A kidney.
Helen: But there is one thing I would like to know. I can see how blood can run down-hill into our fingers and toes, but I can’t see how it can climb back up to the heart again. Will you please tell me?
Mother: The heart is the power that sends it through the arteries to every part of the body, whether it is up-hill or down. Now when the blood has come to the end of its journey, and has reached the tiny hair-like veins of which I told you, more blood keeps coming down and pushes it on till it starts back through the larger veins. The blood keeps crowding behind, and the veins are made in such a way as to help it climb up.
Percy: But how are they different from the arteries?
“Veins have tiny pockets in them.”
Mother: Did you ever see little watch-pockets hung in bedrooms in which to put watches? Well, the veins have tiny pockets in them, as you see in the picture.
Amy: But I don’t see how that helps the blood in climbing.
Mother: It is this way: If you had a tube with little pockets and should hold it so the top of the pockets was down, you could pour anything through it and they would not stop it from passing. But turn the tube the other way, with the pockets up, as you see in the picture, and they would catch and hold anything you tried to pour through the tube. It is the same way with the veins and the blood. If the blood should try to go back, the pockets would fill full and hold it, but when it is passing up toward the heart, they let it slip by without holding it back.
Elmer: Then the blood keeps going round and round in the body, and never stops.