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Disorders of Hepatobiliary and Exocrine Pancreas Function
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hormones and drugs, synthesizes plasma proteins and
blood clotting factors, stores vitamins and minerals,
maintains blood glucose levels, and regulates very–low-
density lipoprotein (VLDL) levels. In its capacity for
metabolizing drugs and hormones, the liver serves as an
excretory organ. In this respect, the bile, which carries
the end products of substances metabolized by the liver,
is much like the urine, which carries the body wastes
filtered by the kidneys. These functions, many of which
are disrupted by the liver diseases discussed in this chap-
ter, are summarized in Table 30-1.
Metabolic Functions
The liver is involved in many metabolic pathways
including carbohydrate metabolism and maintenance
of blood glucose, lipid metabolism, and protein synthe-
sis and conversion of ammonia to urea. In addition to
nutrients, several vitamins (e.g., A, D, K) are also taken
up from the bloodstream and then stored or biochemi-
cally converted in the liver.
Carbohydrate Metabolism.
The liver plays an essential
role in carbohydrate metabolism and glucose homeosta-
sis (Fig. 30-4A). It stores excess glucose as glycogen and
synthesizes glucose from amino acids and other sub-
strates as a means of maintaining blood glucose dur-
ing periods of fasting or increased need. The liver also
converts excess carbohydrates to triglycerides for stor-
age in adipose tissue.
Pathways of Lipid Metabolism.
Although most cells
of the body metabolize fat, certain aspects of lipid
metabolism occur mainly in the liver. These include the
oxidation of free fatty acids to ketoacids; synthesis of
cholesterol, phospholipids, and lipoproteins; and for-
mation of triglycerides from carbohydrates and proteins
(Fig. 30-4B). To derive energy from neutral fats (tri-
glycerides), the molecule must first be split into glycerol
and fatty acids, and then the fatty acids split into two-
carbon acetyl-coenzyme A (acetyl-CoA units). Acetyl-
CoA is readily channeled into the citric acid cycle to
produce adenosine triphosphate ([ATP], see Chapter 1,
Understanding Cell Metabolism). Because the liver can-
not use all the acetyl-CoA that is formed, it converts the
excess into acetoacetic acid, a highly soluble ketoacid
that is released into the bloodstream and transported
to other tissues, where it is used for energy. The acetyl-
CoA derived from fat metabolism is also used to synthe-
size cholesterol and bile acids. Cholesterol has several
fates in the liver. It can be esterified and stored, exported
bound to VLDLs, or converted to bile acids.
Protein Synthesis and Conversion of Ammonia
to Urea.
In addition to its role in carbohydrate and
lipid metabolism, the liver is also an important site for
TABLE 30-1
Functions of the Liver and Manifestations of Altered Function
Function
Manifestations of Altered Function
Production of bile salts
Malabsorption of fat and fat-soluble vitamins
Elimination of bilirubin
Elevation in serum bilirubin and jaundice
Metabolism of steroid hormones
Sex hormones
Disturbances in gonadal function, including gynecomastia in the male
Glucocorticoids
Signs of increased cortisol levels (i.e., Cushing syndrome)
Aldosterone
Signs of hyperaldosteronism (e.g., sodium retention and hypokalemia)
Metabolism of drugs
Decreased drug metabolism
Decreased plasma binding of drugs owing to a decrease in albumin
production
Carbohydrate metabolism
Hypoglycemia may develop when glycogenolysis and
gluconeogenesis are impaired
Stores glycogen and synthesizes glucose from
amino acids, lactic acid, and glycerol
Abnormal glucose tolerance curve may occur because of impaired
uptake and release of glucose by the liver
Fat metabolism
Formation of lipoproteins
Impaired synthesis of lipoproteins
Conversion of carbohydrates and proteins to fat
Synthesis, recycling, and elimination of cholesterol
Altered cholesterol levels
Formation of ketones from fatty acid
Protein metabolism
Deamination of proteins
Formation of urea from ammonia
Elevated blood ammonia levels
Synthesis of plasma proteins
Decreased levels of plasma proteins, particularly albumin, which
contributes to edema formation
Synthesis of clotting factors (fibrinogen,
prothrombin, factors V, VII, IX, X)
Bleeding tendency
Storage of minerals and vitamins
Signs of deficiency of fat-soluble and other vitamins that are stored in
the liver
Filtration of blood and removal of bacteria and
particulate matter by Kupffer cells
Increased exposure of the body to colonic bacteria and other foreign
matter
