Unit Thirteen: Metabolism and Temperature Regulation Chapter 68: Lipid Metabolism Guyton and Hall, Textbook of Medical Physiology, 12th edition

Lipid Metabolism Classification of Lipids Triglycerides (neutral fats) Phospholipids 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 Chylomicrons-primarily triglycerides, they contain small amounts of cholesterol, phospholipids and protein 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 Chylomicron triglycerides are hydrolyzed by lipoprotein lipase, and the fat is stored in adipose tissue 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 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 Large quantities of fat are stored in two major tissues of the body: adipose and liver Adipocytes- modified fibroblasts that store almost pure triglycerides

Transport of Lipids in the Body Fluids Tissue lipases- when activated by hormones, catalyze the splitting of triglycerides to release free fatty acids 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: Degrade fatty acids into small compounds that can be used for energy Synthesize triglycerides, mainly from cbh Synthesize other lipids from fatty acids (i.e. cholesterol and phospholipids)

Lipids Metabolism Use of Triglycerides for Energy: ATP Formation Hydrolysis of triglycerides Entry of fatty acids into the mitochondria Degradation of fatty acids to acetyl coA by beta-oxidation Oxidation of acetyl-CoA to enter the citric acid cycle 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

Synthesis of Triglycerides from Carbohydrates 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 Fat- about 15% of the energy is lost as heat, so 85% is transferred to the stored triglycerides Regulation of Energy Release 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) Epinephrine and norepinephrine Glucocorticoids Growth hormone Thyroid hormone

Lipid Metabolism Phospholipids and Cholesterol Formation of phospholipids-essentially synthesized in all cells but 90% in the liver Specific uses-constituent of lipoproteins, thrombo- plastin, nervous system, all membranes Cholesterol-exogenous from the diet and endogenous from the liver

Lipid Metabolism Factors That Affect Plasma Cholesterol Concentration Amount ingested each day Highly saturated fat diet Ingested of unsaturated fat in diet depresses the blood cholesterol concentration Lack of insulin or thyroid hormone increases levels Genetic disorders

Lipid Metabolism Fig. 68.6 Cholesterol

Lipid Metabolism Specific Uses of Cholesterol Used by the adrenal glands to produce adreno- corticol hormones Used by the ovaries to produce estrogen and progesterone Used by the testes to form testosterone 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: Increased LDL Physical inactivity and obesity Diabetes mellitus Hypertension Hyperlidpidemia Cigarette smoking Low HDL (ratio)