Affiliate Marketing Secrets

A university medical school is affiliated with a teaching hospital. In this hospital patients have a physician (generalist) who, on needs basis, refers them to one or more specialists depending on their health problem.
The hospital has its own laboratory for performing various tests. When a patient requires tests (e.g., blood test), the specialist writes these requests and the patient presents then to the laboratory administrator. Test requests include the patient’s name, medicare card number, birth date, all the tests that are required, as well as the identification of the specialist requesting the tests and the generalist who referred the patient to the respective specialist.
This is the case, i am confused to what are the entity in this case? my guess is physicians, patients specialists and lab administrator..
I also need to create a Data flow diagram and a data dictionary.. i have no clue what those are.. help!