Portal of entry

Or you can also call it 'route of exposure' in toxicology it is the body site at which substances enter the body

I can think of the following routes of exposure (maybe you can imagine others too)

Oral - gastrointestinal tract

Inhalation - respiratory tract (the latter including the alveoli)

Dermal - skin 

Mucosal surfaces - e.g. in the nose - I guess the eyes could be placed here too (if not having its own entry - ocular)