How Much Acceleration Can the Human Body Take?

Hey guys, Matt here with Science or Fiction. Is there a speed that would harm humans? There are a bunch of human speed records that are going to be broken in the coming decades as we develop the technology to travel thousands of miles in minutes by land, send commercial flights into low-earth orbit, and attempt to get humans to Mars. So with all of these possibilities on the horizon, a question you might be asking yourself is “How fast is too fast?” Well, first of all, wherever you are, no matter what you are doing, you are traveling nearly 67,000 mph around the sun. It just doesn’t feel that way because the Earth, and therefore humans, have been traveling at that constant rate for, well, you know, kind of a while. We can actually survive any speed, as long as it remains constant. It’s when acceleration enters the equation that things can get pretty tricky. In fact, if you accelerate at a high enough rate, you will pass out and very possibly die. This is because of what happens to the human body when it experiences g-force, or the unit of measure for acceleration.

It’s based on Earth’s gravitational pull. One “G” is the force applied to an object by gravity while on the Earth’s surface. It’s also the force that determines how much we weigh. So, simply put, if a human experiences three Gs, they experience a force on their bodies that is three times their weight. We can experience the G force in different ways. If we accelerate horizontally like in a car or train, we can experience g-force either front-to-back or side-to-side. We can actually withstand quite a bit of g-force when it’s hitting us horizontally. It’s when things go vertical, which mostly happens only on airplanes, roller coasters, or during space launches, that things get much more dangerous much more quickly. Vertical g-force can be experienced negatively (foot to head) or positively (head to foot). Both can be experienced in an airplane or rocket, and both can pose serious threats to our health because of what happens to all the liquid in our bodies under these circumstances.

With a high level of negative Gs, blood can end up collecting in our heads. Sometimes a “red out” will occur, with the lower eyelids swelling up so much that they cover the pupils. If enough blood pools in the head, which can happen around 2 or 3 negative Gs, a person will lose consciousness. Yikes. And other scary stuff happens when force is applied from head-to-foot and blood starts to pool in the lowest parts of our bodies. At 1 G, or normal conditions, our bodies require 22 millimeters of mercury blood pressure to pump blood from the heart to the brain. That requirement doubles at 2 G, triples at 3 G, and so on. There’s a certain point at which our bodies can no longer muster the energy needed to pump the blood, which happens around 4 or 5 Gs. Blood ends up pooling in the feet, and the rest of the body receives such a lack of oxygen that a person first loses vision due to a lack of blood flow to the eyes and then consciousness altogether when the brain loses enough of its blood flow.

This phenomenon is known as G-LOC, or G-induced loss of consciousness. Both head-to-foot and foot-to-head vertical g-forces can eventually lead to death if they last long enough. So, given that humankind is on the precipice of breaking many speed records, and given that many of the airplanes and spacecrafts that will be involved could one day fly your average Joe across the country at record speeds, how can we make sure that we, you know, don’t die? Pilots do a number of things to train for serious g-s. The better shape they are in, the more able they are to control their body’s blood flow with their muscles. Leg and abdominal muscles must be particularly strong so that when pilots tense these muscles, they can keep the blood from draining to their legs while experiencing positive g-force. Well, it’s sort of unrealistic to expect everyone to have abs of steel, so other solutions might be needed if we are to expect average people to eventually accelerate at rates currently only safe for fighter pilots. One solution could anti-g suits, which pilots have been using since World War II. They utilize air bladders to keep the blood in our bodies where it should be.

Fortunately, we haven’t quite reached the point where people like you or me (unless there are a lot of fighter pilots watching this episode, then I take that back) are going to be accelerating at deathly rates. There is still time for us to figure out how to manage all that. For more epic stories of innovation that shaped our future check out THE AGE OF AEROSPACE.COM Did you know we learned a lot about how g-force affects the body from one guy shooting himself down a track in a rocket sled? It’s true, check it out here. Before you do that, make sure to subscribe and be sure to keep coming back to Seeker for more videos..

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