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Chapter 68: Lipid Metabolism Guyton and Hall, Textbook of Medical Physiology, 12th edition Lipid Metabolism • Classification of Lipids a. Triglycerides (neutral fats) b. Phospholipids c. Cholesterol Lipid Metabolism • Basic Chemical Structure of Triglycerides- three most common are stearic aicd, oleic acid, and palmitic acid Transport of Lipids in the Body Fluids • Transport of Triglycerides from the GI Tract by Lymph a. Chylomicrons-primarily triglycerides, they contain small amounts of cholesterol, phospholipids and protein b. Transported upward through the thoracic duct and emptied into the venous circulation at the juncture of the subclavian and jugular veins Transport of Lipids in the Body Fluids • Removal of Chylomicrons from the Blood a. Chylomicron triglycerides are hydrolyzed by lipoprotein lipase, and the fat is stored in adipose tissue b. Chylomicrons have a half-life of less than one hour in plasma Transport of Lipids in the Body Fluids Fig. 68.1 Summary of major pathways for metabolism of chylomicrons synthesized in the intestine and VLDL synthesized in the liver Transport of Lipids in the Body Fluids • Free Fatty Acids- transported in the blood in combination with albumin • Lipoproteins- 95% of the lipids in plasma are in the form of lipoproteins • Types of Lipoproteins a. b. c. d. VLDL- very low density lipoproteins IDL- intermediate density LDL- low density HDL- high density Transport of Lipids in the Body Fluids • Formation and Function of Lipoproteins- almost all are formed in the liver; primary function is to transport lipid components in the blood • Fat Deposits a. Large quantities of fat are stored in two major tissues of the body: adipose and liver b. Adipocytes- modified fibroblasts that store almost pure triglycerides Transport of Lipids in the Body Fluids c. Tissue lipases- when activated by hormones, catalyze the splitting of triglycerides to release free fatty acids d. Triglycerides in fat cells are renewed every 2-3 weeks Transport of Lipids in the Body Fluids • Liver Lipids- principal functions of the liver in lipid metabolism are to: a. Degrade fatty acids into small compounds that can be used for energy b. Synthesize triglycerides, mainly from cbh c. Synthesize other lipids from fatty acids (i.e. cholesterol and phospholipids) Lipids Metabolism • Use of Triglycerides for Energy: ATP Formation a. Hydrolysis of triglycerides b. Entry of fatty acids into the mitochondria c. Degradation of fatty acids to acetyl coA by beta-oxidation d. Oxidation of acetyl-CoA to enter the citric acid cycle e. Large amounts of ATP are formed by the oxidation of fatty acids Lipid Metabolism Fig. 68.2 Beta-oxidation of fatty acids to yield acetyl coenzyme A Lipid Metabolism • Synthesis of Triglycerides from Carbohydrates Fig. 68.4 Overall schema for synthesis of triglycerides from glucose Lipid Metabolism • Efficiency of Carbohydrate Conversion into Fatabout 15% of the energy is lost as heat, so 85% is transferred to the stored triglycerides • Regulation of Energy Release a. Cbhs are preferred over fats for energy when excess cbhs are available b. Absence of cbh leads to fat utilization for energy Lipid Metabolism • Hormonal Regulation of Fat Utilization (7 hormones in addition to insulin lack) a. b. c. d. Epinephrine and norepinephrine Glucocorticoids Growth hormone Thyroid hormone Lipid Metabolism • Phospholipids and Cholesterol a. Formation of phospholipids-essentially synthesized in all cells but 90% in the liver b. Specific uses-constituent of lipoproteins, thromboplastin, nervous system, all membranes c. Cholesterol-exogenous from the diet and endogenous from the liver Lipid Metabolism • Factors That Affect Plasma Cholesterol Concentration a. Amount ingested each day b. Highly saturated fat diet c. Ingested of unsaturated fat in diet depresses the blood cholesterol concentration d. Lack of insulin or thyroid hormone increases levels e. Genetic disorders Lipid Metabolism Fig. 68.6 Cholesterol Lipid Metabolism • Specific Uses of Cholesterol a. Used by the adrenal glands to produce adrenocorticol hormones b. Used by the ovaries to produce estrogen and progesterone c. Used by the testes to form testosterone d. Precipitated in the corneum of the skin for waterproofing Lipid Metabolism • Atherosclerosis- disease of the larger arteries in which fatty lesions develop on the inside surfaces of the vessels; basic causes and risk factors include: a. b. c. d. e. f. g. Increased LDL Physical inactivity and obesity Diabetes mellitus Hypertension Hyperlidpidemia Cigarette smoking Low HDL (ratio)