Download changes in oxygen delivery to muscle during exercise

LECTURE : CHANGES IN OXYGEN
DELIVERY DURING EXERCISE
BY : DR FARIHA RIZWAN
CHANGES IN OXYGEN DELIVERY TO MUSCLE
DURING EXERCISE
 During intense exercise, the metabolic need for oxygen in
skeletal muscle increases many times over the resting value
 To meet this rise in oxygen demand blood flow to the
contracting muscle must increases.
 As mentioned earlier, increased oxygen delivery to
exercising skeletal muscle is accomplished via two
mechanisms
 (I) an increased cardiac output and
 (2) a redistribution of blood flow from inactive organs
to the working skeletal muscle
Changes in Cardiac Output
During Exercise
 Cardiac output increases during exercise in
direct proportion to the metabolic rate
required to perform the exercise task.
 Note that the relationship between cardiac
output and percent maximal oxygen uptake is
essentially linear.
 However, in untrained or moderately trained
subjects, stroke volume does not increase
beyond a workload of 40% to 50% of V02 max
 Therefore, at work rates greater than 40% to
50% V02 max, the rise in cardiac output in
these individuals is achieved by increases in
heart rate .
Pressure changes in blood pressure,stroke
volume,cardiac output, heart rate, and the
arterial-mixed venous oxygen difference as
asfunction of relative work rates.
Pressure changes in blood pressure,stroke
volume,cardiac output, heart rate, and the
arterial-mixed venous oxygen difference as
asfunction of relative work rates.
 Maximal cardiac output tends to decrease in a
linear fashion in both men and women after
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thirty years of age .
This is primarily due to a decrease in maximal
heart rate with age.
For example, because cardiac output equals
heart rate times stroke volume, any decrease in
heart rate would result in a decrease in cardiac
output.
The decrease in maximal heart rate with age can
be estimated by the following formula
Max H R = 220 - age (years)
 According to this formula, a twenty-year-old
subject might have a maximal heart rate of
200 beats per minute
 (220 - 20 = 200),
 whereas a fifty-year-old would have a
maximal heart rate of 170 beats per minute
(220 -50 = 170)
Changes in Arterial-Mixed
Venous O2 Content During
Exercise
 arterial-mixed venous oxygen difference (a - V O2 diff) that
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occurs during exercise.
The a V O2 difference represents the amount of O2 that is
taken up from 100 ml of blood by the tissues during one trip
around the systemic circuit.
An increase in the a - V02 difference during exercise is due
to an increase in the amount of O2 taken up and used for
the oxidative production of ATP by skeletal muscle.
The relationship between cardiac output (Q). a - V O2 diff.
and oxygen uptake is given by the Fick equation
V02 = Q x (a - VO2 diff)
 the Fick equation says that V02 is equal to the product of
cardiac output and the a - V02 difference
 This means that an increase in either cardiac output or
a -VO2 diff would elevate V02
Redistribution of Blood Flow
During Exercise
 To meet the increased oxygen demand of the
skeletal muscles during exercise, it is necessary to
increase muscle blood flow while reducing blood
flow to less-active organs such as the liver, kidneys,
and GI tract.
 the change in blood flow to muscle and the
splanchnic (pertaining to the viscera) circulation is
dictated by the exercise intensity (metabolic rate).
 That is, the increase in muscle blood flow during
exercise and the decrease in splanchnic blood flow
change as a linear function of % VO 2 max
the change in blood flow to various organ
systems between resting conditions and during
maximal exercise
Several important points need to be stressed.
First, at rest, approximately 15% to 20% of
total cardiac output is directed toward
skeletal muscle
However, during maximal exercise, 80% to
85% of total cardiac output goes to
contracting skeletal muscle.
This is necessary to meet the huge increase in
muscle oxygen requirements during intense
exercise.
 the absolute blood flow that reaches the brain is
slightly increased above resting values; this is
due to the elevated cardiac output during
exercise .
 Further, although the percentage of total cardiac
output that reaches the myocardium is the same
during maximal exercise as it is at rest.
 the total coronary blood flow is increased due to
the increase in cardiac output during heavy
exercise.
SHIFTING OF BLOOD FLOW
 Finally, note in the graph the reduction in
blood flow to the skin and the abdominal
organs that occurs during intense exercise
when compared to resting conditions.
 This reduction in abdominal blood flow
during heavy exercise is an important means
of shifting blood flow away from "less-active"
tissues and toward the working skeletal
muscles.
 What regulates blood flow to various organs during exercise?
 muscle as well as other body tissues have the unique abiIity to
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regulate their own blood flow in direct proportion to their
metabolic needs.
Blood flow to skeletal muscle during exercise is regulated in the
following way
First. the arterioles in skeletal muscle have a high vascular
resistance at rest
This is due to adrenergic sympathetic stimulation, which causes
arteriole smooth muscle to contract (vasoconstriction)
This produces a relatively low blood flow to muscle at rest (4-5 ml
per minute per 100 grams of muscle),
but because muscles have a large mass this accounts for 20% to
25% of total blood flow from the heart
 At the beginning of exercise, the initial skeletal muscle
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vasodilation that occurs is due to an intrinsic metabolic control
.
This type of blood flow regulation is termed autoregulation,
and it is thought to be the most important factor in
regulating blood flow to muscle during exercise.
The increased metabolic rate of skeletal muscle during
exercise causes local changes such as decreases in oxygen
tension, increases in CO2 tension, nitric oxide, potassium and
adenosine concentrations, and an increase in acidity
These local changes work together to cause vasodilation of
arterioles feeding the contracting skeletal muscle
Vasodilation reduces the vascular resistance and therefore
increases blood flow
 As a result of these changes,blood delivery to
contracting skeletal muscle during heavy
exercise may rise fifteen to twenty times above
that during rest .
 Further, arteriole vasodilation is combined with
"recruitment" of the capiIlaries in skeletal
muscle.
 At rest, only 5% to 10% of the capillaries in
skeletal muscle are open at anyone time;
 however, during intense exercise, almost all of
the capillaries in contracting muscle may be
open.
 The level of vasodilation that occurs in arterioles and small
arteries leading to skeletal muscle is regulated by the
metabolic need of the muscle.
 That is, the intensity of exercise and the number of motor
units recruited determine the overall need for blood flow to
the muscle
 For example, during low-intensity exercise, a relatively
small number of motor units will be recruited into action
resulting in a relatively small demand for blood flow to
these active muscle fibers.
 In contrast. high-intensity exercise would result in the
recruitment of a large number of motor units and,
therefore, result in increased production of locaI
vasodilatory factors.
 Collectively, these changes would result in increased
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vasodilation of arterioles /small arteries and promote
increased blood flow to the contracting muscle in order to
match the metabolic demand.
While the vascular resistance in skeletal muscle decreases
during exercise,
vascular resistance to flow in the visceral organs and other
inactivity tissue increases.
This occurs due to an increased sympathetic output to these
organs, which is regulated by the cardiovascular control
center.
As a result of the increase in visceral vasoconstriction during
exercise (i.e, resistance increases), blood flow to the viscera
can decrease to only 20% to 30% of resting values
SUMMARY
 Oxygen delivery to exercising skeletaI muscle
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increases due to
(I) an increased cardiac output
(2) a redistribution of blood flow from inactive
organs to the contracting skeletal muscles
Cardiac output increases as a linear function of
oxygen uptake during exercise.
During exercise in the upright position, stroke
volume reaches a plateau at approximately 40% of
V02 max.
therefore, at work rates above 40% V02 max, the rise
in cardiac output is due to increases in heart rate.
 During exercise, blood flow to contracting
muscle is increased and blood flow to less-active
tissues is reduced.
 Regulation of muscle blood flow during exercise
is primarily regulated by local factors (called
autoregulation)
 Autoregulation refers to intrinsic control of blood
flow by change in local metabolites(e.g., oxygen
tension, pH, potassium,adenosine, and nitric
oxide) around arterioles
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