The hormone that defends against low glucose
Glucagon is insulin's counterpart. It is released by the alpha cells of the pancreatic islets when blood glucose falls, for example between meals or during fasting and exercise. Where insulin moves glucose into storage, glucagon mobilizes it back out, so the pair keeps blood sugar within a narrow range.
The diagram labels this counter-regulation because glucagon's job is to oppose a downward drift in glucose before it becomes dangerous, restoring the level rather than lowering it further.
Two liver routes to raise blood sugar
Glucagon acts mainly on the liver through two pathways. Glycogenolysis breaks stored glycogen back down into glucose for a fast release. Gluconeogenesis makes new glucose from non-carbohydrate sources such as amino acids and glycerol, sustaining levels when glycogen runs low during longer fasts.
Glucagon is one of several counter-regulatory signals, working alongside epinephrine, cortisol, and growth hormone. This is general educational information, not medical advice; blood sugar concerns should be managed with a qualified professional.