Blood Circulation.........

A teacher was giving a lesson about the circulation of blood. Attempting to make the subject clearer, he said to his class, "Now students, if I were to stand on my head, as you know, the blood would rush into it and I would turn red in the face."The students all nodded in agreement."Then why is it that when I am standing in an upright position, the blood doesn't rush into my feet?" asked the teacher.A student at the back of the class yelled out, "Probably because your feet aren't empty!"