When do veins functions as blood reservoirs?

Veins function as blood reservoirs when the body is in a resting state. During this time, the body does not require as much oxygen and nutrients, so the veins are able to store some of the blood that would otherwise be circulating. This helps to maintain blood pressure and prevent the body from becoming dehydrated.

When the body is in an active state, the veins constrict and the blood is forced back into the heart. This increases the blood pressure and helps to deliver oxygen and nutrients to the muscles and organs.