chapter
7
Cardiorespiratory
Responses to
Acute Exercise
Cardiovascular Response to
Acute Exercise
• Heart
rate (HR), stroke volume (SV), and cardiac output
.
(Q) increase.
• Blood flow and blood pressure change.
• All result in allowing the body to efficiently meet the
increased demands placed on it.
Resting Heart Rate
• Averages 60 to 80 beats per minute (bpm); can range
from 28 bpm to above 100 bpm
• Tends to decrease with age and with increased
cardiovascular fitness
• Is affected by environmental conditions such as altitude
and temperature
Maximum Heart Rate
• The highest heart rate value one can achieve in an allout effort to the point of exhaustion
• Remains constant day to day and changes slightly from
year to year
• Can be estimated: HRmax = 220 – age in years
Changes in Heart Rate as a Subject Walks,
Jogs, Then Runs on a Treadmill
Steady-State Heart Rate
• Heart rate plateau reached during constant rate of
submaximal work
• Optimal heart rate for meeting circulatory demands at
that rate of work
• The lower the steady-state heart rate, the more efficient
the heart
Increase in Heart Rate With Increasing
Power Output and Oxygen Uptake
Reprinted, by permission, from P.O. Astrand et al., 2003, Textbook of work physiology, 4th ed. (Champaign,
IL: Human Kinetics), 285.
Stroke Volume
• Determinant of cardiorespiratory endurance capacity at
maximal rates of work
• May increase with increasing rates of work up to
intensities of 60% of max
• May continue to increase up through maximal exercise
intensity
• Depends on position of body during exercise
Changes in Stroke Volume (SV) as a
Subject Exercises on a Treadmill
Stroke Volume Increases
During Exercise
• Frank Starling mechanism: more blood in the ventricle
causes it to stretch more and contract with more force
• Increased ventricular contractility (without end-diastolic
volume increases)
• Decreased total peripheral resistance due to increased
vasodilation of blood vessels to active muscles
Cardiac Output
• Resting value is approximately 5.0 L/min.
• Increases directly with increasing exercise intensity to
20 to 40 L/min.
• Value of increase varies with body size and endurance
conditioning.
• When exercise
. intensity exceeds 60%, further
increases in Q are more a result of increases in HR
than SV.
.
Changes in Cardiac Output (Q) as a
Subject Exercises on a Treadmill
Changes in (a) Heart Rate, (b) Stroke
Volume, and (c) Cardiac Output With
Changes in Posture
a
b
c
Cardiovascular Drift
• Gradual decrease in stroke volume and systemic and
pulmonary arterial pressures and an increase in
heart rate
• Occurs with steady-state prolonged exercise or
exercise in a hot environment
CARDIOVASCULAR DRIFT
Blood Pressure
Cardiovascular Endurance Exercise
• Systolic BP increases in direct proportion to
increased exercise intensity.
• Diastolic BP changes little, if at all, during
endurance exercise, regardless of intensity.
Resistance Exercise
• It exaggerates BP responses to as high as
480/350 mmHg.
• Some BP increases are attributed to the Valsalva
maneuver.
Blood Pressure Responses to Both Leg
and Arm Cycling at the Same Relative
Rates of Oxygen Consumption
Adapted, by permission, from P.-O. Astrand et al., 1965, "Intraarterial blood pressure during exercise with
different muscle groups," Journal of Applied Physiology 20: 253-256.
Distribution of Cardiac Output at Rest and
During Exercise
Data from A.J. Vander, J.H. Sherman, and D.S. Luciano, 1985, Human physiology: The mechanisms of body
function, 4th ed. (New York: McGraw-Hill Companies).
Key Points
Cardiovascular Response to Exercise
• As exercise intensity increases, heart rate, stroke
volume, and cardiac output increase to get more
blood to the tissues.
• More blood forced out of the heart during exercise
allows for more oxygen and nutrients to get to the
muscles and for waste to be removed more quickly.
• Blood flow distribution changes from rest to
exercise as blood is redirected to the muscles and
systems that need it.
(continued)
Key Points (continued)
Cardiovascular Response to Exercise
• Cardiovascular drift is the result of decreased
stroke volume, increased heart rate, and
decreased systemic and pulmonary arterial
pressure due to prolonged steady-state exercise or
exercise in the heat.
• Systolic blood pressure increases directly with
increased exercise intensity while diastolic blood
pressure remains constant.
• Blood pressure tends to increase during highintensity resistance training, due in part to the
Valsalva maneuver.
Arterial–Venous Oxygen Difference
• Amount of oxygen extracted from the blood as it travels
through the body.
• Calculated as the difference between the oxygen
content of arterial blood and venous blood.
• Increases with increasing rates of exercise as more
oxygen is taken from blood.
• The Fick equation represents the relationship of the
body’s oxygen consumption (VO2) to the arterial–
venous oxygen difference ((a-v)O2 diff) and cardiac
output (Q); VO2 = Q (a-v)O2 diff.
–
Changes in the (a-v)O
2 Difference From
Low Levels to Maximal Levels of Exercise
Blood Plasma Volume
• Reduced with onset of exercise (goes to interstitial
fluid space).
• More is lost if exercise results in sweating.
• Excessive loss can result in impaired performance.
• Reduction in blood plasma volume results in
hemoconcentration.
Key Points
Blood Changes During Exercise
• The (a-v)O
2 difference increases as venous oxygen
concentration decreases during exercise due to the
body extracting oxygen from the blood.
• Plasma volume decreases during exercise due to water
being drawn from the blood plasma and out of the body
as sweat.
• Hemoconcentration occurs. Plasma fluid is lost,
resulting in a higher concentration of red blood cells per
unit of blood and, thus, increased oxygen-carrying
capacity.
• Blood pH decreases due to increased blood lactate
accumulation with increasing exercise intensity.
Did You Know . . . ?
The increase in (a-v)O
2 difference during strenuous
exercise reflects increased oxygen use by muscle
cells. This use increases oxygen removal from arterial
blood, resulting in a decreased venous oxygen
concentration.
The Ventilatory Response to Light,
Moderate, and Heavy Exercise
Breathing Problems During
Exercise
Dyspnea is shortness of breath. During exercise this is
most often caused by inability to readjust the blood
PCO2 and H+ due to poor conditioning of respiratory
muscles.
Hyperventilation is an increase in ventilation that
exceeds the metabolic need for oxygen. Voluntary
hyperventilation reduces the ventilatory drive by
increasing blood pH.
Valsalva maneuver is a breathing technique to trap
and pressurize air in the lungs; if held for an extended
period, it can reduce cardiac output. This technique is
often used during heavy lifts and can be dangerous.
Did You Know . . . ?
Ventilation tends to match the rate of energy
metabolism during mild steady-state activity. Both
vary
. in proportion to the volume of oxygen consumed
(VO2) and .the volume of carbon dioxide produced by
the body (VE).
Ventilatory Equivalent for Oxygen
•
•
•
•
•
.
.
The ratio between VE and VO2 in a given time frame.
Indicates breathing economy.
. .
At
. rest, VE/VO2 = 23 to 28 L of air breathed per L VO2
per minute.
. .
.
At maximal exercise, VE/VO2 = 30 L of air per L VO2
per minute.
. .
Generally VE/VO2 remains relatively constant over a
wide range of exercise levels.
Ventilatory Breakpoint
• The point during intense exercise at which ventilation
increases disproportionately to the oxygen
consumption.
.
• When work rate exceeds 70% VO2max, energy must
be derived from glycolysis.
• Glycolysis increases CO2 levels, which triggers a
respiratory response and increased ventilation.
Ventilatory Threshold
• Point during intense exercise at which metabolism
becomes anaerobic
• Reflects the lactate threshold under most conditions,
though the relationship is not always exact
. .
• Identified by noting an increase in VE/VO2 without a
concomitant increase
. . in the ventilatory equivalent for
carbon dioxide (VE/VCO2)
Changes in Pulmonary Ventilation During
Running at Increasing Velocities
Key Points
Pulmonary Ventilation
• The respiratory centers in the brain stem set the rate
and depth of breathing.
• Chemoreceptors respond to increases in CO2 and H+
concentrations or to decreases in blood oxygen levels
by increasing respiration.
• Ventilation increases upon exercise due to inspiratory
stimulation from muscle activity, which causes an
increase in muscle temperature and chemical changes
in the arterial blood (which further increase ventilation).
(continued)
Key Points (continued)
Pulmonary Ventilation
• Breathing problems associated with exercise include
dyspnea, hyperventilation, and the Valsalva maneuver.
• During mild, steady-state exercise, ventilation parallels
oxygen uptake.
• The ventilatory breakpoint is the point at which
ventilation increases though oxygen consumption does
not.
• Anaerobic
threshold is identified as the point
. .
. . at which
VE/VO2 shows a sudden increase, while VE/VCO2 stays
stable. It generally reflects lactate threshold.
Respiratory Limitations to
Performance
• Respiratory muscles may use more than 15% of total
oxygen consumed during heavy exercise and seem to
be more resistant to fatigue during long-term activity
than muscles of the extremities.
• Pulmonary ventilation is usually not a limiting factor for
performance, even during maximal effort, though it can
limit performance in highly trained people.
• Airway resistance and gas diffusion usually do not limit
performance in healthy individuals, but abnormal or
obstructive respiratory disorders can limit performance.
Respiratory Regulation of Acid–Base
Balance
• Excess H+ (decreased pH) impairs muscle contractility
and ATP formation.
• The respiratory system helps regulate acid–base
balance by increasing respiration when H+ levels rise.
The increase in respiration allows more CO2 to be
released in the blood (bound to bicarbonate) to be
transported to the lungs for exhalation.
• Whenever H+ levels begin to rise, from carbon dioxide
or lactate accumulation, bicarbonate ions can buffer the
H+ to prevent acidosis.
Tolerable Limits for Arterial Blood pH and
Muscle pH at Rest and at Exhaustion
Effects of Active and Passive
Recovery on Blood Lactate Levels