gravity for those above your heart and valves for those below
Veins have valves which stop the blood from flowing backwards
Valves
The flow of blood from the heart to the lungs and back to the heart is called the pulmonary circulation.The blood flowing from the heart to the lungs would be deoxygenated and blood flowing towards the heart from the lungs would be oxygenated.
diastole
In the cardiac cycle, blood flowing into the heart is called the systole.
There are valves in the heart which open and close that allow the blood to be pumped in one direction only. See the related link for more information.
Your heart keeps your blood "flowing"
There are valves in the heart - and the larger blood vessels that stop the blood flowing backwards.
Heart valves help prevent blood from flowing backwards through the chambers of the heart. They open only one way to allow blood to flow through to the next chamber, and close against flow of blood from the other directon.
the blood flowing in a vein is under a lot less pressure than in an artery, generally veins carry blood back towards the heart from the various parts of your body whereas arteries are the main outlet distributers from the heart itself.
When the blood is flowing back to the heart, the blood has already given off its oxygen to the rest of the body which means it contain less oxygen concentration in the blood. and when blood is flowing from the heart to the rest of the body, the blood contains high level of oxygen because they were being given oxygen by the lungs which passes along the heart and the heart pumps it to the rest of the body.
There are two ways that blood is kept flowing in the same direction. The heart is pumping it in that direction, and blood vessels contain valves which prevent blood from flowing in the wrong direction.