How does blood flow back to the heart from the lower extremities, since it is fighting against gravity?
The vein anatomy has a big part to play in this by using valves.
The blood pressure coming off the heart drops as the blood flows through the arteries, arterioles and capillary beds. Coming back the pressure is about 8 mm Hg.
Compare that to the 120 mm Hg leaving the ventricle.
To help bring that back, the veins have valves which help "lift" the blood. Notice that muscles help, too. Walking or exercising is a big benefit.
A big problem can occur when those valves break down, especially if we stand for a long time.