Form items can be referred to as bind variables in Forms PL/SQL, e.g.
pmrn := :PATIENT_BLOCK.MRN_FIELD;
:PATIENT_BLOCK.FNAME := var_ptuple.p_fname;
etc.
Be aware, however, that you most probably don't need to write all this code. Just set the block source to be the table and execute a query on it - Forms will take care of loading the records for you.