Car accident: What happens to the human body & the physiology behind it

Knowing certain basics of a car crash could help us make sound decisions that can potentially save our lives.

BHPian GKR9900 recently shared this with other enthusiasts.

The Ministry of Road Transport and Highways (MoRTH) is observing road safety week from January 11th to 17th, 2023, to promote awareness about road safety to the masses. Various activities are being organised throughout the country involving citizens and officials to promote road safety. Being a medico by profession and an automotive enthusiast by passion, I wanted to contribute something from my side that could help in this cause. I am better known for my medical educator role among my circles. So I decided to put that into effect. This is my humble effort at educating about one of the most common scenarios that occur on our roads on a daily basis – a car crash, from the viewpoint of a medico trying to incorporate principles of physics and biology, or should I say physiology! What happens to the human body in a car crash? Why does it happen? What are the steps that we can take to modify fatal outcomes? I try to answer these questions in this video.

For the uninitiated, let us discuss that in the following section.

An estimated 4,00,000 road traffic accidents and 1,50,000 deaths occur on our roads every year. This equates to about 15-20 deaths an hour. Those who are critically injured are even more. Although we might think that two-wheeler accidents make up the bulk of these statistics, car accidents are not so far behind. They are the second most common cause of injuries and fatalities. And the reason for almost 75% of these accidents is over-speeding. Knowing certain basics of a car crash could help us make sound decisions that can potentially save our lives.

A large percentage of car crashes are head-on collisions with a stationary or moving object. For the sake of simplicity, this is what we are going to focus on here and I will try to keep the technical and medical terminologies to a minimum for better understanding.

Physics behind a car crash

When we mash that accelerator pedal and build up speed in a car, we are creating kinetic energy. When we slow down, we apply brakes which helps in dissipating this kinetic energy as heat. In a normal braking scenario, the energy is dissipated gradually over a period of time in a manageable way. What happens when we brake hard? Here, the time available to come to a stop is less but energy has to be dissipated nevertheless. Deceleration occurs rapidly as the energy needs to be dissipated faster, leading to the violent shedding of speed. You might remember that last instance when hard braking threw you forward. Nothing personal here, just energy trying to dissipate itself. Keeping that in mind, imagine a scenario where the entire kinetic energy of a moving car and its occupants need to be dissipated in a matter of milliseconds and almost nil distance. This is what happens in a head-on collision. When there is not enough time available to dissipate the entire kinetic energy as heat via brakes, it finds other ways to dissipate itself, one of which is a massive amount of mechanical energy. Some transfer of kinetic energy into heat and sound also takes place. The transfer of this mechanical energy from a metal cage (the car) into a fleshy capsule (the human body) is what happens during a crash. The energy released can be so huge that it can rip through steel, not to mention fragile human tissue. That is why I said in the beginning- this is where physics meets biology; tremendous amounts of physical forces exert their effects on biological systems.

So what are these effects?

To answer that, we need to look at a crash from a different perspective. In a single-car crash, there are 3 collisions which take place.

Collision no.1: Metal hits Metal

Here the car collides with an object and comes to a stop. Based on the nature of the collision, and the speed and size of the involved vehicle, it can come in many forms. The kinetic energy of the car dissipates by breaking the car’s structure, the object that comes into contact and the rest is transmitted to the occupants.

Collision no.2: Body hits Metal

This is felt directly when the occupant collides with an object inside the car like a steering wheel or dashboard. When a car is mobile, the occupants inside are moving at the same velocity as that of the car. When the car suddenly stops, unless some restraining mechanisms are in place, the occupants continue their motion until they hit an object. This is in accordance with Newton's 2nd law of motion, an object in motion remains in motion until acted upon by an external force.

Collision no.3: Body hits Body

Here, the internal organs bump into the inner lining of the body cavities or bump against each other. Our internal organs are housed inside cavities, whose walls are made of bones, muscle and skin. Similar to the occupants inside the car being subjected to kinetic forces, the organs inside the body experience the same. When the occupant hits an object inside and comes to a stop, the internal organs are still moving along with the original velocity, bumping into each other. This can cause a variety of damages.

So what kind of damages are we talking about? We are fragile creatures. And there are a lot of things that can break when crash energy enters into our body. We will try to understand some of the most common and potentially fatal injuries that can occur.

Biology behind a car crash

Every tissue in our body has got an inherent strength to it. Collisions during a car crash release shock waves throughout the body. These shockwaves exert varying amounts of tensile, compressive, shear and torsional stress on the occupants. When the forces generated by these waves exceed the native strength of the tissues, injuries occur involving everything from cells to organ systems.

Head injuries are one of the most dreaded injuries that can occur in a car crash. Our brain is a soft and pliable structure that is safeguarded by multiple layers of protection. We have the skull, the skin and the muscles over the head that help in this regard. The brain is cushioned and suspended inside the skull in a fluid known as cerebrospinal fluid (CSF). To start with, blunt force trauma can cause skull fractures and hematomas on the outside of the brain. If the forces are stronger, it can lead to the dreaded traumatic brain injury (TBI). There are multiple ways in which TBI can manifest.

When the head suddenly decelerates as in the case of a car crash, the brain suspended in the CSF continues moving forward and hits the front side of the cranial cavity, then rebounds to hit the backside. This is known as a coup-contra coup injury.

This can cause shearing of the blood vessels and nerves inside the brain. Shear stress can tear blood vessels causing bleeding into the brain tissue. The problem here is that the bony skull is a closed container which is already packed tightly with the brain, nerves, vessels and other structures. When blood leaks out into this closed cavity, as there is no further space to expand, the pressure inside begins to rise. This can compress the fragile brain tissue causing damage. Loss of blood also results in inadequate perfusion of the brain tissue leading to its death. Shear stress can damage the nerves arising from the brain, leading to something known as a diffuse axonal injury which results in neurological deficits. The shockwaves in a crash can cause changes at a cellular level also, with various chemicals and ions leaking out of the damaged nerve cells. These multiple insults cause brain tissue to die off, in a process known as necrosis, leading to the loss of precious grey matter. Depending on the extent of damage, the occupant can have a myriad of presentations from memory loss to coma and even death.

The neck connects the head to the body. The sudden deceleration can cause whiplash injuries as a result of neck hyperextension and flexion, although they are more common in rear-end crashes. This can damage the neck structures like muscles, ligaments, blood vessels and cervical spine.

The thorax is another major cavity that houses important organs. During sudden deceleration, the heart and lungs can hit the inside of the ribcage or breastbone causing contusions. This can also cause rib fractures. A fractured rib can puncture the lung causing air to leak into the thin space between the lung and its outer lining, a condition called pneumothorax. Just like TBI in the head, a dreaded complication that can arise in the thorax is traumatic aortic injury (TAI). Aorta is the major blood vessel that supplies blood from the heart to various areas of the body. Its initial part (ascending aorta and arch) is mobile and the rest is fixed (descending aorta). During sudden deceleration, the mobile part continues moving forward tearing itself away from the fixed part, leading to TAI. If the forces are strong enough, it can even lead to the rupture of the aorta, which is as fateful as it sounds.

Your abdomen contains a lot of delicate and important organs, which unlike the brain, heart and lungs are not protected by a bony cage. This puts them directly in the line of fire of the shockwaves arising out of a car crash. Injuries ranging from contusions to lacerations can occur depending upon the strength of the crash. Imagine your liver or spleen getting sheared in half. Not a pretty picture right?

During a crash, it is natural for the occupants to adopt a defensive posture. The limbs that stick out of our body – the arms and legs, are among the first structures that get struck. The bones inside can break, and they can tear away neighbouring vessels and nerves causing a whole lot of problems.

I know, we might be getting a bit too ‘medical’ here. This is just the tip of the iceberg. Depending on the nature of the collision, the velocity of impact, the position of the occupants and a whole lot of other variables, there can be umpteen injuries that can occur in a car crash. Not to mention the post-traumatic stress disorders that some can be crippled with.

And you may not even need to be subjected to a crash to realise the horror. I still remember that one case I came across during my residency training 10 years ago, where a 20-something-year-old bike crash victim came into the ER, unconscious and mangled up. Some cloth was wound around his right forearm. I took off the cloth to assess his injuries and I still can’t shake off that feeling. The force of the crash had shattered his forearm revealing the anatomy of his bones, arteries and nerves – albeit fractured and mangled. I was 23 at the time, the proud owner of a Yamaha FZ and I dreaded taking the motorcycle out for a few weeks. The kid survived, but his arm was amputated.

This begs the question – is there anything that can be done to ensure occupant safety in case of a crash? Where do we start? We start in our heads.

How to modify a car crash?

First, we need to rejig our perspective. We need to stop referring to crashes as accidents. You don’t have to take my word for it. BMJ - British Medical Journal, is one of the iconic communicators in medicine. In 2001, they banned the word ‘accidents’ to be used in association with traffic incidents like these. Referring to crashes as accidents don’t tell you how to prevent them. That takes away the predictability factor. Realise that it is a crash. It is predictable. There are people, objects and scenarios that are involved. There are modifiers that can change the outcome.

Remember when I said that speeding is the cause of almost 75% of car crashes? That is the first and most important modifier. Speed, or in better terms, velocity! The impact and the resultant shockwaves disrupting the car and occupants are all a by-product of the kinetic energy trying to redistribute itself. How do we calculate kinetic energy? Your high school physics teacher says “hi”. KE = ½ mv2. It is directly proportional to the square of the velocity.

When you’re involved in a 50 km/h crash in a 1.5-ton vehicle, the energy released is close to 150,000 Joules. That is like 1000 people trying to punch you all at once. Double your speed and you are left with 4 times this energy to dispose of. Imagine fending off 4000 people trying to have a go at you. That is the size of an academy award crowd! And Chris Rock could hardly fend off a slap from Will Smith.

Imagine if he had to face this

Believe me, no matter how strong you think you are, that is just way too much energy to handle. Reducing your speeds to 40 km/h decreases this kinetic energy to just 2/3rd of its initial value. Reducing speed also gives us enough reaction time to apply brakes and slow down even further. Now, the kinetic energy might look more manageable, but it can still cause considerable damage. That is where engineering comes into play.

Just to have some fun, I decided to play around with a simple experiment that I found in the crash science section of the IIHS (Insurance Institute for Highway Safety) website: The eggselerator!

For those who are interested, this is their URL. Their resources are really well laid out.

My rendition of this experiment was a little different from theirs. I decided to use a toy car instead of building one out of wheels and mini axles. The idea is simple. To design a mechanism using paper that can prevent the egg from cracking when it hits a stationary wall. I also decided to have some fun with my phone’s slo-mo cameras while at it. And these are the results.

What I ended up doing was creating a rudimentary crumple zone.

Whatever energy is generated during a crash can be diverted onto other structures away from the passenger compartment with the help of crumple zones. Modern cars are engineered with crumple zones to absorb as much energy of the impact as possible and allow sufficient time for the occupants to decelerate so as to minimize injuries. Remember collision 1? Crumple zones help in mitigating this. This is why choosing cars that are specifically designed to absorb this impact energy is important. It is not exactly the weight of the car that determines its crashworthiness. It is the way in which everything is designed and fitted to divert that energy away from the occupants that matter. Cars like these are the ones which usually score well in various crash tests.

The second collision, where the occupant’s body hits an object inside the car, can be taken care of by using restraint systems. That is why buckling your seat belts are very important. In the event of a crash, the seat belts slow down the occupant from hitting on the inside of the car. But simply wearing seat belts is not enough, you have to wear them properly.

There’s a really good thread (Guide: How to fasten your seatbelt properly)already on this topic in Team-BHP.

Seat belts anchor us to the car at two of the strongest points of our body-the chest and the pelvis. The lap belt should be close-fitting to your hips, and the shoulder belt should be centred over the opposite collarbone. If the belts are not positioned properly, they can wander off into the nearby soft tissues like the neck or abdomen, which can prove to be disastrous in case of a crash. The next time you see someone walking off a car crash with a broken collar bone, ask him to thank his stars because the seat belt just did its job!

Then we have the supplemental restraint system – the airbags. When the sensors in the car detect that the crash is severe enough, the airbags are inflated in an explosive manner. The occupants are cushioned against impact and they slowly deflate providing ample time to dissipate energy. Here also, wearing seatbelts is important as:

  1. Most cars enable airbags only if the seat belts are fastened
  2. The occupant should contact the airbags only after complete inflation.

If not, the occupant will be subjected to the considerable amount of force that is produced while inflating an airbag leading to injuries.

Regarding the third collision, there’s nothing that can be done to avoid the organs bumping into each other. That is inevitable. But there is one thing that can be done. Reduce the energy, reduce the speed. Reducing speed mitigates all 3 types of collisions that occur in a car crash.

If you are among the really tolerant ones who have read this long thread till this very moment, you might have noticed that in many instances I have mentioned about how the extent of injury varies with the magnitude of force, which ultimately depends on the amount of energy to be dissipated. And this energy depends on…you guessed it….the velocity times itself.

Remember: 2 x velocity, 4 x energy; ½ x velocity, ¼ x energy.

Engineering prowess has enabled us to find ways for reducing impact energy and extending impact time. But the best solution would be to prevent crashes in the first place by following safe driving practices. No matter how skilled we are as drivers, or how capable our machines are, we still cannot beat physics or its effects on human physiology.

Remember: Drive slower to live longer.

Here's what BHPian V.Narayan had to say on the matter:

Dear @GKR9900,

Thank you for a well-written and well-illustrated explanation of this important topic. Going by the number of readers who have visited this thread sadly it is a subject not on the minds of most. More tragic given that India is the global leader in road accidents.

At 120 km/h your kinetic energy is 2.25X that at 80 km/h and 3X that at 70 km/h and 4X that at 60 km/h. This fundamental point is missed by most. Combine this with the fact that at 120 km/h, our reaction time available is significantly lower than at 80 km/h and we understand the challenge better.

Several members argue in favour of 120 km/h or higher speeds. But till we remain a low-income country our roads will serve both the rich {almost all on this forum} as well as the rural folks, the wheels of trade and animal-drawn vehicles. That is a fact of life. One usually does not reach a destination earlier by driving faster. Not in India at least. That toll plaza, that cow, that tractor, that micro traffic snarl on the road all ensure the seconds gained by driving faster get lost in a flash.

I drive inter-city a fair amount and in all my years I find 80 km/h to be the ideal speed that balances safety with reaction time and time to destination. To each his own.

Here's what BHPian vedirah had to say on the matter:

Thanks for the video! I've circulated it among my family and friends. I have a few cousins who are almost repelled by the idea of wearing seat belts for some reason. The other day we were going to Varkala and I told her to wear a seat belt. She had such a frown on her face like I asked her to jump off a cliff or something. They labelled me 'lame' for wearing my seat belt. Eventually, they budged and agreed to wear a seat belt when I told them I wouldn't pay the fine if we got caught.

Today she got her own new car, a Swift. I sent her this video and many others describing the importance of seat belts. Now, my question to this forum is - is there any way to make the seat belt reminder beeps louder or more persistent? I would like the sound to stay on permanently until the time she wears the seat belt.

Here's what BHPian locusjag had to say on the matter:

There is a major cause for brain injuries that most folks don't realize - the insides of the skull aren't a smooth surface. When the brain collides against the Cerebrospinal Fluid sack, it gets pressed against several hard ridges, undulations and needle-like sharp surfaces that the skull has on its insides.

This URL explains why our skull has ridges and holes - to hold the brain in one place.

And alas, I've seen a brilliant answer on Quora by a Neurologist and I'm unable to locate it right now; he had shared a pic of a skull's internal surface that has proper bony needles in it. For a brain to collide against that would be outrightly horrific. We must think way beyond concussions with the pin cushions that our skulls have in some of their internal surfaces!

Check out BHPian comments for more insights and information.

Seat belts save lives