The body does not run one steady program; it switches between a fed state and a fasted state, and each state is dominated by a different hormone. In the fed state, blood glucose rises after a meal, beta cells of the pancreatic islets release insulin, and insulin drives glucose into liver, muscle, and adipose tissue while suppressing glucose release from the liver. In the fasted state, blood glucose begins to fall, insulin secretion drops, alpha cells release glucagon, and glucagon pushes the liver to break down glycogen and to manufacture new glucose. The two hormones are not simply opposite in direction; they are reciprocal in timing, so that a rise in one is normally accompanied by a fall in the other. The transition between states is gradual, and the crossover point is what keeps glucose from drifting.
Learn MotionInsulin, Blood Sugar, and Diabetes: A Systematic Course
Blood Glucose Homeostasis and the Endocrine Players
Fed and Fasted: Two States, Two Dominant Hormones
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Look at the two curves. After a meal, blood glucose climbs and the insulin line rises with it, while glucagon falls. That is the fed state: insulin moves glucose into liver, muscle, and fat, and it shuts down glucose release from the liver. Hours later the glucose line drifts down, insulin falls, and glucagon rises. Now the liver breaks down its glycogen and manufactures new glucose. The important point is not that the hormones are opposites in direction, but that they cross over in time. A rise in one is normally paired with a fall in the other, and that crossover is what keeps glucose from drifting during the switch.
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