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.
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