book lung vs respiratory organ

book lung and respiratory organ both are nouns.

Word	Noun	Adjective	Verb	Adverb
book lung	Yes	No	No	No
respiratory organ	Yes	No	No	No

As nouns, respiratory organ is a hypernym of book lung; that is, respiratory organ is a word with a broader meaning than book lung:

book lung: organ in many arachnids containing many thin folds of membrane resembling the leaves of a book
respiratory organ: any organ involved in the process of respiration

book lung (noun)	respiratory organ (noun)
organ in many arachnids containing many thin folds of membrane resembling the leaves of a book	any organ involved in the process of respiration