A channel is the host of an elongate cavity within a (complex) system of fluid circulation. A morphological (and functional) extension is needed for this definition.
(=> (_Channel ?A ?B) (Exists (?Y) (And (_Cavity ?Y True) (_Elongate ?Y True) (Host-Of ?A ?Y) (Exists (?Z) (And (_Channel ?Z True) (Connected ?A ?Y))))))