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When you think about it, it’s kind of mind-blowing that open-heart surgery is even a thing. In most of these surgeries, doctors crack open the patient’s rib cage and literally stop their heart, sometimes for hours at a time.

And it’s only possible thanks to the heart-lung bypass machine, a device that keeps you alive by basically pumping your blood and breathing for you. It’s essentially an artificial heart and a set of lungs. But it’s more than just a pump and some oxygen.

Scientists have been trying to pull blood out of the body and put it back in again since the early 1800s, but it took until the 1950s before they figured out how to build a bypass machine safe enough for actual surgeries. And they’re still tweaking the design today. Every organ in your body needs oxygen from your blood to survive.

Your brain, for example, can only last somewhere between 3 and 10 minutes without oxygen before it experiences severe damage. So if you can’t keep someone’s blood oxygenated and circulating while their heart’s stopped … well, you can’t do open-heart surgery. Work on this started back in the 19th century, with surgeons who were trying to accomplish something much simpler: preserving individual organs after death by pumping blood through them.

But their attempts quickly failed because blood starts to coagulate, or clot, pretty much as soon as it leaves your blood vessels. Normally, coagulation is good, because bleeding forever is, uh, bad. But when doctors tried to temporarily remove blood for their work, it would harden before it could be of any use.

The solution to this problem came in 1916, when a medical student accidentally discovered a compound we now call heparin. Heparin prevents a clot from forming by keeping the proteins dissolved in your blood from turning into fibrin, the stuff that eventually thickens and hardens into a full-blown clot. That keeps the blood in its flowy, liquid state long enough to safely leave the body for a little bit.

Heparin was super useful, but it didn’t solve the other major problem: getting oxygen into the blood. Those 19th-century surgeons had tried literally shaking blood and air together in a balloon before pumping it back in again. It actually … kind of worked, but only for small, single organs.

And the blood was pretty beat up by the process. So doctors looked for new designs. Two types of machines emerged: bubble oxygenators, which bubbled pure oxygen through the blood, and film oxygenators, which spread a very thin film of blood over a rotating disk to expose it to oxygen.

But even then, the blood wasn’t absorbing enough oxygen to keep an entire body alive. It wasn’t until the 1930s that an American surgeon named John Gibbon found a way. At first, he tried swapping the disk for a drum, because it created a larger surface area for gas exchange into and out of the blood.

But he soon realized that to oxygenate enough blood to keep someone alive, you’d need a drum way too massive to be practical. So instead, he designed a machine that used a series of screens to keep a large volume of blood thin enough to absorb lots of oxygen. His machine also had better pumps, along with sensors that could regulate the blood flow and check for clots or air bubbles that could potentially block circulation.

In 1953, Gibbon used his design to perform the first successful open-heart surgery where the patient was put on bypass — a huge milestone. But there was still one big problem: Gibbon’s machine and others of its kind were direct contact oxygenators. In other words, they exposed the patient’s blood directly to air.

The direct exposure sometimes led to blood absorbing too much oxygen, which can be just as deadly as not having enough. Plus, the different flow speeds and pressures would stretch and eventually damage blood cells, which could then die or cause clotting disorders. Which meant that even though they kept people alive, these bypass machines were doing a lot of damage to their blood.

Doctors had to find a way to treat blood more gently, so the next generation of bypass machines were designed to oxygenate blood through a thin membrane instead of exposing it directly to air. It was a lot like what happens in your lungs, where blood absorbs oxygen through the tissue in tiny air sacs. These, like the machine’s membranes, provide just enough of a barrier to protect the blood while still allowing oxygen in and carbon dioxide out.

For a while, layers of silicone membranes were all the rage for bypass machines because they were strong, but still thin enough to let air through. The whole process treated blood much more gently, which led to less damage. But these days, doctors mostly work with machines that use microporous hollow fibers instead.

These fibers have extremely tiny holes that are just a few thousandths of a millimeter wide, which allows for even better gas exchange. The only problem with them is that they tend to leak over time, which … is not something you want happening during an open-heart surgery. So some machines still use the stronger silicone membranes.

Meanwhile, researchers are looking for new, more durable microporous fibers to use in future machines. So, there are some problems left to solve when it comes to keeping people alive for long periods of time while their heart is stopped. There are still some risks to using bypass machines, and even our most advanced artificial hearts can’t keep someone alive for more than a few days, max.

But they work well enough to support patients during thousands of life-saving open-heart surgeries every year. And we’ve definitely come a long way since shaking blood and air together in balloons. Thanks for watching this episode of SciShow!

If surgical triumphs are your thing, you might also like our episode on xenotransplantation: people transplanted with animal parts! [♪ OUTRO].