Answer:
to ensure that blood flows in one direction only
Step-by-step explanation:
Here, we want to select which of the options is the function of valves within hearts and veins.
Basically, valves serve as control mechanisms in the circulatory system.
By control, it means that they actually dictate the direction in which the blood will flow.
Hence, valves ensures that blood types which is supposed to flow from the heart does not flow towards it and blood types that are meant to flow towards it does not flow away from it
An example of this can be seen in the case of oxygenated and deoxygenated blood.
Both have different flow mechanisms which are controlled by the actions of the valves