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The main components of the human circulatory system are the heart, the blood, and the blood vessels. The circulatory system includes: the pulmonary circulation, a "loop" through the lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood.
Originally posted by TheRedneck
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.
Originally posted by wtf2000
but what cycles it back through im not gonna sit there and be like well how much non oxidized blood needs to be transported from the heart to the lungs, but i always always told in school the heart does the conversion and routes the blood to the needed subnet, but how does the subnet reroute the packet back to the server since there is not two way communication