Why is anatomy and physiology hard to understand?
There is nothing hard and easy to understand. Both anatomy and physiology are easy to understand.
In life science there a principle known as "form follows the function" Structure of the tissue, organ or system is its anatomy. Functional system is its physiology. There are many examples to follow. Eye structure that is anatomy of the eye and physiology of the eye goes hand in hand. First follow the structure then try to concentrate on its physiology .