Usually there are valves in the internal jugulars. As many as 10% of people may have none. If a vein is above the heart, and the person is upright, gravity will help get blood back to the heart and lungs. If it is below the heart, muscles alongside it will squeeze the blood upward, which the veins allow more easily than the arteries, because they are more flexible, having thinner walls, with less of their own smooth muscle to make them stiff. If a vein is below the heart and must get blood upward, against gravity, the valves inside the vein can keep gravity from pulling the blood back down. If you stand on your head, the valves below your heart don't help anymore, and gravity does the work, but the ones in your neck will protect your brain. Perhaps we have lost the neck ones sometimes, in evolution from primates, because they help most when you are upside-down, and we don't hang upside-down from trees as much anymore.
The smooth muscle, inside the vein, is for adjusting the diameter of the vein, not moving the blood. The squeezing action that moves the blood upward, against gravity, is done by external muscles, like in the legs and chest. I think the heart does not bring blood upwards from below it, against gravity, or downwards from above, since it has enough work to pump blood away from itself, to the organs and lungs. It relies on gravity to get blood from above, and other muscles to bring the blood back from below. Valves keep blood from falling or going backward.
Or so I believe. Your mileage may vary.
"Try - Just A Little Bit Harder" - Janis Joplin
CCSVI procedure Albany Aug 2010
'MS' is over - if you want it
Patients sans/without patience
I am not a doctor. Do not take anything I say as medical advice.