Homeostatic Control of Blood Glucose Levels

Steady state control of blood glucose levels Glucose is essential to the body because glucose is the primary source of energy for all biological functions and is actually the only energy form that the brain and central nervous system can use. Ideal blood glucose is 80-90 mg of glucose for 100 ml of blood. However, this level is not static - it will fluctuate due to physical changes such as eating, exercising, or not eating for a long time.

Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. If the blood sugar level is too high, insulin will be secreted from the pancreas and if it is too low glucagon will be secreted from the pancreas. The flat lines shown represent steady state set points. The sign line shows blood sugar level. The pancreas has about 3 million cell clusters called pancreatic islets. There are four main types of cells of these islets involved in regulating blood glucose levels. Different types of hormones are secreted for each cell type. Alpha alpha cells secrete glucagon (elevate blood glucose), beta beta cells secrete insulin (reduce blood glucose), delta delta cells secrete somatostatin β cells and PP cells or γ (Γ) cells secrete pancreatic polypeptide.

Beta cells are sensitive to glucose concentration, also known as blood sugar level. At high glucose levels, beta cells secrete insulin in the blood, and low glucose levels inhibit insulin secretion. Those adjacent alpha cells secrete glucagon in the blood in the opposite way by obtaining a signal from the beta cells: secretion increases when the blood glucose is low and secretion decreases when the glucose concentration is high . Glucagon stimulates the liver to release glucose by degrading glycogen and gluconeogenesis, but insulin has the opposite effect. The main mechanism of insulin and glucagon in response to the secretion of blood glucose into the blood is glucose homeostasis.

Figure 1: An example of controlling blood glucose levels as negative feedback. Increase in postprandial blood glucose (stimulation). Insulin releases insulin, which promotes the transport (response) of glucose into the selected tissue in the blood. Then lower the blood glucose concentration and lower the original irritation. After that insulin is secreted into the bloodstream. Positive feedback is less common in biological systems. The role of positive feedback is to accelerate the direction of change. An example of positive feedback is breast-feeding (milk production). When the baby sucks, the nerve information from the breast causes hormone prolactin, which is secreted from the pituitary gland. The more baby gets, the more prolactin is released. And it stimulates further increases in milk production.