How the pancreas releases insulin

The pancreas does not release insulin on a schedule: it releases it when blood sugar goes up. Blood glucose is the main regulator, and the response is very sensitive: a modest rise is enough to raise blood insulin a lot.

The cell that measures and responds is the beta cell, which lives in the islets of the pancreas. Glucose gets into it through transporters, which are doorways specific to that sugar. Inside, the cell burns it, and that drives up ATP, the energy currency of every cell. And there is the trick: there are potassium channels that close precisely when ATP goes up. They work like a switch that senses energy.

When those channels close, the cell membrane depolarizes, which is an electrical change, and that change lets calcium in. Calcium is the final signal: it makes the little packets of insulin stored inside fuse with the membrane and spill straight into the capillary of the islet, a process called exocytosis.

Along with insulin another piece comes out, C-peptide, and exactly one is released for every insulin molecule. No function of its own has been established for it, but that exact ratio makes it useful: it serves as a marker of how much insulin is being secreted.