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Blood glucose or blood sugar, as it is commonly called, is a tightly regulated biochemical parameter in normal humans and animals. The body maintains the blood sugar within a narrow range. There are several interacting systems that regulate blood sugar. Of these, regulation of blood sugar by the hormone insulin is the most important.
Insulin is synthesized in significant quantities only in beta cells in the pancreas. When the beta cell is appropriately stimulated, insulin is secreted from the cell by exocytosis. The insulin then diffuses into small blood vessels of the pancreas.
Insulin is secreted in primarily in response to elevated blood concentrations of glucose. Thus insulin is secreted as the body detects high blood glucose and helps regulate the levels of glucose.
There are some other stimuli like sight and taste of food, increased blood levels of amino acids and fatty acids that may also promote the release of insulin. During digestion (around one or two hours following a meal), insulin release is not continuous, but occurs in bursts.
Other hormones that regulate blood sugar include glucagon, growth hormone, cortisol and catecholamines. These increase blood glucose by reducing uptake of the sugar by the various organs of the body. These are termed catabolic hormones. Insulin is the anabolic hormone that decreases blood glucose.
As blood glucose rises after a large carbohydrate meal, a glucose transporter GLUT 2 increases its affinity for glucose. These transporters GLUT 1, 2, and 3 are proteins and not enzymes.
GLUT 2 and the enzyme glucokinase coordinate glucose control in liver. This converts Glucose to Glucose 6 Phosphate. The reaction utilizes ATP or energy. This conversion causes utilization of the Glucose 6 phosphate in the glycolysis reaction.
The liver can also take up fructose similarly from honey, soda sweeteners, sucrose, etc. and metabolize it to release its carbon skeleton as glucose in the post-absorptive phase (gluconeogenesis).
When blood glucose rises the glucose transporters rise this causes increased glucose transport inside the β cell by GLUT 2 and results in raised G-6-P concentration, which increases glucose metabolism, which in turn stimulates the β cell to secrete insulin.