20. The structure of the vein aids its function in the following ways:
a) the wall of the vein is thin as the blood does not flow in pulses, wo the vein wall can't pump it.
b) The lumen or central space of the vein is wide so that a large amount of slow-moving blood may flow through.
c) The vein walls are thin, allowing them to be compressed by adjacent muscles, which can press venous blood forward when they contract.
d) The layers of collagen in the outer wall of the vein are thin since there is little danger of bursting due to the low pressure.
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