Now this was tricky. You can't have one system without the others. I knew there would be no going to the teacher's desk to voice my perplexity over this apparent contradiction. That's when the answer became clear. Each system relies on fuel, nutrients in order to perform its function.
Life is sustained, it is carried in our blood.
It wasn't the most obvious answer, but it was the right one. I got the extra credit points and later a better understanding of why God views blood with such reverence, such importance. If blood represents life and the penalty for sin is death, then no wonder blood is the price tag, the debt owed for sin. Before that first Easter, God accepted the blood of animals as a temporary substitution for our sin. But animals weren't the ones from whom the penalty was due.