In medicine, blood sugar is a term used to refer to levels of glucose in the blood. Blood sugar concentration, or serum glucose level, is tightly regulated in the human body. Glucose, transported via the bloodstream, is the primary source of energy for the body's cells.
Normally, blood glucose levels stay within narrow limits throughout the day: 4 to 8 mmol/l (70 to 150 mg/dl). Levels rise after meals and are usually lowest in the morning, before the first meal of the day.
Diabetes mellitus is the most prominent disease related to failure of blood sugar regulation.
Though it is called "blood sugar", other sugars besides glucose are found in the blood, like fructose and galactose. However, only glucose levels are regulated via insulin and glucagon.
If blood sugar levels drop too low, a potentially fatal condition called hypoglycemia develops. Symptoms may include lethargy, impaired mental functioning, irritability, and loss of consciousness.
If levels remain too high, appetite is suppressed over the short term. Long-term hyperglycemia causes many of the long-term health problems associated with diabetes, including eye, kidney, and nerve damage.
Blood sugar levels are regulated by negative feedback in order to keep the body in homeostasis. The levels of glucose in the blood are monitored by the cells in the pancreas. If the blood glucose level falls to dangerous levels (as in very heavy exercise or lack of food for extended periods), some of those cells release glucagon, a hormone whose effects on liver cells act to increase blood glucose levels. They convert glycogen storage into glucose (this process is called glycogenolysis). The glucose is released into the bloodstream, increasing blood sugar levels.
There are also several other causes for an increase in blood sugar levels. Among them are the 'stress' hormones such as adrenalin, several of the steroids, infections, trauma, and of course, the ingestion of food.
When levels of blood sugar rise, whether as a result of glycogen conversion, or from digestion of a meal, a different hormone is released from beta cells found in the Islets of Langerhans in the pancreas. This hormone, insulin, causes the liver to convert more glucose into glycogen (this process is called glycogenesis), and to force about 2/3 of body cells (primarily muscle and fat tissue cells) to take up glucose from the blood, thus decreasing blood sugar levels. Insulin also provides signals to several other body systems, and is the chief regulatory metabolic control in humans.
Diabetes mellitus type 1 is caused by insufficient or non-existent production of insulin, while type 2 is primarily due to a decreased response to insulin in the tissues of the body ("insulin resistance"). Both types of diabetes result in too much glucose remaining in the blood ("hyperglycemia") and many of the same complication.
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