295
CHAPTER NINE
Muscular System
degree of oxygen debt also refl ects the oxygen required to
restore blood and tissue oxygen levels to preexercise levels.
The metabolic capacity of a muscle may change with ath-
letic training. With high-intensity exercise, which depends
more on glycolysis for ATP, a muscle will synthesize more
glycolytic enzymes, and its capacity for glycolysis will
increase. With aerobic exercise, more capillaries and mito-
chondria develop, and the muscles’ capacity for the aerobic
reactions of cellular respiration increases.
may decrease during muscular contraction when contract-
ing muscle F
bers compress blood vessels
(f
g. 9.13)
.
Oxygen Debt
When a person is resting or moderately active, the respira-
tory and cardiovascular systems can usually supply sufF
cient
oxygen to the skeletal muscles to support the aerobic reac-
tions of cellular respiration. However, when skeletal muscles
are used more strenuously, these systems may not be able to
supply enough oxygen to sustain the aerobic reactions of cel-
lular respiration.
Chapter 4 (pp. 120–122) discussed how the anaero-
bic reactions break down glucose into pyruvic acid, which
then reacts to produce lactic acid. This shift in metabolism
is referred to as the anaerobic threshold, or the
lactic acid
threshold.
The lactic acid diffuses out of the muscle F bers
and is carried in the bloodstream to the liver. Liver cells can
react the lactic acid to form
glucose,
but this requires energy
from ATP
(f g. 9.14)
. During strenuous exercise, available
oxygen is primarily used to synthesize ATP for muscle con-
traction rather than to make ATP for reacting lactic acid to
yield glucose. Consequently, as lactic acid accumulates, a
person develops an
oxygen debt
that must be repaid at a
later time. The amount of oxygen debt roughly equals the
amount of oxygen that liver cells require to use the accumu-
lated lactic acid to produce glucose, plus the amount that the
muscle cells require to resynthesize sufF
cient ATP and cre-
atine phosphate to restore their original concentrations. The
FIGURE 9.13
The oxygen required to support the aerobic reactions of cellular respiration is carried in the blood and stored in myoglobin. In the
absence of suF
cient oxygen, anaerobic reactions use pyruvic acid to produce lactic acid. The maximum number of ATPs generated per glucose
molecule varies with cell type; in skeletal muscle, it is 36 (2 + 34).
ATP
2
Energy
Lactic acid
Glucose
ATP
Synthesis of 34
CO
2
+
H
2
O
+
Energy
Pyruvic acid
Citric acid
cycle
Heat
In the absence of
sufficient oxygen,
glycolysis leads to
lactic acid
accumulation.
2
Oxygen carried from
the lungs by
hemoglobin in red
blood cells is stored
in muscle cells by
myoglobin and is
available to support
aerobic respiration.
1
Electron
transport
chain
Mitochondria
Cytosol
Energy to
synthesize
Glycolysis and
lactic acid formation
(in muscle)
Synthesis of glucose
from lactic acid
(in liver)
Energy
from
ATP
ATP
Glucose
Glycogen
Lactic acid
Pyruvic acid
FIGURE 9.14
Liver cells can react lactic acid generated by muscles
anaerobically to produce glucose.
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