Detailed Explanation of Blood Lipid Classification: From Chylomicrons to High-Density Lipoprotein

Because these lipids in blood plasma are poorly soluble in water and cannot directly participate in lipid metabolism, they cannot exist independently in the blood. They must rely on certain transport vehicles to be transported and utilized in the blood; these vehicles are called "apolipoproteins." Complexes of lipids and apolipoproteins are called "lipoproteins." Clinically, cholesterol levels in the blood are often determined by measuring the amount of various lipoproteins present. ① Chylomicrons (CM): Chylomicrons are the largest lipoprotein particles in the blood, formed after the absorption of lipids from the diet. The core lipid of chylomicrons is 95% triglycerides, resulting in extremely low density. Their main function is to transport exogenous triglycerides (TG). TG absorbed from food is resynthesized into CM in the endoplasmic reticulum of the small intestinal epithelial cells, and then transported to the blood via the lymphatic system. Through blood circulation, TG is delivered to fat cells and muscle tissue. In normal individuals, after a 12-hour fast, there are no chylomicrons in the blood, so chylomicrons are not detected in blood lipid tests. Chylomicrons are the main form by which the blood transports triglycerides and cholesterol from the diet. ② Very Low-Density Lipoprotein (VLDL): VLDL is the main form of triglycerides produced internally by the body and is synthesized by the liver. The main function of VLDL is to transport endogenous triglycerides (TG) synthesized in the liver. Its core components include 76% TG, 24% cholesterol, 15% phospholipids, and 10% protein. VLDL and chylomicrons are called triglyceride-rich lipoproteins due to their high triglyceride content. Normal VLDL does not have an atherogenic effect, but in diabetic patients, due to abnormal VLDL metabolism, cholesterol ester deposition in the arterial walls can occur, leading to an atherogenic effect. ③ Low-Density Lipoprotein (LDL): LDL is converted from VLDL and contains 40% cholesterol, 10% free cholesterol, 6% triglycerides, 20% phospholipids, and 24% protein. LDL is the lipoprotein with the highest cholesterol content in plasma and is a degradation product of VLDL secreted by the liver. LDL has a small molecular weight and can penetrate the blood vessel wall. Excessive LDL penetration beyond the body's clearance capacity can lead to atherosclerosis. Currently, LDL is considered the primary lipoprotein contributing to atherosclerosis and is therefore the most actively controlled lipoprotein. ④ High-density lipoprotein (HDL): The main source of HDL in the blood is the components released from the degradation of lipoproteins such as CM and VLDL in the plasma. The main function of HDL is to transport cholesterol and phospholipids across cell membranes. HDL transports cholesterol in tissues through various pathways and is metabolized in the liver into bile acids for excretion. Therefore, HDL is currently considered negatively correlated with the occurrence of coronary heart disease and has an anti-atherosclerotic effect, acting as a protective factor against coronary heart disease. The pancreas is a gland with both endocrine and exocrine functions. The pancreatic lobules contain numerous pancreatic acini and their ducts, which constitute the exocrine portion of the pancreas, accounting for approximately 90% of its structure. The exocrine portion secretes pancreatic juice, containing proteases, amylases, and lipases, which can digest the three major nutrients: proteins, fats, and carbohydrates. Pancreatic islets are clusters of endocrine cells scattered among the acini of the pancreas, within the pancreatic parenchyma, with the highest concentration in the tail. They float within the pancreas like small islands on the ocean, hence the name "islets." The human pancreas contains hundreds of thousands to over a million islets. Based on their staining and morphological characteristics, islet cells are mainly divided into A cells, B cells, D cells, and PP cells. A cells account for about 20% of islet cells and secrete glucagon; B cells account for about 75% and secrete insulin; D cells account for about 5% and secrete somatostatin; PP cells are few in number and secrete pancreatic polypeptides. The most important endocrine functions of the islets are the secretion of insulin and glucagon, which regulate blood glucose concentration. Blood glucose refers to glucose in the blood; it is our body's most direct and primary basic energy source, consumed almost constantly and continuously replenished. Therefore, the blood glucose level in healthy individuals is relatively constant, remaining between 3.89 and 6.11 mmol/L when fasting. There are three main sources of blood glucose: first, the digestion and absorption of carbohydrates in food by the gastrointestinal tract, which is the main source of blood glucose; second, the breakdown of glycogen stored in the liver into glucose, which enters the bloodstream; and third, the conversion of non-carbohydrate substances in the liver (amino acids from protein breakdown, glycerol from fat breakdown, and lactic acid produced from muscles) into glucose through gluconeogenesis under certain conditions.