Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
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24
into blood. However, there is more than a push-pull mechanism
here because rising amino acid levels in the blood stimulate the
release of both insulin and glucagon.
Tis effect is insignificant when we eat a balanced meal, but
it has an important adaptive role when we eat a high-protein,
low-carbohydrate meal. In this instance, the stimulus for in-
sulin release is strong, and if it were not counterbalanced, the
brain might be damaged by the abrupt onset of hypoglycemia
as glucose rushes out of the blood. Simultaneous release of glu-
cagon modulates the effects of insulin and helps stabilize blood
glucose levels.
Te sympathetic nervous system also plays a crucial role
in supplying fuel quickly when blood glucose levels drop sud-
denly. Adipose tissue is well supplied with sympathetic fibers,
and epinephrine released by the adrenal medulla in response
to sympathetic activation acts on the liver, skeletal muscle, and
adipose tissues. ±ogether, these stimuli mobilize fat and pro-
mote glycogenolysis—essentially the same effects prompted by
glucagon.
Injury, anxiety, or any other stressor that mobilizes the fight-
or-flight response will trigger this control pathway, as does ex-
ercise. During exercise, large amounts of fuels must be made
available for muscles, and the metabolic profile is essentially the
same as that of a fasting person when glucagon and the sympa-
thetic nervous system are in control except that the facilitated
diffusion of glucose into muscle is enhanced. (Te mechanism
of this enhancement is not yet understood.)
In addition to glucagon and epinephrine, a number of other
hormones—including growth hormone (GH), thyroxine, sex
hormones, and corticosteroids—influence metabolism and
nutrient flow. Prolonged fasting or rapid declines in blood
glucose levels enhance growth hormone (GH) secretion,
which exerts important anti-insulin effects. For example, GH
reduces the ability of insulin to promote glucose uptake in fat
and muscle.
However, the release and activity of most of these hormones
are not specifically related to absorptive or postabsorptive meta-
bolic events.
Table 24.5
summarizes typical metabolic effects of
various hormones.
Check Your Understanding
21.
Which three organs or tissues are the primary effector organs
determining the amounts and directions of interconversions
in the nutrient pools?
22.
Generally speaking, what kinds of reactions and events
characterize the absorptive state? The postabsorptive state?
23.
Which hormone is glucagon’s main antagonist?
24.
Which event increases both glucagon and insulin release?
For answers, see Appendix H.
The Metabolic Role of the Liver
Describe several metabolic functions of the liver.
Differentiate between LDLs and HDLs relative to their
structures and major roles in the body.
Plasma glucose
(and rising amino
acid levels)
Stimulates
Plasma glucagon
Plasma fatty acids
Fat used by tissue cells
= glucose sparing
Plasma glucose
(and insulin)
Liver
Alpha cells of
pancreatic islets
Stimulates
glycogenolysis
and gluconeogenesis
Negative feedback:
rising glucose
levels shut off
initial stimulus
Stimulates
fat breakdown
Reduces, inhibits
Increases, stimulates
Adipose tissue
Initial stimulus
Physiological response
Result
Figure 24.21
Glucagon is a hyperglycemic hormone that
stimulates a rise in blood glucose levels.
The dashed arrow
indicates negative feedback control exerted by rising plasma glucose
levels on glucagon secretion.
One of the most biochemically complex organs in the body, the
liver processes nearly every class of nutrients and plays a major
role in regulating plasma cholesterol levels. In a pinch, mechanical
contraptions can stand in for a failed heart, lungs, or kidney, but
the only thing that can do the versatile liver’s work is a hepatocyte.
Te hepatocytes carry out some 500 or more intricate meta-
bolic functions. A description of all of these functions is well
beyond the scope of this text, but
Table 24.6
provides a brief
summary.
Cholesterol Metabolism and Regulation
of Blood Cholesterol Levels
Cholesterol
has received little attention in this discussion so far,
primarily because it is not used as an energy source. It serves in-
stead as the structural basis of bile salts, steroid hormones, and
vitamin D and is a major component of plasma membranes.
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