Everything you need to heal is contained within your blood. Your organs, muscles, tendons, bones, your entire body, needs high quality access to oxygen, nutrients, hormones and a plethora of other vital substances to heal. Decrease in blood flow can cause serious damage, and increase in blood flow, especially microcirculation, can bring all the right substances to an area to allow it to heal.
Acupuncture release the bodies natural opioids, endorphins, and enkephalins.
Our autonomic nervous system contributes to something like 'polar opposite' affect for our nervous system. You've likely heard of the phrase 'fight or flight' and 'rest and digest'. This is your autonomic nervous system broken up into your sympathetic (fight or flight) and parasympathetic (rest, digest and heal). An easy way to understand this is, if you were being chased by a tiger, it's not important to sleep, heal, digest, or reproduce at that time. Instead you need to stay away and run, go, go, go, resting will come later, healing will come later. Fortunately, most of us don't come into contact with tigers very often, however, our stressful lives, work, relationships, politics, financial situations and so forth, simulate this reaction and we can often be caught up in a fight or flight mode, and so our body is not in rest and digest and heal mode very often. Acupuncture appears to be able to modulate the autonomic nervous system and bring it back into a rest, digest and heal mode.
When the local pain areas are needled, a micro trauma occurs. This sends signals for the body to 'pay attention' to this area. As the body addresses the micro trauma, it addresses the tissue around it as well, thus healing past damaged tissue that has been causing a person pain.
Emerging studies are showing how acupuncture can help reset the nervous systems ability to address pain. There are two main types of nerves involved in a pain reaction, one that involves experiencing pain (nocioceptive) and the other that tells the body where the pain is (proprioceptive) and thus where to address healing. If these two nerves are firing correctly, the body should be able to know that it is injured and know where to address that injury and thus release pain killers and start the natural process of healing. Your body should be able to heal itself, however, this doesn't always go according to plan. In chronic pain conditions your body is sending pain signals (nociocpetive) but not communicating correctly where the problem is (proprioceptive). Thus, you are in pain and your body isn't able to react appropriately.