406
U N I T 5
Circulatory Function
facilitates the clearance of cholesterol from atheromatous
plaques and transports it to the liver, where it may be
excreted rather than reused in the formation of VLDL.
The mechanism whereby HDL takes up cholesterol from
peripheral cells has recently been elucidated. A lipid trans-
porter (adenosine triphosphate [ATP]–binding cassette
transporter A class 1, or ABCA1) promotes the move-
ment of cholesterol from peripheral cells to the lipid-poor
HDL cholesterol.
7,9
Defects in this system (resulting from
mutations in the ABCA1 transporter gene) are respon-
sible for a condition called
Tangier disease
, which is char-
acterized by accelerated atherosclerosis and little or no
HDL cholesterol.
9
Hypercholesterolemia
Hypercholesterolemia refers to increased levels of cho-
lesterol in the blood. The Third Report of the National
Cholesterol Educational Program (NCEP) Expert Panel
on Detection, Evaluation, and Treatment of High Blood
Cholesterol in Adults classification system describes opti-
mal to very high levels of LDL cholesterol and triglycerides,
and low and high levels of HDL cholesterol
10
(Table 18-2).
Intestine
Dietary
triglycerides
and cholesterol
Bile acid
and cholesterol
Chylomicron
Liver
Chylomicron
fragments
Blood vessels
Adipose and skeletal muscle tissue
HDL
VLDL
IDL
LDL
LDL
receptor
Cholesterol
Triglycerides
Extrahepatic
tissue
Scavenger
pathway
Exogenous pathway
Endogenous pathway
Reverse cholesterol transport
Receptor-
dependent
pathway
HDL
FIGURE 18-4.
Schematic
representation of the exogenous
and endogenous pathways for
triglyceride and cholesterol
transport. HDL, high-density
lipoprotein; IDL, intermediate-
density lipoprotein; LDL, low-
density lipoprotein; VLDL,
very–low-density lipoprotein.
TABLE 18-2
NCEP Adult Treatment Panel III
Classification of LDL,Total, and
HDL Cholesterol (mg/dL)
LDL Cholesterol
<
100
Optimal
100–129
Near optimal/above optimal
130–159
Borderline high
160–189
High
≥
190
Very high
Total Cholesterol
<
200
Desirable
200–239
Borderline high
≥
240
High
HDL Cholesterol
<
40
Low
≥
60
High
From National Institutes of Health Expert Panel. Third Report
of the National Cholesterol Education Program [NCEP] Expert
Panel on Detection, Evaluation, andTreatment of High Blood
Cholesterol in Adults [AdultTreatment Panel III]. NIH Publication
No. 01-3670. Bethesda, MD: National Institutes of Health; 2001.
