How to render picklist values according to those available in the related object?

I am a newbie to salesforce and I am doing a project on hospital management system in it. I have Objects for Hospitals, Wards & Patients. Patient is in a lookup relationship with Ward and Hospital. In Ward I have a picklist called ward type(values: General, ICU, HCU, ICCU, Cabin). In Patient Object I also have a similar picklist called ward type but here I want to display only those wards which are available in the hospital where the patient is getting admitted(for e.g, ICCU and HCU are not present in every hospital so it is meaningless to show those options in ward type of such hospitals while filling a new patient detail).