PotassiumK
The main electrolyte inside cells, tightly controlled because the heart depends on it.
Potassium is the counterpart to sodium: most of the body's potassium sits inside cells rather than in the blood, and the small amount in blood is held within tight limits by the kidneys.
It matters more than its narrow range suggests, because the difference in potassium either side of a cell membrane is what lets nerve and muscle cells fire. Heart muscle is especially sensitive, which is why both high and low values are taken seriously.
In practice, this is one of the markers most easily distorted by the blood draw itself: if red cells are damaged in the sample they release potassium and the result reads falsely high. An unexpected value is usually repeated before anything is concluded from it.