Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

Endogenous pathway

Exogenous pathway

Reverse cholesterol transport

HDL

Dietary triglycerides and cholesterol

Receptor- dependent pathway

Bile acid and cholesterol

Scavenger pathway

HDL

Extrahepatic tissue

LDL receptor

Intestine

Liver

LDL

VLDL

Cholesterol Triglycerides

Chylomicron

Chylomicron fragments

IDL

Blood vessels

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.

Adipose and skeletal muscle tissue

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).

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.