Unit Thirteen: Metabolism and Temperature Regulation

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 mostcommon 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 containsmall amounts of cholesterol, phospholipids and protein

b. Transported upward through the thoracic duct and emptied into the venous circulation at the junctureof 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 adiposetissue

b. Chylomicrons have a half-life of less than onehour 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 theform of lipoproteins

• Types of Lipoproteins

a. VLDL- very low density lipoproteinsb. IDL- intermediate density c. LDL- low densityd. HDL- high density

Transport of Lipids in the Body Fluids

• Formation and Function of Lipoproteins- almostall are formed in the liver; primary function isto 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 almostpure triglycerides

Transport of Lipids in the Body Fluids

c. Tissue lipases- when activated by hormones,catalyze the splitting of triglycerides torelease free fatty acids

d. Triglycerides in fat cells are renewed every 2-3weeks

Transport of Lipids in the Body Fluids

• Liver Lipids- principal functions of the liverin lipid metabolism are to:

a. Degrade fatty acids into small compounds that can be used for energy

b. Synthesize triglycerides, mainly from cbhc. Synthesize other lipids from fatty acids (i.e.

cholesterol and phospholipids)

Lipids Metabolism

• Use of Triglycerides for Energy: ATP Formation

a. Hydrolysis of triglyceridesb. Entry of fatty acids into the mitochondriac. Degradation of fatty acids to acetyl coA by

beta-oxidationd. Oxidation of acetyl-CoA to enter the citric acid

cyclee. 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 Fat-about 15% of the energy is lost as heat, so 85% istransferred 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 hormonesin addition to insulin lack)

a. Epinephrine and norepinephrineb. Glucocorticoidsc. Growth hormoned. Thyroid hormone

Lipid Metabolism

• Phospholipids and Cholesterol

a. Formation of phospholipids-essentially synthesizedin all cells but 90% in the liver

b. Specific uses-constituent of lipoproteins, thrombo-plastin, 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 dayb. Highly saturated fat dietc. Ingested of unsaturated fat in diet depresses the blood

cholesterol concentrationd. Lack of insulin or thyroid hormone increases levelse. Genetic disorders

Lipid Metabolism

Fig. 68.6 Cholesterol

Lipid Metabolism

• Specific Uses of Cholesterol

a. Used by the adrenal glands to produce adreno-corticol 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 forwaterproofing

Lipid Metabolism

• Atherosclerosis- disease of the larger arteries inwhich fatty lesions develop on the insidesurfaces of the vessels; basic causes and riskfactors include:

a. Increased LDLb. Physical inactivity and obesityc. Diabetes mellitusd. Hypertensione. Hyperlidpidemiaf. Cigarette smokingg. Low HDL (ratio)