The heart pumps oxygenated blood from the left atrium through the arteries to all organs and tissues that need oxygen and nutrients. Deoxygenated blood enters the heart through a vein (right atrium). Thus, the first process, that is, the pumping of blood to the organ, is an anatomically smooth process, because of gravity (towards the toe), the second blood flows with the decrease of the pressure gradient, and the third is oxygenated (without clots), but when it comes to deoxygenated blood flowing back to the heart. It’s like climbing up, from the toes to the heart. The pumping force of the heart dissipated in the early process, another force must take over, and it comes from the legs. The muscles of the legs contract, creating pressure on the walls of blood vessels and squeezing blood. And prevents the muscles from flowing backwards when relaxed. Tiny venous valves snap into the closure, creating a beat that releases pressure.  This is known as the “milking action” of the muscles and is known as the second heart.