it doesn't " flow" in one direction exactly the blood (using the heart as a starting point is pumped to the lungs where it is oxygenated then moves throughout the body via the arterial system, to the limbs and other organs. Once the blood has supplied this oxygen to the tissues and organs through capilliaries it passes into the veins and returns to start the cycle again. This is a very simplified model of something that has developed over many millenia.