posted on Mar, 26 2009 @ 09:53 PM
Blood is a continually pumped liquid that moves via pressure form the heart through the arterial system and then back through the veins. There is no
'two-way communication' required since the pressure does not release at the cells. The blood never leaves the blood vessels.
Oxygen, nutrients, etc. are gathered through the process when the blood comes into contact with the pulmonary walls or the digestive walls. These are
exchanged for waste products whenever the blood comes close enough to a cell that needs such a transfer. The waste products are then given off to the
liver, kidneys, lower digestive system, and lungs when the blood comes close to them.
It sounds like you are thinking each blood cell gets some sort of target which it travels to and then has to make its way back to the heart for more
instructions. It doesn't work that way; it's more of a 'get when you can, give when it's needed' set-up.