How the body maintains a relatively constant blood sugar


Regulation of blood glucose concentration: The reason that blood concentration can be kept relatively constant is due to the existence of a set of highly efficient regulation mechanisms in the body, which precisely control the source and the way of blood glucose to achieve a dynamic balance.
  (I) Regulatory effect of the nervous system The regulation effect of the nervous system on blood glucose concentration is mainly achieved by the secretion of controlled hormones by the hypothalamus and the autonomic nervous system, which in turn affects the activity of blood glucose sources and key enzymes. The regulation of the nervous system finally achieves its purpose through the regulation of the cellular level.
  On the one hand, the hypothalamus acts on the adrenal medulla through the visceral nerves to stimulate the secretion of adrenaline; on the other hand, it also acts on the islet α-cells to secrete glucagon; at the same time it can also directly affect the liver. As a result of the three aspects, the phosphorylation enzymes of liver cells are activated, which accelerates the decomposition of glycogen; the activity of key gluconeogenesis enzymes is increased, and the gluconeogenesis effect is increased, thereby increasing the blood glucose concentration.
  The hypothalamus can excite the vagus nerve to cause pancreatic β-cells to secrete insulin, and at the same time, it can directly act on the liver, activate glycogen synthase in liver cells, and promote liver glycogen synthesis; in addition, it can also inhibit the gluconeogenesis pathway and promote Oxidation and conversion of sugars, on the whole, increase the way of blood sugar, reduce the source, and finally achieve the purpose of reducing blood sugar concentration.
  (B) the role of hormone regulation: hormones that regulate
  blood sugar concentration can be divided into two categories, namely, hormones that lower blood sugar concentration and hormones that increase blood sugar concentration. The mechanisms by which various hormones regulate glucose metabolism and thus affect blood glucose concentration are briefly described in Table 3-1.
  1. Insulin Insulin is a protein hormone secreted by beta cells of the islets and consists of 51 amino acids. When the blood glucose or amino acid concentration is high, it can promote insulin secretion.
  The mechanism of insulin’s regulation of blood glucose is to increase the permeability of muscle and adipose tissue cell membranes to glucose, which is beneficial for blood glucose to enter these tissues for metabolism. Insulin can also induce the synthesis of glucokinase, phosphofructokinase and pyruvate kinase, and accelerate the breakdown and utilization of glucose in cells. Insulin reduces the intracellular cAMP content, activates glycogen synthase and pyruvate dehydrogenase systems, inhibits phosphorylase and gluconeogenesis key enzymes, etc., increases glycogen synthesis, the oxidation utilization of sugar, and the conversion of sugar to fat The response increases, the blood glucose goes faster; the glycogen decomposition and gluconeogenesis are reduced or suppressed, the blood glucose source is reduced, and eventually the blood glucose concentration is reduced.
  Insulin-like growth factor (IGF, also called somatomedins) isolated from human serum in recent years has a chemical structure and biological characteristics similar to insulin, but the immunological properties of IGF are completely different from insulin. IGF through the IGF receptor and insulin ?? equivalent to a portion of insulin, for example: ① promote fat cell transformation, uptake and oxidized glucose, and the strength of synthesis of fat is only 1/50 or 1/100 of insulin; ② uptake of myocardial cells The effect of glucose is 1/2 or 1/5 of insulin; the effect on skeletal muscle uptake, glucose oxidation and glycogen synthesis is only 1/20 of insulin. The long-term effect of IGF is to promote growth.
  2. Glucagon is a peptide hormone composed of 29 amino acids, synthesized and secreted by islet α cells, and has a molecular weight of 3500. Its primary structure is similar to some gastrointestinal active peptides such as secretin, intestinal inhibitory gastric peptide (GIP) and so on. Glucagon secretion increases when blood sugar drops, but decreases after a high-sugar diet.
  Glucagon mainly regulates blood glucose concentration by increasing cAMP content in target cells. Intracellular cAMP can activate cAMP-dependent protein kinases, which alter the activity of intracellular enzymes through covalent modification of enzyme proteins, that is, the key enzymes that activate glycogenolysis and gluconeogenesis, and the key to inhibit glycogen synthesis and sugar oxidation Enzymes that raise blood sugar. The protein kinase also activates hormone-sensitive lipase in adipose tissue, accelerates fat mobilization and oxidative energy supply, reduces tissue use of sugar, and thereby increases blood sugar. Glucagon is currently considered to be the most important hormone that raises blood glucose levels.
  The precursor of glucagon is inactive proglucagon. Glucagon-like substances produced and secreted by intestinal epithelial cells are called glucagon. Therefore, the glucagon measured by the general immunological method is composed of glucagon, proglucagon, and intestinal glucagon, and the basic concentration in normal plasma is 50-100ng / L.
  Of the hormones that play a role in regulating plasma concentrations, the most important are insulin and glucagon. Adrenaline plays a role in stress, while adrenocortical hormones, growth hormones, etc. can affect blood sugar levels, but they play only a minor role in physiological regulation.
  In summary, insulin and glucagon are the main hormones that regulate blood glucose concentration. Keeping blood sugar levels constant is the result of coordinated sugar, fat, and amino acid metabolism.
  ⒊The role of liver in the regulation of glucose metabolism The liver is the main organ that regulates blood glucose concentration, not only because there are many ways of glucose metabolism in the liver, but the key is that some metabolic pathways are unique to the liver.
  After meals, the sugars in the food are digested and absorbed, and a large amount enters the blood in the form of glucose, which causes the blood glucose concentration to rise slightly temporarily. At this time, glucose directly promotes the uptake of glucose by liver and other tissues, significantly increases glycogen synthesis in liver cells, and also inhibits the breakdown of liver glycogen, reducing its release of glucose to the blood, and also converts sugar to fat. The blood glucose concentration increased only slightly and quickly returned to the normal range. When starving, the liver breaks down stored liver glycogen into glucose through its own unique glucose-6-phosphatase to provide blood sugar, but the muscle glucose principle cannot be converted to glucose.
  The liver is also the main organ of gluconeogenesis (Table 3-2). Under physiological conditions, non-sugar substances such as glycerin and amino acids are mainly converted into glucose in the liver cell bone to supplement the insufficient blood source caused by fasting blood glucose. This is because the key enzymes of the gluconeogenesis pathway: pyruvate carboxylase and phosphoenol propionate carboxykinase have the highest activity in the liver. When starving or strenuous exercise, the liver uses non-sugar substances to convert sugar into sugar. In addition, the fructose diphosphatase and glucose-6-phosphatase possessed by the liver also play an important role in the conversion of other monosaccharides into glucose.
  It can be seen that the liver plays a more comprehensive role in the source and route of blood glucose than other organs, so it is the key organ to maintain blood glucose constant. When the body needs it, through the action of neuro-hormones, the enzyme activities of various sugar metabolism pathways in liver cells are changed to achieve the purpose of maintaining constant blood glucose concentration in the liver. When liver function is severely impaired, temporary hyperglycemia or even diabetes may occur when eating sugar or infusion of glucose, and hypoglycemia may occur when starvation.


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