Answer:
1. heart : heart is a muscular organ that pumps the blood for the body
2. blood vessels : blood vessels carry oxygenated and de-oxygenated blood from heart to the body and vice versa
3. blood : blood has oxygen and other nutrients to supply essential substances for the body
4. brain : receive, process and send signals and commands for the body
5. spinal cord : spinal cord serves as the highway to transport messages to and from the brain
6. nerves : nerves receives and send signals for different tissues to function well
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