Your heart keeps your blood "flowing"
i think you mean bypass
a flap of tissue that prevants blood from flowing backward is
valves in the veins
If blood starts flowing backwards through a valve, stroke volume and ejection fraction would decrease. The heart's pumping action would be less efficient.
your skin of course!
heart valves
These are called valves.
The heart valves keep blood from flowing backward. This increases the efficiency of the pumping of the heart.
Valves in the veins prevent the blood from flowing backward or from pooling.
a valve
The four heart valves.
The flap of tissue that prevents blood from flowing backward is called a valve. Valves are found in veins and in the heart to ensure that blood flows in one direction and does not pool or backflow.