kidney vs excretory organ

kidney and excretory organ both are nouns.

Word	Noun	Adjective	Verb	Adverb
kidney	Yes	No	No	No
excretory organ	Yes	No	No	No

As nouns, excretory organ is a hypernym of kidney; that is, excretory organ is a word with a broader meaning than kidney:

kidney: either of two bean-shaped excretory organs that filter wastes (especially urea) from the blood and excrete them and water in urine
excretory organ: an organ that separates waste substances from the blood and discharges them

Other hypernyms of kidney include urinary organ.

kidney (noun)	excretory organ (noun)
either of two bean-shaped excretory organs that filter wastes (especially urea) from the blood and excrete them and water in urine	an organ that separates waste substances from the blood and discharges them