Explaining a cardiac system (a heart):
F-eng: build a mechanism that can do what the heart can do
R-eng: F-eng + explain (and/or build) the structure and the function
of the biological heart itself:
what the heart is (made out of)
how it in particular happens to do what hearts can do.

Explaining a conscious system (the brain):
F-eng: build a mechanism that can do what the brain can do
R-eng: F-eng + explain (and/or build) the structure and the function
of the biological brain itself:
what the brain is (made out of)
how it in particular happens to do what brains can do.

Either way, the causal explanation is a structural/functional one.