Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
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24
it is not enough to simply measure total cholesterol. Te form
in which cholesterol is transported in the blood is more impor-
tant clinically.
As a rule, high levels of HDLs are considered
good
because
the transported cholesterol is destined for degradation (think H
for healthy). HDL levels above 60 are thought to protect against
heart disease, and levels below 40 are considered undesirable. In
the United States, HDL levels average 40–50 in males and 50–60
in women.
High LDL levels (130 or above) are considered
bad
(think L
for lousy) because when LDLs are excessive, potentially lethal
fill with cholesterol picked up from tissue cells and “pulled”
from artery walls. HDL also provides the steroid-producing or-
gans, like the ovaries and adrenal glands, with their raw material
(cholesterol).
Recommended Total Cholesterol, HDL,
and LDL Levels
For adults, the recommended total cholesterol level is 200 mg/
dl of blood (or substantially lower). Blood cholesterol levels
above 200 mg/dl have been linked to atherosclerosis, which
clogs the arteries and causes strokes and heart attacks. However,
Table 24.6
Summary of Metabolic Functions of the Liver
METABOLIC PROCESSES TARGETED
FUNCTIONS
Carbohydrate Metabolism
Particularly important in
maintaining blood glucose
homeostasis
Converts galactose and fructose to glucose
Stores glucose as glycogen when blood glucose levels are high; in response to hormonal controls,
performs glycogenolysis and releases glucose to blood
Gluconeogenesis: converts amino acids and glycerol to glucose when glycogen stores are exhausted
and blood glucose levels are falling
Converts glucose to fats for storage
Fat Metabolism
Although most cells are capable of
some fat metabolism, liver bears
the major responsibility
Primary body site of beta oxidation (breakdown of fatty acids to acetyl CoA)
Converts excess acetyl CoA to ketone bodies for release to tissue cells
Stores fats
Forms lipoproteins for transport of fatty acids, fats, and cholesterol in blood
Synthesizes cholesterol from acetyl CoA; catabolizes cholesterol to bile salts, which are secreted in bile
Protein Metabolism
Without liver metabolism of
proteins, severe survival problems
ensue: many essential clotting
proteins would not be made, and
ammonia would not be disposed
of, for example
Deaminates amino acids for their conversion to glucose or for ATP synthesis; amount of
deamination that occurs outside the liver is unimportant
Forms urea for removal of ammonia from body; inability to perform this function (e.g., in cirrhosis
or hepatitis) allows ammonia to accumulate in the blood
Forms most plasma proteins (exceptions are gamma globulins and some hormones and enzymes);
plasma protein depletion causes rapid mitosis of the hepatocytes and actual growth of liver, which
is coupled with increased synthesis of plasma proteins until blood values are again normal
Transamination: interconversion of nonessential amino acids; amount that occurs outside liver is
inconsequential
Vitamin/Mineral Storage
 
Stores vitamin A (1–2 years’ supply)
Stores sizable amounts of vitamins D and B
12
(1–4 months’ supply)
Stores iron; other than iron bound to hemoglobin, most of body’s supply is stored in liver as ferritin
Biotransformation Functions
 
Metabolizes alcohol and drugs by inactivating them for excretion by the kidneys, and performs
reactions that may result in products which are more active, changed in activity, or less active
Processes bilirubin resulting from RBC breakdown and excretes bile pigments in bile
Metabolizes bloodborne hormones to forms that can be excreted in urine
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