Insulin (from the Latin, insula meaning island) is a peptide hormone produced by beta cells of the pancreatic islets, and by the Brockmann body in some teleost fish. It has important effects on the metabolism of carbohydrates, fats and protein by promoting the absorption of, especially, glucose from the blood into fat, liver and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both. Glucose production (and excretion into the blood) by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. In high concentrations in the blood it is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism.
The pancreatic beta cells (ß cells) are known to be sensitive to the glucose concentration in the blood. When the blood glucose levels are high they secrete insulin into the blood; when the levels are low they cease their secretion of this hormone into the general circulation. Their neighboring alpha cells, probably by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: high secretion rates when the blood glucose concentrations are low, and low secretion rates when the glucose levels are high. High glucagon concentrations in the blood plasma powerfully stimulate the liver to release glucose into the blood by glycogenolysis and gluconeogenesis, thus having the opposite effect on the blood glucose level to that produced by high insulinconcentrations. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism responsible for keeping the glucose levels in the extracellular fluids within very narrow limits at rest, after meals, and during exercise and starvation.
We researched this topic for you and found the following best online resources. They are categorized into basic, advanced, and research level based on the extent of information you need. You will be taken to the respective websites by pressing on the links below.
http://www.aafp.org/afp/2001/0315/p1159.html insulin resistance syndrome by goutham rao, m.d., university of pittsburgh medical center–st. Margaret, pittsburgh, pennsylvania am fam physician. 2001 mar 15;63(6):1159-1164.
https://www.hindawi.com/journals/bmri/2010/476279/ muhammad a. Abdul-ghani and ralph a. Defronzo, “pathogenesis of insulin resistance in skeletal muscle,”journal of biomedicine and biotechnology, vol. 2010, article id 476279, 19 pages, 2010. Doi:10.1155/2010/476279
https://www.hindawi.com/journals/bmri/2014/145846/ silvija cvitic, gernot desoye, and ursula hiden, “glucose, insulin, and oxygen interplay in placental hypervascularisation in diabetes mellitus,” biomed research international, vol. 2014, article id 145846, 12 pages, 2014. Doi:10.1155/2014/145846