How veins and arteries contribute to erection

Erections are triggered by influx of blood into the penis.
In order to obtain erection, blood must reach the penis in sufficient quantity,
and stay long enough in the penis. This is the role of arteries and veins.

The penis is like a bathtub
to be filled for erections to ocure.

Normal erection

To fill-up the bathtub,  water must arrive through the tap (arteries), and remain in the bathtub (blocked veins).

Impotence due to
vein leakage

Vein leakage =

No erection

Water arrives through the tap but escapes at the bottom (venous leakage). Bathtub never gets filled-up.

Impotence due to
arterial disease

Clogged arteries =

No erection

Water does not arrive through the tap. Bathtub does not fill-up.