Veins

The veins are the primary capacitance vessels of the cardiovascular system. Their relatively compliant walls allow them to accommodate large volumes of blood at low pressure, giving them a substantially greater storage capacity than arteries.

Under resting conditions, approximately 54% of the total blood volume is contained within the systemic venous circulation, compared with about 11% within the systemic arterial circulation. This high capacity reflects the low intravascular pressure of veins, which are located farther from the cardiac ventricles.

Mean blood pressure in large arteries averages 90–100 mmHg, with systolic peaks reaching approximately 120 mmHg, whereas venous pressure averages about 10 mmHg. As a result, blood flow in veins is steady and non-pulsatile, in contrast to the pulsatile flow characteristic of arteries.

When not distended with blood, veins tend to collapse, producing a flattened and irregular lumen in histological sections and cross-sectional imaging.