(Bio) 關於Vein的問題 ?

Resistance is any force that oppose the movement of the object/substance
concerned while friction is one of this kind of forces.

In veins, the resistance to blood flow is mainly the friction between the vessel wall and the blood itself.
For a larger lumen, the ratio between the inner-wall
surface area(which causes resistance) to cross-sectional area
(which allow blood to flow)of the vessel is smaller.
In other words, the contact area between the blood and the
inner-wall of the vein is smaller in relative means for the case of large lumen.
(PS: Inner-wall surface area= contact area between blood and the wall itself)

That is why a large lumen has a smaller resistance to blood flow.

Up to here, of cause blood vessels do have resistance.

the larger the internal diameter of the tube,
the less resistance it has.
just imagine a thick water pipe and a thin water pipe.
which one will have larger resistance?
of course, thinner one has larger resistance.
so the same effect is on the lumen of the vein.