Blood Circulation

A teacher was giving a lesson on the circulation of blood. Trying to make the matter clearer, he said: "Now class, if I stood on my head, the blood as you know, would run into it and I should turn red in the face."

"Yes, sir," the class said.

"Then why is it that while I am standing upright in an ordinary position the blood doesn't run into my feet?"

A class member shouted, "'Cause your feet ain't empty."