I remember a theory about veins being hydrophobic and the electric activity going on in blood resulted in it flowing through the body on its own. The heart is there to regulate it, rather than to pump it. The doctor claiming this explained that there was no way the heart pumped hard enough to push the blood all the way theough the miles of veins throughout the human body. It also referenced the fact that people who were nearly frozen to death who had the heart stop did not require cpr. As soon as the blood reached a certain temperature it just started flowing again and the heart began beating once more.