Question

what prevents backflow of blood when the heart contracts

Answer 1

Valves in the heart prevents the backward flow of blood.
They help in unidirectional flow of blood by opening in one way.

Hope this will help you :)

Answer 2

Hi friend !

Valves prevents the backflow of blood , when the heart contracts.
Valves open and close , to let the blood flow , and also prevent it from flowing back