Why do blood vessels and lymph vessels have valves? to control the volume of blood and lymph to ensure blood and lymph flow back to the heart to regulate hormone release in blood and lymph to block the flow of harmful substances in blood and lymph

The most appropriate answer would be "to ensure blood and lymph flow back to the heart".

Blood vessels (especially veins) and lymph vessels have bicuspid or two flapped valves in them.

Veins are the type of blood vessels which transport blood from the body to the heart.

The presence of valves in veins and lymph vessels prevent the backflow of the blood into the organs or muscles.

In addition, the muscles surrounding these deep veins also help in preventing the backflow and aid in forcing the blood towards the heart.

Thus, these valves ensure that the blood and lymph flow back to the heart.

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.

sorry to break it to ya but no one can answer your question it's just

jk but i am serious about the part that most people cant answer it beacuse you did not mention the section you are talking about. but it would if you added some detail and were a bit more specific.