Circulation of blood

A teacher was giving a lesson on the circulation of the blood. Trying to make them understand, she said, 'Now, class, if I stood on my head, the blood, as you know, would run into it, and I would turn red in the face.'
 

'Yes,' the class said.

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

A little fellow shouted, 'Cause your feet ain't empty.'