Blood is the one tissue that comes into contact with every other tissue of the body, and it is through blood that our disparate parts communicate, through blood that our organs cooperate. Without a circulatory system, there would be no internal civilization, no means of ensuring orderly devotion to the common cause that is us.

“It’s an enormous communications network,” Dr. Schafer said — the original cellphone system, if you will, 100 trillion users strong.

Blood can also be thought of as a private ocean, a recapitulation of what life was like for all the years we spent drifting as microscopic, single-celled organisms, “taking up nutrients from sea water and then eliminating waste products back into sea water,” Dr. Schafer said. Not only is blood mostly water, but the watery portion of blood, the plasma, has a concentration of salt and other ions that is remarkably similar to sea water.

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