Blood Circulation_英语笑话

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."