Blood glucose — the negative feedback loop
Both arms of the loop around a 4.0–5.5 mmol L⁻¹ set point: a meal pushing glucose up, β cells releasing insulin and GLUT4 pulling it into muscle and fat, then α cells and glucagon driving glycogenolysis and gluconeogenesis when it falls — with the two ways the loop breaks and the HbA1c threshold that detects them.
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