume at low pressure. This makes the venous system a capacitance reservoir: a store the body can draw on when more blood is needed in active circulation.
The arteries, by contrast, hold a much smaller fraction but at high pressure, acting as the pressurized delivery side of the system.
How the body shifts blood centrally
When demand rises, for example during exercise, blood loss, or standing up, the sympathetic nervous system constricts the veins (venoconstriction). This squeezes stored blood out of the reservoir and toward the heart, increasing the volume returning to it (venous return) and helping maintain cardiac output and pressure.
Skeletal muscle contraction and the one-way valves in veins also assist this central return. This is general educational physiology, not medical advice; consult a qualified professional for circulation-related concerns.