Two hormones, insulin and glucagon, which are both secreted by the pancreas, are responsible for glucose homeostasis. Insulin acts to reduce the levels of blood glucose, and glucagon has the opposite effect, causing an increase in blood glucose. The pancreas contains different populations of secretory cells. Approximately 1 million cell clusters called islets of Langerhans exists in the pancreas. Each islet is composed of alpha cells that produce glucagon, beta cells that produce insulin, and delta cells that produce somatostatin, a hormone that inhibits s

After eating a meal, blood glucose levels increase, and within minutes, insulin is released from beta cells in the pancreas. The immediate effect (in seconds or minutes) of insulin is to increase the rate of glucose update from the blood into target cells- primarily liver and muscle cells. Once inside cells, glucose molecules can be metabolized or linked together to produce glycogen, a storage form of glucose. As blood glucose levels fall below the normal range, glucagons is released by alpha cells of the pancreas in response.

Insulin and glucagon are antagonistic hormones that regulates the concentration of glucose in the blood. This is a critical bioenergetic and homeostatic function, because glucose is a major fuel for cellular respiration and a key source of carbon skeletons for the synthesis of other organic compounds. Metabolic balance depends on the maintenance of blood glucose at a concentration near a set point, which is about 90mg/mL in humans (see picture next page).

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