18. Exercise

Question 1

Describe the physiological changes that occur in response to exercise.

O2 Consumption at rest

During exercise

Answer 1

In healthy individuals,
predictable physiological changes
occur during exercise.

At rest, oxygen consumption
is approximately 250 mL/min.

During strenuous exercise
oxygen consumption may rise
to over 4000 mL/min.

This massive increase in
oxygen consumption requires a
cardio-respiratory
response to increase oxygen delivery.

Question 2

Respiratory changes:

Answer 2

On initiation of exercise,

minute ventilation increases dramatically

from a basal rate of approximately

5 L /min to over 20 L /min

via a combination of increase in respiratory rate
and tidal volume.

This initial increase is thought
to occur in response to afferent
impulses from proprioceptors in muscle.

As exercise progresses,
minute ventilation continues to rise
linearly with work rate and
may reach in excess of 150 L /min.

Oxygen consumption also increases linearly
with work rate,
until the subject reaches their VO2 . max,

at which point it becomes constant.

VO2. max is the maximum amount
of oxygen a subject can utilise at a
cellular level to produce ATP to
power the exercise.

It is measured in mL/kg/min.

VO2  max is the best and
most reproducible index of
cardiopulmonary fitness.

Any increase in work rate
above VO2 . max
can only occur via anaerobic glycolysis.

Minimal changes occur in
arterial pH, PaCO2 and PaO2 during exercise.

Question 3

VO2. max

Answer 3

is the maximum amount
of oxygen a subject can utilise at a
cellular level to produce ATP to
power the exercise.

Question 4

Cardiovascular changes:

Answer 4

Cardiac output increases as a consequence of a
rise in heart rate
and
augmentation of stroke volume

because of increased force of systolic contraction.

These chronotropic and inotropic
effects on the heart are the
result of activation of the
sympathetic nervous system

and a

rise in LVEDP resulting from
increased venous return from
muscle and capacitance beds.

In trained athletes,
the left ventricle hypertrophies
and a resting bradycardia
is often present.

Trained athletes may achieve
cardiac outputs in excess of
30 L /min during exercise.

Muscle blood flow is increased
during exercise as a consequence
of accumulation of metabolites
such as adenosine and potassium.

Consequently, SVR is reduced.
Oxygen extraction by the muscle is also
increased during exercise.

The oxyhaemoglobin dissociation curve
is shifted to the right because of the

reduced local pH in exercising muscle and

the increased temperature.

Blood is also redistributed to the
skin to enable heat loss.

Question 5

What metabolic changes occur during exercise?

Answer 5

The primary source of fuel to
produce energy in the
early stages of exercise
is carbohydrate,

stored as glycogen
and
liberated into glucose.

As glycogen stores become depleted
during prolonged exercise, the
metabolic substrate switches
to fatty acids.

If fat is fully oxidised via the Krebs cycle
it leads to the generation of
129 molecules of ATP.

The rate of ATP re-synthesis from fat
is too slow to be of
great importance during high intensity
exercise such as sprinting

however it is
important during endurance exercise.

Under normal circumstances
protein metabolism does not
contribute to ATP generation

because it is an essential
structural component of the body.

However, in extreme conditions
(e.g. ultra marathon runners or starvation)
protein can be used to generate ATP

Question 6

What is the anaerobic threshold (AT)?

Answer 6

The AT marks the onset of
anaerobic metabolism as a
result of inadequate oxygen delivery.

At this point lactate begins to
accumulate in the blood.

The VO2  at this point is called
the anaerobic threshold.
It is measured in mL/Kg/min of O2.

The anaerobic threshold typically occurs 
between 45% and 65% of the VO2  max 
in healthy untrained individuals 
and 
generally does not exceed 80% 
even in endurance-trained athletes.

Training can result in an increase in both
VO2  max and anaerobic threshold.

Question 7

What is the respiratory exchange

ratio (RER)?

Answer 7

The RER is the ratio of
CO2 production to O2 consumption:

       .            . RER = VCO2/VO2 

The RER represents the metabolic exchange
of gases in the body’s tissues
and is dependent in part on

the predominant fuel
(carbohydrate vs. fat) used for cellular metabolism.

At rest and with early exercise,
the V.CO2 curve runs
slightly below the V.O2 curve (RER 0.8)

but once the anaerobic threshold is passed,
additional non-metabolic CO2 is produced,
resulting in a steep rise in VCO2
and an accompanying rise in the
RER, ultimately exceeding 1.0.