Greetings
Eventhough the HRD and every departments are playing their own roles and every departments have their own credibility, the recruitment, posting, appraisal, promotion, retirement, resignation, training & Development, Discipline and other activities are to be done only with the HR Department and without the HR's involvement, the activities done will not have the completeness.
The Medical Department has their own role in that, obviously no one can interfere. If the Medical Department recommends for sick leave, the HR department has to sanction and they cannot deny. But if other line managers or some Department Head recommends for leave, that can be denied by the HR Department with proper reason.