Which part of the cardiovascular system contains the most blood?

This question can be addressed in different aspects. If a localized region of the cardiovascular system is considered, it is possibly the heart itself which has a large collection of blood at any time. If you compare the arterial and venous system as two major components of the cardiovascular system, more blood will be there in the venous system at any given time because of the larger capacitance and slower velocity of blood flow. If you consider the amount of blood flowing per unit weight of tissue, it is the kidneys, which receive one fifth of the cardiac output, in spite of being relatively smaller organs. For this reason, the renal veins (veins draining deoxygenated blood from the kidneys) have the highest oxygen saturation as the blood flow is out of proportion to the metabolic demand of the kidneys and the oxygen extraction is quite less. The blood flow to the kidneys is mainly for the purpose of filtration of waste materials.