A while ago I had someone tell me that they were a little... annoyed? Mildly irritated? I don't know (they might have been joking, but the truth tends to lie in jest), about the fact that the summary of this says that there'll be fact files, but I mainly put out one shots. Today, I will put out a fact file, on meridians!
Most of this might not be related to what's actually theorised about meridians, it's just how I see them working in this universe.

Also thanks to geminiloveswriting for favouriting!

Meridians are believed to be a system that transports Chi around your body. It's partially correct. Meridians transport magical energy around the body, not just a Descendant's, but anyone's, it's just that due to the nature of a Descendant's genetics that their meridians are naturally larger and function from birth. In normal humans they need to actively pursue magic in order to properly activate their meridians. Therefore anyone can use Atlantian magic if they can find a teacher and develop their meridians.

However, Descendants start with a different kind of magical energy in their meridians, so if they try to use a different kind of magic on top of the energy that's already there, it overloads their meridians. This would then cause the original energy to push back the foreign magic, in those that control some form of element (ice, fire, air, earth, sand, shadows, metal etc.) this would mean parts of their body turning into the element. If the foreign energy is used too often, or too much is used at once, their entire body will turn into their element. There is no known reversal.

Practice can develop meridians, either by increasing size and/or efficiency, for example Jackie's meridians have a fairly even development across her body, but are most developed and concentrated in her arms and hands. Meridians also function in a similar fashion to the lymphatic system, e.g. they don't have a pump like the heart in the circulatory system, the energy is circulated through them by physical and mental exercise.

This is why meditation is such a large part of how the Descendants of Sandman work. Due to the nature of their magical energy (sandy or grainy) they were more likely to have their meridians clogged or blocked, mediation helps to keep them clear. Presumably sand using Descendants of Pitch do the same thing.

When the opposite metal is used against a Descendant (Lead from the Earth's Core or meteor/stardust metal) it blocks the meridians, and therefore the energy flow for as long as it's inside the body. The wound itself would cause terrible pain, as the nearby energy rejects the metal and the blockage causes a build up of energy, which may eventually become unusable or will kill the Descendant. During this the energy becomes visible, and has different colours depending on the energy type and lineage.

If basic bodily functions are altered to adapt to the type of energy, a blockage of magical energy will cause the body to start to revert to normal functions, which can be catastrophic once the energy flow begins again. The resulting stress on the body could lead to death, or permanent power loss.

If the metal can suck in some form of energy (e.g. light energy in terms of Lead from the Earth's Core) it can also attract and in severe or prolonged cases suck in a Descendant's magical energy. The energy can be replenished over a period of time, but it depends on the individual's level of power. A more powerful individual would have more energy to lose, so would take longer to recover the energy lost.

In most Descendants, their meridians are most developed and numerous in the hands, which is why handcuffs lined with the opposite metal works. The magical energy is suppressed in the area where it's used most, suppressing the energy in the rest of the body as well. As this is external there are no adverse physical effects. However there is still the question of mental, as some Descendants describe it to be similar to temporally losing the use of a limb.

It is presumed that immortals have much more developed and resilient meridians, but they still can take damage.

While energy recognised by science cannot be created or destroyed, only converted, magical energy doesn't quite work in the same way. As one of the properties of magic is creating something out of nothing, magical energy can also function in the same way, which is how it replenishes itself if some is lost.

Thought I'd add a bit from the DA comments between myself and MIMitation Balance

MimTo clarify on metal effects for immortals;

Immortal Meridians are naturally much larger and their energy store folds on that of a mortal or a Descendant exponentially. External contact with the lead has no adverse effects on immortals and they can handle it with bare hands without issue, although trying to channel power past it is extremely difficult. Internally, the lead can't kill them. But since all immortals are made of a specific energy in and of themselves, it can put them in a coma, such as in Mim's case. Mastery of immortal energy partially comes with age and talent. Mim's extreme youth put her at a disadvantage but her self control and resulting recovery make her the youngest immortal to date to have achieved the feat of overcoming the lead. An Immortal like North would sustain little damage from internal lead, save for extreme pain as he is older more experienced etc. This highlights the space between immortal and Descendant greatly. Lead nearly always kills Descendants, but can't kill immortals. Needless to say, immortals don't enjoy it but will take the hit to save someone else.

MeSmall correction I'm afraid. Lead will nearly always kill Descendants if treatment cannot be given in time, but small amounts (e.g. a short stab/cut with a knife) are survivable. The smaller the injury or area exposed to the metal, the larger the time gap there can be between the original injury and the treatment