Physicians have been trying to stick fluids into patients—or experimental animals—for hundreds of years. In 1656, Sir Christopher Wren wanted to see what would happen if he took a quill and a pig's bladder and used it to send intoxicants—beer and wine and opium—directly into a dog's blood. (The dog was intoxicated.) That was just the beginning of a series of 17th-century experiments to infuse blood with other substances—mostly blood. Experimentally minded researchers and physicians inject animal blood into other animals, animal blood into humans, human blood into other humans.
At the same time, often physicians were trying to get blood out of sick humans, through bloodletting. This was a common practice in the 1830s, when a cholera epidemic hit England. The Royal College of Physicians, though, wanted to look more closely at how cholera affected its victims's bodies, and it sent a young Scottish physician, William O'Shaughnessy, to investigate.
Building on work in Russia, which had shown that cholera patients's blood was much more concentrated than normal—it had 30 percent less water in it—O'Shaughnessy reported back to the college that, to help cholera patients, doctors should attempt to bring blood back to its natural state. Adding water didn't seem to be enough, though: The blood needed the right saline content, too.