Is this Duolingo presenting us with its funny, sweeping statements, or is this how a Dutch doctor might describe a patient?
I had to think about it for a second, but I think they mean it in the general sense. The body needs blood (in order to survive/function/etc.), but I'd love for a native to weigh in in case I'm wrong.
I generally think of the "hebben nodig" construction as "to have need of" something. In this case, "The body has need of blood."
it's a bit more like 'the body has the blood necessary' which sounds stupid i guess so think if it like 'the blood is necessary to the body'. 'nodig' is an adjective, not a verb.