The systemic venous system contains the most blood because veins are stretchable due to presence of elastic tissue, so they act as natural blood reservoirs. So the systemic venous system contains approximately 64% of blood, while the arterial system contains about 13%, the pulmonary circuit holds 9%, and the smallest amounts are held by the systemic arteries and heart which is 7%. However, these amounts can fluctuate by the environment or activity, as the vein performs venoconstriction to move the blood when needed.
