Biology Lesson

A teacher was giving a lesson about the circulation of blood. He said, “Now who can help me solve this mystery. As you know, if I stood on my head, the blood would run into it and I would turn red in the face.”
“Yes, sir,” the kids chorused.
“So why is it,” asked the teacher, “that when I am standing upright, the blood doesn’t run into my feet and make them turn red, like my head?”
The kids were silent for a moment, when a voice from the back called out, “I know, because your feet aren’t empty!”