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chapter 22
Metabolic Homeostasis
oxidation of primary foodstuffs is determined by energy
need and cannot occur simply to convert excess foodstuffs
to products that can be readily eliminated. As a conse-
quence, regulation of body weight is determined by three
factors:
1. Food intake,
2. Heat loss, and
3. Energy expenditure (i.e., exercise).
The metabolic activity of each organ in the body de-
pends on its specific functions and the energy required to
perform those functions. The amount of energy needed can
be determined from the amount of ATP generated since
most of the oxygen is used by mitochondria to produce
ATP. At rest, the adult brain (~ 2% of body weight) uses
2 0
% of the oxygen consumed; in contrast, skeletal muscle
(~40% of body weight) consumes only 30% of the oxy-
gen. During heavy muscular work, oxygen consumption
by the body may increase 5-10 fold with a correspond-
ing 10- to 25-fold increase in oxygen consumption by the
skeletal muscle. During heavy muscular work, however,
oxygen consumption by the brain does not change. These
differences between tissues illustrate the metabolic flex-
ibility that can be achieved. The large change in oxygen
consumption by muscle tissue is accompanied by large
changes in fuel utilization and in production of metabo-
lites while overall homeostasis is maintained.
Skeletal muscle has the highest glycolytic capacity re-
flecting the need to generate ATP rapidly for muscle con-
traction. Skeletal muscle relies on glycolysis for short
bursts of high-intensity work under conditions where oxy-
gen is in short supply (Chapter 21). Other tissues that also
possess a high glycolytic capacity are the brain and the
heart. However, the heart, which has a much greater ca-
pacity than skeletal muscle to derive energy oxidatively
from sources other than glucose, rarely relies on glycoly-
sis for energy. The brain, on the other hand, preferentially
uses glucose for energy, first converting it to pyruvate and
then to CO
2
.
The liver and kidney, which do not metabolize glu-
cose to a significant degree, rely primarily on fatty acid
I
Ketone bodies
_^
1 Amino acids I-----
Glucose
Proteins
Glycerol
|-
Glucose
acids
|
-
Ketone bodies I«—
Lactate
Amino acids
|
Glycogen
Arrows indicate preferred
routes in the well-led state
Brain
Heart
tf
H
Glucose
ratt\ acids
( 1 UCOSC
m
Ketone
Ketone nodi
\dipose tissue
Acets C
oA
Ґ
Fatty ac
l
Æ
ij-
Glycogen
Psruvate
( 1 UCOSC
Proteins
Muscle
F IG U R E 22-1
Interrelationships of organs to maintain homeostasis. [Reproduced with permission from R. H. Garrett and
C. M. Grisham,
B io ch em istry,
2nd ed. Harcourt College Publishers, 1995, p. 940.]
