Volume in Blood Vessels
The blood volume (approximately 5.0 liters [5.3 qt]) is unevenly distributed among the arteries, veins, and capillaries at any given time. Most of the blood (60%–65%) is contained in the venous system, as the walls are thinner and more distensible. For this rea-son, the veins are also referred to as capacitancevessels. The veins serve as reservoirs of blood (blood reservoirs) and, when the blood volume drops as insevere bleeding or dehydration, they constrict and in-crease the blood volume in the arteries and capillar-ies. The veins of the abdominal organs, such as the liver and spleen and the veins of the skin, are impor-tant blood reservoirs.
At rest, about 5% of the blood volume is present in the capillaries. The systemic arteries hold about 15% of the blood volume. About 8% of the volume is in the heart and 12% in the pulmonary vessels.