Answer: ensure blood flows toward your heart
Explanation:
These valves are what ensure blood flows toward your heart. Vein valves work hard, against gravity, to bring the blood that flowed down via arteries back up to your heart. When your vein valves are open and healthy, they are able to do their job properly, as the blood is easily able to flow up.
The answer is: Use right / the right to use
Use right could only exist if a person who own a certain property or resources willingly (without any threat) give permission to other people to use that property/resources.
In business environment, this type of rights mostly given for artistic products. For example, in order to input a music that you didn't create on your you tube video, you need to obtain the use right from the artist who created the music.