Why can blowing up a balloon make you pass out? Because we hyperventilate and have too much oxygen and not enough carbon dioxide. Counterintuitively, it is too much oxygen and not enough carbon dioxide that can cause people to faint when they blow up a balloon. One would think that it is a lack of oxygen, but it is the reverse.
Fainting is technically called syncope, which comes from Ancient Greek via Latin and meant “cut off”. If you get very lightheaded but don’t actually faint, it is called presyncope. Fainting is caused because of a sudden drop of blood pressure, reducing the amount of blood that reaches the brain, or a rise in blood pressure, stopping blood from reaching the brain.
When you inflate a balloon, you are breathing in very quickly and then breathing out all of the air you have just breathed in. You are basically hyperventilating, which means you are breathing in lots of oxygen and breathing out lots of carbon dioxide. That sounds like it should be a good thing because we all know that too much carbon dioxide can be dangerous, but it isn’t. Your blood has to be between a pH of 7.35 and 7.45. The pH scale goes from 0 (very acidic) to 14 (very alkaline), which means blood is in the middle and ever so slightly on the alkaline side of neutral. Carbon dioxide is acidic and keeps the blood at the pH level it is supposed to be. When you breathe out so much carbon dioxide, your blood becomes too alkaline and you get something called respiratory alkalosis.
The brain monitors how much carbon dioxide is in the blood in order to regulate breathing. When the level of carbon dioxide falls, the brain directs the kidneys to begin making the blood more neutral. It also believes that it doesn’t have enough oxygen because the carbon dioxide level is so low, so it shunts blood to vital areas and constricts the veins. This is so that blood can be sent to parts of the lungs that have a higher oxygen content and to deliver oxygen to the tissue. However, because the vessels are constricted, the heart has to work extra hard to pump blood around the body. The brain notices this and sends an order to the heart to relax. The heart does what it’s told and relaxes. The brain obviously hasn’t thought this through because blood flow stops, the brain is without blood, and you pass out. If all you have done is blow up a balloon, your carbon dioxide levels will return to normal very quickly and you will recover.
Because the drop in carbon dioxide has made the blood more alkaline, there is a drop in calcium ions, which excites the nerves and gives pins and needles in the hands, feet, lips, and other tingling sensations. This is the reason why people who are hyperventilating are told to breathe into a paper bag. Rebreathing in their own exhaled carbon dioxide will fix the problem and reduce the feeling of panic.
This is why we pass out when we blow up a balloon. However, it is not the most common reason why people faint. That would be divided into two categories and both are because of low blood pressure. The first category is when you faint because you stand up too quickly. When you crouch or kneel down, the distance from your feet to your brain is not very far and blood tends to pool in your legs. When you suddenly stand up from this position, the distance from the brain to the feet is suddenly greater, yet there is still a lot of blood pooled in the legs. The heart has to quickly work hard to overcome gravity and get the blood out of the legs, so blood pressure drops and there is not enough blood in the brain. Without blood, the brain can’t work, and you pass out. When you are lying down, the heart doesn’t have any trouble pumping blood to the brain because it is not fighting gravity and you wake up pretty quickly.
The second category is when you pass out because you have been straining. Maybe weightlifting, or, and this is more common than it sounds, on the toilet. When you strain, there are pressure receptors inside the blood vessels in the neck that detect it. They misunderstand the signs as high blood pressure and react by sending a signal to slow the heart down to lower the blood pressure. This results in not enough blood reaching the brain, and fainting. And this is what I learned today.
Sources
https://en.wiktionary.org/wiki/syncope
https://www.healthpartners.com/blog/what-happens-to-your-body-when-you-faint/
https://en.wikipedia.org/wiki/Syncope_(medicine)
https://my.clevelandclinic.org/health/symptoms/21699-fainting
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/fainting
https://my.clevelandclinic.org/health/diseases/21657-respiratory-alkalosis