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Regulation of Blood Glucose Levels
Blood glucose is essential for cellular respiration and should be between 80-90mg/100ml before meals. If blood glucose levels rise above or fall below these levels negative feedback sets in to return levels to normal as determined by the 'set point' in the control centre.
Blood glucose level is determined by the balance between the amount of glucose entering the bloodstream and the amount leaving it. This is controlled by the endocrine system, specifically the pancreas.
The pancreas contains receptors in the Islets of Langerhans that detect changes in blood sugar levels. As blood flows through the capillaries located in the pancreas, levels of glucose are detected by the alpha and beta cells which are present in the Islets of Langerhans. These cells will secrete hormones depending on how high or how low the blood glucose is.
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When we eat carbohydrates they are broken down into glucose init the duodenum. The glucose is then absorbed into the blood stream through the ilium of the small intestine. Blood glucose rises and this is detected when it passes through the pancreas. The beta cells in the Islets of Langerhans release the hormone insulin, which is essential for allowing the passage of glucose into the cells.
Glucose is too large to diffuse easily through the cell membrane and therefore needs the help of a carrier protein. This is called facilitated diffusion, and it is the insulin that activates the carrier protein to bring the glucose into the cell. Without insulin the glucose would not be able to enter the cells, and would remain in the bloodstream.
Once in the cells, the glucose can be used for cellular respiration. Glucose can also be converted to glycogen and stored in the liver and muscle cells until required. Because the glucose has now left the blood stream, the blood glucose levels lower and return to normal. This takes about two hours after eating carbohydrates.
If blood glucose levels fall e.g. if skip a meal, this is detected by the alpha cells located in the Islets of Langerhans. The alpha cells release a hormone called glucagon which speeds up the the conversion of the stored glycogen in the liver, back to glucose. This is then released back into the bloodstream raising the blood glucose levels back to normal.
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Blood Sugar Abnormalities
Diabetes Mellitus is caused by a reduction of insulin secretion from the beta cells in the islets of Langerhans., or by the lack of response by cells to insulin. A reduction in insulin results in abnormal metabolism of carbohydrates, fats and proteins because the glucose can not get into the cells.
This can have an effect on many tissues and organs of the body. Fortunately, it doe not affect all cells for example:
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Insulin is not required for transport of glucose in the brain, which is useful because neurons need glucose as a constant source of energy
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Insulin is not required for glucose absorption in the digestive tract
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Skeletal muscle doesn't require large amounts of insulin to utilize glucose. This can be beneficial because if there is insufficient insulin, controlled exercise can be a good way of reducing blood glucose levels. However, this can be problematic during excessive exercise because this can result in hypoglycaemia (low blood sugar) if all of the glucose is depleted.
If there is a reduction in the secretion of insulin, the transportation of glucose into the cells is reduced. This causes the level of glucose in the blood to rise (hyperglycaemia). Excess glucose spills into the urine which results in increased urination, and water is drawn from the cells resulting in dehydration and increased thirst.
If the insulin deficit is severe or prolonged more serious consequences can arise which can be life-threatening. Lack of glucose in the cell will result in the breakdown of fats and proteins to produce ATP. A waste product of this is ketones. An increase in ketones in the blood causes ketoacidosis, a condition that can be life-threatening.
Hypoglycaemia (low blood sugar) can also occur in diabetics if they take too much insulin medication, exercise strenuously, skip meals or vomit. These factors result in low blood sugar and particularly affect the brain because neurons can not use fat or protein as a source of energy. This can result in brain damage and also be life-threatening.
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For information on HYPOGLYCAEMIA click on link to NHS choices
For information on HYPERGLYCAEMIA click on link to NHS choices