I love hot food so I looked into the whole effect-of-chilis thing a couple years ago and was really intrigued and pleased at what I discovered.
If you could watch cells and nerves on your tongue through a powerful microscope as someone burnt them with a flame you would actually see them get physically damaged as they sent a mad “heat and pain!” signal to your brain. If you watched them as you ate a habanero chili the same urgent message would be sent, but NO HARM would be happening to the cells! No harm whatsoever.
Human tongue up close
Once you process this info you can really start to relax and enjoy hot food. It’s “capsaicin” that’s responsible for that burn and pain sensation. Marvellous stuff! Capsaicin selectively binds to a protein known as TRPV1 that resides on the membranes of pain and heat-sensing neurons. TRPV1 is a heat-activated calcium channel that usually opens between 37 and 45°C. When capsaicin binds to TRPV1, it causes the channel to open at or below normal human body temperature, which is why capsaicin is linked to the sensation of heat. Glorious heat! So what happens when you add chillis to your grub is your body thinks it is being exposed to heat. Your tolerance for heat in the exposed tissues goes so low that the temperature of your own body is mistaken for a serious burn. Your brain says ouch! and eina! at first, but once you know what’s happening you learn to say Ooh! and Aah! and you get to LURV that sensation and the feeling of well-being that follows.
So: You can go on a long boring run for your endorphin pleasure, or you can sit and enjoy a delicious meal for the same effect!
That’s the brain’s response. Meantime, your body responds to this heat like it would normally by trying to cool you. Capillaries expand in the area of contact and redness and swelling begin so the blood can carry heat away and bring in healing factors to fix the injury (in this case: “injury”). The chemical only works locally on the tissues it directly touches (on nerve endings). This is why your tongue and lips will burn, as they have lots of nerve endings close to the surface. But the pain may also cause general stress sweating as your body is chemically reacting to pain and trying to counteract it. Your body thinks it is in the presence of a dangerous level of heat and responds with the sensation of pain even though there is no danger.
Incidentally, your body’s ability to feel temperature is a separate sense. TPRV1 receptors are an “Ouch! that’s hot” sense not a “Hmm, this is hot or cold” sense. Normally heat only begins to generate pain if it goes over about 37°C and the pain climbs as the heat does!
You get used to (and learn to love) the effect of chillis. Repeated exposure to capsaicin causes the chemicals that communicate pain to be depleted from your nerve endings, making you tougher and less of a ninny. Did I mention: Marvellous stuff!?
Aside: Chillies originally come from South and Central America, and were taken to Europe first and then to India and Asia, where they became an integral part of Asian cuisine.
(Thanks to Ariel Williams on quora for a lot of this!)
Remember: 'Tis a boring man who can only spell a word one way.