That is because being prokaryotic or eukaryotic doesn't depend on the size of the organism, but rather on the cellular structure. Both are eukaryotic because they have clearly defined nuclei and functioning organelles that are differentiated, unlike prokaryotic organisms.
Well its a little deeper than that. The oxygen transport is made through Hemoglobin (red blood cells) which is pumped from the heart
Explanation:
I think it all of the above