Blood sugar regulation
Blood sugar regulation is the process by which the levels of blood sugar, primarily glucose, are maintained by the body within a narrow range. This tight regulation is referred to as glucose homeostasis. Insulin, which lowers blood sugar, and glucagon, which raises it, are the most well known of the hormones involved, but more recent discoveries of other glucoregulatory hormones have expanded the understanding of this process.
Blood sugar levels are regulated by negative feedback in order to keep the body in balance. The levels of glucose in the blood are monitored by many tissues, but the cells in the pancreatic islets are among the most well understood and important.
If the blood glucose level falls to dangerous levels (as during very heavy exercise or lack of food for extended periods), the alpha cells of the pancreas release glucagon, a hormone whose effects on liver cells act to increase blood glucose levels. They convert glycogen into glucose (this process is called glycogenolysis). The glucose is released into the bloodstream, increasing blood sugar. Hypoglycemia, the state of having low blood sugar, is treated by restoring the blood glucose level to normal by the ingestion or administration of dextrose or carbohydrate foods. It is often self-diagnosed and self-medicated orally by the ingestion of balanced meals. In more severe circumstances, it is treated by injection or infusion of glucagon.
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 through the GLUT4 transporter, thus decreasing blood sugar. When insulin binds to the receptors on the cell surface, vesicles containing the GLUT4 transporters come to the plasma membrane and fuse together by the process of endocytosis, thus enabling a facilitated diffusion of glucose into the cell. As soon as the glucose enters the cell, it is phosphorylated into Glucose-6-Phosphate in order to preserve the concentration gradient so glucose will continue to enter the cell. Insulin also provides signals to several other body systems, and is the chief regulator of metabolic control in humans.
There are also several other causes for an increase in blood sugar levels. Among them are the 'stress' hormones such as epinephrine (also known as adrenaline), several of the steroids, infections, trauma, and of course, the ingestion of food.
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, if untreated, result in too much glucose remaining in the blood (hyperglycemia) and many of the same complications. Also, too much insulin and/or exercise without enough corresponding food intake in diabetics can result in low blood sugar (hypoglycemia).
Hormones that influence blood glucose level
|Hormone||Tissue of Origin||Metabolic Effect||Effect on Blood Glucose|
|Insulin||Pancreatic β Cells||1) Enhances entry of glucose into cells; 2) Enhances storage of glucose as glycogen, or conversion to fatty acids; 3) Enhances synthesis of fatty acids and proteins; 4) Suppresses breakdown of proteins into amino acids, of adipose tissue into free fatty acids.||Lowers|
|Amylin||Pancreatic β Cells||1) Suppresses glucagon secretion after eating; 2) Slows gastric emptying; 3) Reduces food intake.||Lowers|
|GLP-1||Intestinal L cells||1) Enhances glucose-dependent insulin secretion; 2) Suppresses glucagon secretion after eating; 3) Slows gastric emptying; 4) Reduces food intake. (Only works while food is in the gut)||Lowers|
|Glucagon||Pancreatic α Cells||1) Enhances release of glucose from glycogen (glycogenolysis); 2) Enhances synthesis of glucose (gluconeogenesis) from amino acids or fats.||Raises|
|Asprosin||White adipose tissue||1) Enhances release of liver glucose during fasting.||Raises|
|Somatostatin||Pancreatic δ Cells||1) Suppresses glucagon release from α cells (acts locally); 2) Suppresses release of Insulin, Pituitary tropic hormones, gastrin and secretin.||Lowers|
|Epinephrine||Adrenal medulla||1) Enhances release of glucose from glycogen; 2) Enhances release of fatty acids from adipose tissue.||Raises|
|Cortisol||Adrenal cortex||1) Enhances gluconeogenesis; 2) Antagonizes Insulin.||Raises|
|ACTH||Anterior pituitary||1) Enhances release of cortisol; 2) Enhances release of fatty acids from adipose tissue.||Raises|
|Growth Hormone||Anterior pituitary||Antagonizes Insulin||Raises|
|Thyroxine||Thyroid||1) Enhances release of glucose from glycogen; 2) Enhances absorption of sugars from intestine||Raises|
Some minerals play roles in glucose regulation: see Chromium in glucose metabolism for example.
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