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Unformatted text preview: * Have thinner walls than arteries with less muscular and elastic tissue * They collapse when empty and look flattened or irregular in histologic sections * Expand more easily and accommodate more blood than do arteries * approx. 54% of the blood is found in the systemic veins at rest (called capacitance vessels ) Veins * Pressure in the veins is not high enough to push blood upward against gravity to the heart * flow of blood back to the heart depends on the * messaging action of skeletal muscle * one-way valves that keep blood from dropping down again when the muscles relax * thoracic (respiratory) pump * cardiac suction-chordae tendeneae pulls AV valves cusps downward, slightly expanding the atrial space, creating a slight suction * gravity-blood flow from head * Valves occur especially in medium sized veins of the arms and legs * They are absent from very small and very large veins of the ventral body cavity and brain...
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- Fall '10