The veins are under CNS control, receiving plentiful innervation from the sympathetic branch of the autonomic nervous system. Blood pooling occurs below heart level. Veins are ordinarily collapsed above heart level; for example, the jugular vein in the neck. This large vein becomes obvious when a person becomes angry or does the Valsalva maneuver. Because veins are thin-walled, they are flat when under low or negative pressure. As discussed earlier, transmural pressure remains near zero until the cross section becomes circular, then the pressure increases sharply because the walls are much less compliant when being stretched than when just being filled like a plastic bag.
The capacity for venomotion, variation in wall tone through contraction and relaxation of smooth muscle, is decreased by bed rest, alcohol, fatigue and microgravity (i.e. space flight). Some veins show a pulse, described below. Variations in venous pressure may also be created by the contraction of skeletal muscle (i.e. the skeletal muscle pump).
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