Why does heart failure cause edema?

I'd think that the with less hydrostatic pressure, there'd be enough osmotic pressure to keep much fluid from leaking out of the capillaries (at least on the arterial side). On the venous side, if fluid is leaking out, why isn't it returned back via lymphatics into circulation efficiently? Is the heart's contractions + movement of blood through vessels a type of movement outside of skeletal muscles that helps move lymph and without it, you get edema?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums