The answer is Circulatory system.
your lungs need Oxygen (duh) just think for a second.
your nervous system is like your veins ans stuff like thats so it cant be that.
the digestive system i mean COME ON-
you obviously know what that is. does food go in your lungs? no. no it doesnt. so eliminate that. ok now we have respiratory system and circulatory system. ok i maybe forgot what respiratory system is but i know its not the answer. there.
bOoM.
