The heart – Heart rates spike to over 155 bpm, with the highest heart rates recorded during the slow climb to the top of the first drop, emphasizing the key role of fear and anticipation. While the rising heart rate is not dangerous for young, healthy people, it could set off a heart attack for those with pre-existing heart disease.

The brain - People enjoy scary situations so long as they aren’t in any real danger. Much like watching a scary movie, a roller coaster ride triggers a fight-or-flight response, which releases a mix of hormones into your bloodstream, including adrenaline, dopamine and cortisol, stimulate a natural high and give you a boost of energy that makes you feel more alert.

Blood - Roller coasters rely on gravity which can have some interesting physical effects on the body. On the ground you are experiencing 1G, or one times the normal force of gravity, but the record for the highest G-forces on a roller coaster is 6.3Gs. With increasing gravitational pressure, blood is forced toward your feet, making your heart work even harder to get it back up to your brain. The typical human body can withstand around 5Gs, but at these levels the brain is deprived of blood and oxygen, sometimes leading to light-headedness, vision loss or momentary black outs.

Balance - Interactions between our eyes and our vestibular system, found in the inner ear, detect motion, balance and spatial orientation. However, the jerky and unpredictable movements of a roller coaster can cause the eye and the ear to send mismatched messages to the brain. For kids, this internal confusion feels exciting. But for adults, who are accustomed to more predictable motion, the outcome is often dizziness, vertigo or motion sickness.

Sources:

https://www.ahealthiermichigan.org/2019/07/08/how-roller-coasters-affect-your-body/