Translation:The heart is the organ responsible for the circulation of blood.
There should be more ways to say this sentence, no? 'responsible organ' or 'organ that's responsible' I wouldn't really say the sentence that's above. I don't know why, but I would say either of the two I mentioned. However is duo's sentence more accurate?
‘The heart is the responsible organ for...’ would be a very unnatural sentence. Duo's translation is better.
Duolingo's sentence is the most literal translation possible. Often that it is what they want, even though there are alternative sentences, as you suggest, which are semantically identical.