ELI5: When someone loses a hand/arm etc., and the wound is cauterised, what happens to the blood flow? Wouldn't the arteries leading to the wound clog up with blood, leading to extremely high blood pressure and a heart attack?

Our circulatory system is a lot like a big road system. Now imagine you’re taking a trip in a car. Along the way there’s a wreck that no one can pass. It slows cars down and there’s traffic buildup so you take a detour. You exit off the highway and try to find a different way to go. In very simple terms this is how circulation works in the body. Red blood cells behave a lot like people in cars - they hate traffic, and they always want to take the least congested route. In the body, what path these cells take is determined mostly by pressure, traveling from areas of highest pressure (left ventricle) all the way back to areas of lowest pressure (the right atrium). So what happens when there’s a kink in the system? To answer your question about cauterization, this is like a whole road being closed. Red blood cells build up behind the blockage, increasing crowding, which increases the pressure. Blood cells coming into the jam are more likely to be detoured into another vessel branch that has less pressure. Now our circulatory system is incredibly connected by branches that we call “collaterals.” This connectivity prevents the problem of “extremely high pressures” that you’re talking about because there are literally millions of release valves that keep the pressure manageable. They’re essentially branches in parallel, if you know something about circuits.

The amazing thing about collateral circulation is that if there is still high pressure, the body will make completely new vessels to reduce the pressure over time. It will even remodel existing blood vessels to handle more blood than they could originally. A great example of this is the rib notching phenomenon in patients with coarctation of the aorta. This is a disease where the aorta has a spot that looks like someone tied a rubber band around it. So instead of passing through this tight squeeze, blood takes a detour through vessels that go around your ribs. The vessels get bulkier from all the new blood they have to carry, eventually causing them to create “notches” in the rib where the vessel bulges against it.

There are some cases where the catastrophe you described can happen though, and this typically only occurs where there are no alternative branches for blood to escape. The best example I can think of is a saddle embolus, often happening from a clot in your leg veins that breaks off and travels all the way to your heart. Some background, there is only one route to get from the heart to the lungs - the single pulmonary trunk that branches off into the right and left pulmonary arteries. If a clot is large enough to get caught at this branch point it will “saddle” both arteries, cutting off blood supply to the lungs and left heart which kills you almost instantly.

So the main idea here is that collateral circulation is the key to keeping your blood flowing when there is a blockage. I mentioned two examples that show the role collaterals play. In coarctation of the aorta, the body may divert blood through pre-existing collaterals as an emergency pressure release, and remodel/make new ones as a long term fix. But in a saddle embolus there are no collaterals and there is no time to make new ones. The pressures build up to dangerous levels on the right side of the heart and fall to near zero on the left side. Eventually the heart stops pumping altogether and you die.

Explain Like I`m Five: good questions, best answers.