system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does adrenaline work so instantly?

Adrenaline, the fight or flight hormone, works in a matter of milliseconds. But being a gland which releases its hormones into the bloodstream, how does it work so fast? I think that blood circulation speed is much slower relatively.
gpt
Adrenaline works very quickly, but the initial response is actually due to your SNS! The SNS is the sympathetic nervous system. 

Basically there is a part of your brain called the amygdala. The amygdala controls responses to distress, feelings of fear, anger, etc. There is another part of your brain called the hypothalamus that controls your bodies homeostastis(keeping your body temp, breathing rate, etc. normal) Once your amygdala recognizes a threat, It signals distress to the hypothalamus which in turn signals for the neurons in your SNS to fire. 

The SNS controls your fight/flight response and sends signals to your adrenal glands(the glands that produce adrenaline) to release it, but initially sends signals to the entirety of your body to increase heart rate, widening the bronchial passages(pockets of air in your lungs to increase efficiency of breathing), etc. 

The cause of this is that the SNS is connected to a network of neurons that release noradrenaline, which is technically a neurotransmitter and a hormone, but when it is released from the SNS It acts more like a neurotransmitter, kind of like a drug, hence the immediate response. 


Your hypothalamus and amgydala respond to the threat and signal these reactions even faster than your brain is even able to visually comprehend what is happening, which is why if you put your hand on a hot stove for example, you’ll pull away before you even feel the burn initially, or you’ll jump back before oncoming traffic is about to hit you. 

The SNS is then kind of like a gas pedal, and if the gas pedal is still being pressed down, your hypothalamus will then send signals for your adrenal glands to release noradrenaline and adrenaline through your bloodstream. The adrenal glands are sitting right on top of the kidneys which is the organ where blood flow is most prevalent, and by 3 main arteries, where it quickly travels throughout your body and maintains your stress alert phase for longer. If this “gas pedal” is let go though, your PNS or parasympathetic nervous system will be activated by your brain through neuron firings and you will calm down, as it will signal for a release of neurotransmitters that will calm you down. 

A little abridged towards the end but hope it makes more sense OP! feel free to correct me if i’m wrong about anything.