It comes back forever

From The Gift by Lewis Hyde:

Perfect gift is like the blood pumped through its vessels by the heart. Our blood is a thing that distributes the breath throughout the body, a liquid that flows when it carries the inner air and hardens when it meets the outer air, a substance that moves freely to every part but is nonetheless contained, a healer that goes without restraint to any needy place in the body. It moves under pressure--the "obligation to return" that fascinated Marcel Mauss--and inside its vessels the blood, the gift, is neither bought nor sold and it comes back forever.