Theory:

Human circulatory system:
circulatory system.cmap
The role of circulatory system:
2141_CircSyst_vs_OtherSystemsN.jpg
Reference:
http://cmapspublic3.ihmc.us/rid=1J7VDPLLW-1QV77GF-R9R/circulatory%20system.cmap?rid=1J7VDPLLW-1QV77GF-R9R&partName=htmljpeg
https://upload.wikimedia.org/wikipedia/commons/f/fd/2141_CircSyst_vs_OtherSystemsN.jpg