I won't even try to be comprehensive here. I want to focus specifically and briefly on two things - some relevant assumptions we make about human nature in the west, and what is needed to put acupuncture in its proper context.
In the west, the dominant idea is that a human is their physical body. In this model, all consciousness phenomena is the result of exceptionally complex neural activity in the brain, and every physiological phenomena explainable via biochemistry and electrical signal. Abrupt, permanent cessation of existence results when the body ceases to function.
In this model, there is certainly room to understand how acupuncture can work. There is a significant body of Cochrane-grade scientific research that demonstrates how acupuncture can improve neural transmission, adjust inflammatory processes, and cause the production or release of chemicals that inhibit pain, like endorphins.
Entire schools of acupuncture have been developed around this western material sciences model of human nature, and their techniques can certainly be effective.
However, this model is not remotely close to the ontology around which acupuncture was originally designed.
In extremely short form, this is what I understand as the fundamental premise upon which acupuncture is based:
A human consists of a material form, and many layers of progressively more refined energetic forms as well.
Consider the physical body, not as a monolith, but as a composite of many systems. Picture the skeleton, then overlay the muscles, the internal organs, the blood vessels, the lymphatic system, the nervous system, the fascia, and finally the skin. A multitude of systems that all interact with each other, forming, in aggregate, the human body.
This basic organising principle extends beyond the material form. We just need to keep overlaying systems to form a fuller picture of human structure.
The next systems to overlay are systems of physical energy - the organ-meridian complexes - where acupuncture works. Here is a map of how the meridians are arranged in the body:
This diagram indicates just the major pathways that run on the surface of the skin. There are other energy pathways of different sizes that permeate every cubic centimetre of the body.
At this point we are out of the realm of the material and the tangible. However, the energies contained within the organ-meridian complexes have a deep and powerful connection to the function of tangible/material systems in the body.
It is the level of physical energies where acupuncture has its jurisdiction. While a needle certainly causes chemical changes, these are not a direct result of chemical interactions caused by the introduction of a small piece of metal in the skin. These chemical changes are a secondary effect of changes to the physical energies of the body caused by techniques that were specifically designed to influence these energies.
We can consider the spirit to be yet another system to be overlaid onto our understanding of the human body. This is the aspect of the human being that is the most subtle, the most refined. This is the immortal part of ourselves, that survives the decay and death of all our denser systems. Fundamentally, humans are beings of spirit that have chosen to experience life as material beings.
If you place the physical body, physical energies and the spirit on a continuum from dense to subtle, you can also see how there is a progression from form to consciousness. Of course, there are many more points in between on this continuum.
Now that we have sketched out the most fundamental context required for discussing acupuncture, next time we can move on to actually talking about how it works.