Glucose is a sugar which is used as cellular fuel by human cells. Many tissues in the body have several sources of fuel molecules to generate energy in addition to glucose such as fatty acids and amino acids. Glucose is a unique source of energy for certain organs in the body such as the brain and the retina of the eye.
These two organs rely exclusively on the continuous supply of glucose for them. Otherwise a permanent damage can occur to the brain as a result to the fact that its cells do not regenerate. Glucose is so important for the body. This importance is manifested by the number of hormones in the body which participate together in the regulation of its concentration in the blood.
Due to the function of these hormones the concentration of glucose in the blood is tightly regulated and is kept within a narrow range. Under pathological conditions this regulation can be interrupted and states of hypoglycemia and hyperglycemia can occur but these will be discussed later after reviewing the regulation of glucose under normal conditions.
The liver is the organ in which the regulation of glucose occurs. When the level of glucose rises causing a state of hyperglycemia the hormone insulin is secreted by the beta cells of the islet of langerhans in the pancreas. This hormone functions by stimulating the building of the polymer glycogen through the facilitating function of glucose entry into the cells. Glycogen is a polymer of glucose in which degrades to its glucose units upon demand by the body tissues.
In this case the hyperglycemia is corrected to its normal glucose value by the action of insulin. If a state of low glucose level in the blood occurs such as in hypoglycemia glucoagon which is also a protein hormone that is secreted also by the islet cells of langerhans of the pancreas.
This hormone is an antagonist to insulin. It does the opposite effect of insulin on glucose. Namely, it functions by stimulating the degradation of the glucose polymer glycogen. Thus increasing the amount of glucose in the blood which in turn corrects the hypoglycemia.
In the case of hypoglycemia the low level of glucose in the blood is a stimulant for the sympathetic nervous system in which gives signal to the adrenal medulla to synthesize and secrete more epinephrine. This in turn induces the liver to supply more glucose to the blood. Thus correcting the hypoglycemia.
Also two other hormones that are the growth hormone and cortisol function by inducing the production of glucose to correct a hypoglycemic state. They also function by decreasing the amount of glucose that is consumed by the body tissues.
So far I discussed regulations of glucose under non-pathologic conditions. In diabetes mellitus a state of hyperglycemia occurs in which is corrected by the infusion of insulin into the body intravenously. Also islet cells neoplasm which is called insulinoma causes a state of hypoglycemia due to the increased insulin level in the blood. This condition is corrected by surgical intervention to remove the tissue that oversecretes insulin.
Another pancreatic neoplasm which is called glucagonoma is a tumour that oversecretes the hormone glucagon. Thus increasing the concentration of glucose in the blood causing a state of hyperglycemia which is also corrected by surgical intervention to remove the affected tissue. These two disorders are endocrine disorders of the pancreas and are rare conditions.
Pituitary tumours which oversecretes growth hormone such as an adenoma of the anterior pituitary gland is are also causes in certain cases to hyperglycemia. This is especially evident in tall people whose hormonal level of growth hormone is high.
Hypoglycemia can occur due to several reasons. One of them is a tumour to the islet cells of the pancreas which is called insulinoma. The excessive secretion of insulin to the blood causes a state of decreased glucose level in the blood or hypoglycemia. In addition, lack of the hormone cortisol as occurs in Addison's disease is also a cause to hypoglycemia. Administration of drugs to treat diabetes can also precipitate low level of glucose in the blood.