29. Why does blood always flow forward
and never backwards?
Blood is carried towards the heart in a blood vessel known as vein.
The blood in our body circulates through a system of tubes called
blood vessels. The blood in the veins travels a bit slowly.
Many large veins have valves to stop the blood from draining
backwards towards the legs and feet. The blood, which flows
forward, forces the valve flaps to open, as shown here in picture 1.
The blood flowing backwards forces the valves to shut (picture 2).
The valves of the heart also work in a similar fashion. The
contraction of the muscles of arms and legs also helps in flowing of
blood. This is the reason why, if one stands still for a long time, blood
collects in the legs and makes them puffy and painful.
