Transcript Unit One: Introduction to Physiology: The Cell and General

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)