Three tissues carry most of the glucose-handling load, and they do different jobs. The liver is the buffer: it takes up glucose after a meal and stores it as glycogen, then releases glucose into the blood during fasting by breaking down that glycogen and by building new glucose from lactate, glycerol, and amino acids. Muscle takes up glucose mainly in the fed state, stores it as glycogen, and uses that glycogen for its own contraction; it does not release free glucose back into the blood in any meaningful amount, because it lacks the enzyme glucose-6-phosphatase. Adipose tissue takes up glucose and converts it to fat for long-term storage. The pancreatic islets sit above this system as the sensors and signalers: beta cells release insulin when glucose rises, and alpha cells release glucagon when glucose falls. The result is a loop in which the sensed glucose level sets the hormone output, and the hormone output sets what the tissues do.
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Blood Glucose Homeostasis and the Endocrine Players
Where Glucose Goes: Liver, Muscle, and Adipose Tissue
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Follow the loop. The pancreatic islets sit at the top as sensors. When glucose rises, beta cells release insulin; when it falls, alpha cells release glucagon. Those hormones act on three tissues below. The liver is the buffer: it stores glucose as glycogen after a meal and releases it during fasting, both by breaking down glycogen and by building new glucose. Muscle takes glucose up mainly in the fed state and keeps it as glycogen for its own contraction, but it cannot send free glucose back into the blood because it lacks the enzyme needed for that final step. Adipose tissue converts glucose to fat for long-term storage. Notice the shape of the system: the sensed glucose level sets the hormone output, and the hormone output sets what the tissues do, which changes the glucose level again.
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