Tsunade is the head. Without her, the Land of Fire would be nothing. She is the leader of many other parts, some useless where others are needed. She in herself has no special purpose but being in charge, to house the other parts. To lead the other parts that would otherwise be able to do nothing. To give information to the parts that are given to her from the brain, without knowing if this is true or not.
Naruto is the mouth of Team Seven. For all his talk, he can do nothing but make noise. He cannot follow through with whatever he says. Only what he says. He can't think for himself, a job for other parts, and that has nothing to do with his part. His only real job is to relay information to other bodies.
Sakura is the eyes. For all she can watch what happens, she can't do anything about it. She tries, but it is impossible. She can relay what she sees to the other parts that are to do something much more than she could ever think of doing. That would require her to do something other than watch.
Sasuke is the ears. He hears what people says, but does nothing about it. He can't think on what has been said, as that is not his job. Others chatter in his ear and he has learned to turn it off, so that only the things that are "important", that require no thought can go through.
Kakashi is the hands. The hands that teach, that fight, that do what they are told without thinking (as they aren't the brain and therefore cannot think). He can take the information given and go only on that, himself only to be used as a tool by the rest of the body.