Why do we break the into "systems" when discussing it? Why don't we just talk about the entire body all at once?

Sometimes we do talk about the entire body, but you rarely have something to discuss that actually involves all the systems of the body. Usually when you talk about the body, it is about a specific body part such as a limb or organ.

For example, if you are talking about an ankle sprain, you are not necessarily talking about the entire body. You are just talking about the ankle and the ligaments, muscles, and bones around the ankle. You are not also talking about the heart or the eyes of a person as well, just the ankle.