Make a good argument for blood to be placed in an organ system OTHER than cardiovascular. Make a good argument for blood to remain in the cardiovascular system.

I chose lungs because it is positioned in the same level as heart, but don't know what else to say. As far as blood staying in the cardiovascular system, I guess because it pumps blood to the tissues and organs all over the body, and if it is in lungs then we have problem of respiration.... please help me with some strong good arguments for both.