During exercise, the cardiovascular system undergoes changes to meet increased demands. Blood pressure typically increases during moderate exercise but remains stable during intense exercise. Cardiac output, the product of heart rate and stroke volume, also increases substantially during exercise through higher heart rates and larger stroke volumes. Trained athletes can achieve much higher maximum cardiac outputs than untrained individuals through greater maximal stroke volumes. Prolonged exercise can cause cardiovascular drift, where stroke volume decreases and heart rate increases to compensate and maintain cardiac output.

1. Cardiovascular Changes
During Exercise
Dr. Usha (PT)
Assistant Professor

2. Blood Pressure
◦ With each contraction of the left ventricle, a surge of blood enters the
aorta, distending the vessel and creating pressure within it. The stretch
and subsequent recoil of the aortic wall propagates as a wave through the
entire arterial system.
◦ The pressure wave readily appears as a pulse in the following areas: the
superficial radial artery on the thumb side of the wrist, the temporal
artery (on the side of the head at the temple), and the carotid artery along
the side of the trachea. In healthy persons, the pulse rate equals the HR.
Blood pressure = Cardiac output x Total peripheral resistance

3. At Rest
◦ The highest pressure generated by left ventricular contraction
(systole) to move blood through a healthy, resilient arterial
system at rest usually reaches 120 mm Hg. As the heart relaxes
(diastole) and the aortic valves close, the natural elastic recoil
of the aorta and other arteries provides a continuous head of
pressure to move blood into the periphery until the next surge
from ventricular systole.
◦ During the cardiac cycle’s diastole, arterial blood pressure
decreases to 70 to 80 mm Hg.

4. ◦ Arteries “hardened” by mineral and fatty deposits within their walls or
arteries with excessive peripheral resistance to blood flow from kidney
malfunction induce systolic pressures as high as 300 mm Hg and diastolic
pressures above 120 mm Hg.
◦ High blood pressure (hypertension) imposes a chronic strain on normal
cardiovascular function. If left untreated, severe hypertension leads to
heart failure as the heart muscle weakens, unable to maintain its normal
pumping ability. Degenerating, brittle vessels can obstruct blood flow or
can burst, cutting off vital blood flow to brain tissue to precipitate a stroke

5. During Exercise
◦ Rhythmic Exercise- During rhythmic brisk walking, hiking, jogging,
swimming, and bicycling, dilatation of the active muscles’ blood vessels
increases the vascular area for blood flow.
◦ The alternate rhythmic contraction and relaxation of skeletal muscles
forces blood through the vessels and returns it to the heart. Increased
blood flow during moderate exercise increases systolic pressure in the
first few minutes; it then levels off, usually between 140 and 160 mm Hg.
Diastolic pressure remains relatively unchanged.

7. ◦ Figure 10.5 reveals the general pattern for systolic and diastolic blood
pressures during continuous, graded treadmill exercise. After an initial
rapid increase from the resting level, systolic blood pressure increases
linearly with exercise intensity, and diastolic pressure remains stable or
decreases slightly at the higher exercise levels.
◦ Healthy, sedentary, and endurance-trained subjects demonstrate similar
blood pressure responses.
◦ But during maximum exercise by healthy, fit men and women, systolic
blood pressure may increase to 200 mm Hg or higher despite reduced
total peripheral resistance. This level of arterial blood pressure most
likely reflects the heart’s large cardiac output during maximal exercise by
individuals with high aerobic capacity.

8. Resistance Exercise
◦ Figure 10.6 contrasts the blood pressure responses during rhythmic aerobic exercise and
intense resistance exercises that engage small and large amounts of muscle mass.
Straining-type exercise (e.g., heavy resistance exercise, shoveling wet snow) increases
blood pressure dramatically because sustained muscular force compresses peripheral
arterioles, considerably increasing the resistance to blood flow.
◦ The heart’s additional workload from acute elevations in blood pressure increases the
risk for individuals with existing hypertension or coronary heart disease.
◦ In such cases, rhythmic forms of moderate physical activity provide less risk and greater
health benefits. On more positive note, those who regularly engage in resistance training
show less dramatic blood pressure increases than untrained counterparts, particularly
when each exerts the same absolute muscle force.

9. ◦ Resistance Exercise

10. CARDIAC OUTPUT
◦ Cardiac output provides the most important indicator of the circulatory
system’s functional capacity to meet the demands for physical activity. As
with any pump, the rate of pumping (heart rate) and quantity of blood
ejected with each stroke (stroke volume) determine the heart’s output of
blood:
Cardiac output = Heart rate x Stroke volume

11. Resting Cardiac Output: Untrained
Versus Trained
◦ Each minute, the left ventricle ejects the entire 5-L blood
volume of an average sized man. This value pertains to most
individuals, but stroke volume and heart rate vary considerably
depending on cardiovascular fitness status.
◦ A heart rate of about 70 b/min sustains the average adult’s 5-L
(5000 mL) resting cardiac output. Substituting this heart rate
value in the cardiac output equation (Cardiac output= Stroke
volume X Heart rate; Stroke volume= Cardiac output/Heart
rate) yields a calculated stroke volume of 71 mL/b.

12. ◦ The resting heart rate for an endurance athlete averages close
to 50 b/min.
◦ The athlete’s resting cardiac output also averages 5 L/min as
blood circulates with a proportionately larger stroke volume of
100 mL per beat (5000 mL/50 b).
◦ Stroke volumes for women usually average 25% below values
for men with equivalent training. The smaller body size of the
typical woman chiefly accounts for this “gender difference.”

13. ◦ The table in the box below summarizes average values for cardiac output,
heart rate, and stroke volume for endurance-trained and untrained men at
rest:

14. Exercise Cardiac Output: Untrained
Versus Trained
◦ Blood flow from the heart increases in direct proportion to exercise intensity
for both trained and untrained individuals. From rest to steady-rate
exercise, CO increases rapidly, followed by a more gradual increase until it
plateaus as blood flow matches exercise metabolic requirements.
◦ In sedentary, college-age men, cardiac output in maximal aerobic
exercise increases about four times the resting level to an average
maximum of 22 L of blood per minute. Maximum heart rate for these
young adults averages 195 b/min. Consequently, stroke volume averages
113 mL of blood per beat during exercise (22,000 mL/195 b).
◦ In contrast, world-class endurance athletes generate maximum cardiac
outputs of 35 L/min.

15. ◦ The difference between maximum cardiac output of both individuals
relates solely to differences in stroke volume. The table in the box below
summarizes average values for cardiac output, heart rate, and stroke
volume of endurance-trained and untrained men during maximal
exercise:

16. Exercise Stroke Volume
◦ Figure 10.11 relates stroke volume and percentage VO2 max(to better
equate exercise intensity among subjects) for 8 healthy college-age men
during graded exercise on a cycle ergometer.
◦ Stroke volume increases progressively with exercise to about 50% VO2
max and then gradually levels off until termination of exercise.
◦ For several subjects, stroke volume decreased slightly at near-maximal
exercise intensities.

18. Stroke Volume and VO2 max
◦ Stroke volume clearly differentiates people with high and low VO2 max.
For example, three groups of subjects were studied:
(1) patients with mitral stenosis, a valvular disease that causes inadequate
emptying of the left ventricle;
(2) healthy but sedentary men; and
(3) athletes.
◦ Differences in VO2max among the groups closely paralleled differences in
maximal stroke volume. Aerobic capacity and maximum stroke volume of
mitral stenosis patients averaged half the values of sedentary subjects.
This close linkage also emerges in comparisons among healthy subjects; a
60% larger stroke volume in athletes compared with sedentary men
paralleled the 62% larger V.O2 max.

19. ◦ All groups showed fairly similar maximum heart rates; thus, stroke volume
differences accounted for the variations in maximum cardiac output and
VO2 max among groups.

20. Stroke Volume Increases During Rest
And Exercise
◦ Three physiologic mechanisms increase the heart’s stroke volume during
exercise.
1. The first, intrinsic to the myocardium, involves enhanced cardiac filling in
diastole followed by a more forceful systolic contraction.
2. Neurohormonal influence governs the second mechanism, which involves
normal ventricular filling with a subsequent forceful ejection and emptying
during systole.
3. The third mechanism comes from training adaptations that expand blood
volume and reduce resistance to blood flow in peripheral tissues

21. Cardiovascular Drift: Reduced Stroke Volume
and Increased Heart Rate During Prolonged
Exercise
◦ Submaximal exercise for more than 15 minutes, particularly in the heat,
produces progressive water loss through sweating and a fluid shift from
plasma to tissues. A rise in core temperature also causes redistribution of
blood to the periphery for body cooling.
◦ At the same time, the progressive decrease in plasma volume decreases
central venous cardiac filling pressure that reduces stroke volume. A
reduced stroke volume initiates a compensatory heart rate increase to
maintain a nearly constant cardiac output as exercise progresses.

22. ◦ The term cardiovascular drift describes this gradual time-
dependent downward “drift” in several cardiovascular
responses, most notably stroke volume with concomitant heart
rate increase, during prolonged steady-rate exercise.
◦ Under these circumstances, a person usually must exercise at a
lower intensity than if cardiovascular drift did not occur.

23. ◦ One explanation for cardiovascular drift suggests that a stroke
volume decline during prolonged exercise in a thermoneutral
environment relates to an increased exercise heart rate and not
increased cutaneous blood flow, a hypothesized by some
researchers.
◦ More than likely, the progressive increase in exercise heart rate
with cardiovascular drift decreases end-diastolic volume,
subsequently reducing the heart’s stroke volume.

24. Exercise Heart Rate
Graded Exercise
◦ Figure 10.12 depicts the relationship between heart rate and oxygen uptake during
increasing intensity exercise (graded exercise) to maximum for endurance trained
individuals and sedentary counterparts. Heart rate for the untrained person accelerates
relatively rapidly with increasing exercise demands; a much smaller heart rate increase
occurs for the trained person. The trained person achieves a higher level of exercise
oxygen uptake at a particular submaximal heart rate than a sedentary person.
◦ Maximum heart rate and the heart rate–oxygen uptake relationship remain fairly
consistent for a particular individual from day to day, although the slope of the
relationship decreases considerably from the stroke volume increases with aerobic
training.

26. Submaximum Exercise
◦ Heart rate increases rapidly and levels off within several minutes during
submaximum steady-rate exercise. A subsequent increase in exercise
intensity increases heart rate to a new plateau as the body attempts to
match the cardiovascular response to the metabolic demands. Each
increment in exercise intensity requires progressively more time to
achieve heart rate stabilization.

27. Cardiac Output Distribution
◦ Blood flow to specific tissues increases in proportion their metabolic
activities.
At Rest
◦ Figure 10.13A shows the approximate distribution of a 5-L cardiac output
at rest. More than one-fourth of the cardiac output flows to the liver; one-
fifth flows to kidney and muscles; and the remainder diverts to the heart,
skin, brain, and other tissues.

28. Figure 10.13 A Relative distribution of cardiac output during rest

29. During Exercise
◦ Figure 10.13B illustrates the distribution of cardiac output to various
tissues during intense aerobic exercise. Regional blood flow varies
considerably depending on environmental conditions, level of fatigue, and
exercise mode, yet active muscles receive a disproportionately large portion
of the cardiac output in exercise.
◦ Each 100 g of muscle receives 4 to 7 mL of blood per minute during rest.
Muscle blood flow increases steadily during exercise to reach a maxi mum
of between 50 to 75 mL per 100 g of active muscle tissue.

30. Figure 10.13B Relative distribution of cardiac output during strenuous
endurance exercise. In strenuous exercise, however, nearly 85% of the total
cardiac output diverts to active muscles.

31. Blood Flow Redistribution
◦ The increase in muscle blood flow with exercise comes largely from increased
cardiac output.
◦ Neural and hormonal vascular regulation, including local metabolic conditions
within muscles moves blood through active muscles from areas that temporarily
tolerate a reduction in normal blood flow.
◦ Shunting of blood away from specific tissues occurs primarily in intense exercise.
Blood flow to the skin increases during light and moderate exercise, s metabolic
heat generated in muscle can dissipate at the skin’s surface.
◦ During intense, short-duration exercise, however, cutaneous blood flow
decreases even when exercising in a hot environment.

32. ◦ In some tissues, blood flow during exercise decreases four-fifths of the flow at
rest.
◦ The kidneys and splanchnic tissues use only 10% to 25% of the oxygen available
in their blood supply at rest. Consequently, these tissues tolerate a considerably
reduced blood flow before oxygen demand exceeds supply an compromises
organ function.
◦ With reduced blood flow, increased oxygen extraction from available blood
maintains the tissue’s oxygen needs. The visceral organs tolerate substantially
reduced blood flow for more than 1 hour during intense exercise. This “frees” as
much as 600 mL of oxygen each minute for use by active musculature.

33. Blood Flow to the Heart and Brain
◦ The myocardium and brain cannot compromise their blood
supplies. At rest, the myocardium normally uses % of the
oxygen in the blood flowing through the coronary circulation.
With such a limited “margin of safety,” increased coronary blood
flow primarily meets the heart’ oxygen demands.
◦ Cerebral blood flow increases up to 30 with exercise compared
with rest; the largest portion of any “extra” blood probably
moves to areas related to motor functions.

34. Cardiac Output & Oxygen Transport
At Rest
◦ Each 100 mL (deciliter [dL]) of arterial blood normally carries about 20 mL of
oxygen or 200 mL of oxygen per liter of blood at sea level conditions.
◦ Trained and untrained adults circulate 5 L of blood each minute at rest, so
potentially 1000 mL of oxygen becomes available during 1 minute (5L
blood/200mL O2). Resting oxygen uptake averages only about 250 mL/min; this
means 750 mL of oxygen returns “unused” to the heart. This does not represent
an unnecessary waste of cardiac output.
◦ To the contrary, extra oxygen in the blood above the resting needs maintains
oxygen in reserve—a margin of safety for immediate use if the need arises.

35. During Exercise
◦ A person with a maximum heart rate of 200 b/min and a stroke volume of
80 mL per beat generates a maximum cardiac output of 16 L (200 b/min
0.080 L).
◦ Even during maximum exercise, hemoglobin remains fully saturated with
oxygen, so each liter of arterial blood carries about 200 mL of oxygen.
Consequently, 3200 mL of oxygen circulate each minute via a 16-L cardiac
output (16 L x 200 mL O2).
◦ If the body extracted all of the oxygen delivered in a 16-L cardiac output,
VO2 max would equal 3200 mL. This represents the theoretical upper limit
for this person because the oxygen needs of tissues such as the brain do
not increase greatly with exercise, yet they require an uninterrupted blood
supply.

36. ◦ An increase in maximum cardiac output directly improves a person’s
capacity to circulate oxygen and profoundly impacts the maximal
oxygen consumption.
◦ If the heart’s stroke volume increased from 80 to 200 mL while the
maximum heart rate remained unchanged at 200 b/min, the
maximum cardiac output would dramatically increase to 40 L/min.
◦ This means that the amount of oxygen circulated in maximum
exercise each minute increases approximately 2.5 times from 3200
to 8000 mL (40 L x 200 mL O2).

37. Maximum Cardiac Output and VO2max
◦ Figure 10.14 displays the relationship between maximum cardiac output
and VO2 max and includes values representative of sedentary individuals
and elite endurance athletes.
◦ An unmistakable relationship emerges. Whereas a low aerobic capacity
links closely to a low maximum cardiac output, a 30- to 40-L cardiac
output always accompanies the ability to generate a 5- or 6-L VO2 max.

39. Extraction of Oxygen: The a–vo2
Difference
◦ If blood flow were the only means for increasing a tissue’
oxygen supply, cardiac output would need to increase from 5
L/min at rest to 100 L in maximum exercise to achieve a 20-fold
increase in oxygen uptake, an oxygen uptake increase common
among endurance athletes.
◦ Fortunately, intense exercise does not require such a large
cardiac output because hemoglobin releases its considerable
“extra” oxygen from blood perfusing active tissues.

40. ◦ Two mechanisms for oxygen supply increase a person’s oxygen uptake
capacity:
1. Increased tissue blood flow
2. Use of the relatively large quantity of oxygen that remains unused by
tissues at rest (i.e., expand the a–vO2 difference)
◦ The following rearrangement of the Fick equation summarizes the
important relationship between maximum cardiac output, maximum a–
vO2 difference, and VO2 max:
VO2max = Maximum cardiac output x Maximum a–vO2 difference

41. The a–v–O2 Difference During Rest and
Exercise
◦ Figure 10.15 shows a representative pattern for changes in a–vO2
difference from rest to maximum exercise for physically active men.
◦ A similar pattern emerges for women except that the arterial oxygen
content averages 5% to 10% lower because of lower hemoglobin
concentrations. The figure includes value for the oxygen content of arterial
blood and mixed-venous blood during different exercise intensities.
◦ Arterial blood oxygen content varies little from its value of 20 mLdL1 at
rest throughout the full exercise intensity range. In contrast, mixed-venous
oxygen content varies between 12 and 15 mLdL1 at rest to a low of 2 to 4
mLdL1 during maximum exercise.

43. ◦ The difference between arterial and mixed-venous blood oxygen content
(a–vO2 difference) at any time represents oxygen extraction from blood as
it circulates through the body’s tissues.
◦ At rest, for example, a–vO2 difference equals 5 mL of oxygen, or only 25%
of the blood’s oxygen content (5 mL/ 20 mL x100); 75% of the oxygen
returns “unused” to the heart bound to haemoglobin.
◦ The progressive expansion of the a–vO2 difference to at least three times
the resting value occurs from a reduced venous oxygen content, which, in
maximal exercise, approaches 20 mL in the active muscle (all oxygen
extracted).
◦ The oxygen content of a true mixed-venous sample from the pulmonary
artery rarely falls below 2 to 4 mL/dL because blood returning from active
tissues mixes with oxygen-rich venous blood from metabolically less
active regions.

44. ◦ Figure 10.15 also indicates that the capacity of each dL of arterial blood
to carry oxygen actually increases during exercise. This results from an
increased concentration of red blood cells (hemoconcentration) from the
progressive movement of fluid from the plasma to the interstitial space
because of two factors:
1. Increases in capillary hydrostatic pressure as blood pressure increases
2. Metabolic byproducts of exercise metabolism create an osmotic pressure
that draws fluid from the plasma into tissue spaces

45. Factors Affecting The Exercise a–vo2
Difference
◦ Central and peripheral factors interact to increase oxygen
extraction in active tissue during exercise. Diverting a large
portion of the cardiac output to active muscles influence the
magnitude of the a–vO2 difference in maximal exercise.
◦ As mentioned previously, some tissues temporarily compromise
blood supply during exercise by redistributing blood to make
more oxygen available for muscle metabolism. Exercise training
facilitates redirection of the central circulation to active muscle.

46. ◦ Increases in skeletal muscle microcirculation with endurance training also
increase tissue oxygen extraction.
◦ Muscle biopsy specimens from the quadriceps femoris show a relatively
large ratio of capillaries to muscle fibers in individuals who exhibit large
a–vO2 differences in intense exercise.
◦ An increase in the capillary-to-fiber ratio reflects positive training
adaptation that enlarges the interface for nutrient and gas exchange
during exercise. Individual muscle cells’ ability to generate energy
aerobically represents another important factor governing oxygen
extraction capacity.

47. Cardiovascular Adjustments To Upper-
body Exercise
◦ The highest oxygen uptake during upper-body exercise
generally averages between 70% to 80% of the VO2 max in
bicycle and treadmill exercise. Similarly, maximal heart rate and
pulmonary ventilation remain lower in exercise with the arms.
The relatively smaller muscle mass of the upper body largely
accounts for these physiologic differences.

48. ◦ The lower maximal heart rate in exercise that activates a
smaller muscle mass most likely results from the following:
1. Reduced output stimulation from the motor cortex central
command to the cardiovascular center in the medulla (less
feedforward stimulation)
2. Reduced feedback stimulation to the medulla from the smaller
active musculature

49. ◦ In submaximal exercise, the metabolic and cardiovascular
response pattern between upper- and lower-body exercise
reverses.
◦ Figure 10.16 shows that any level of submaximal power output
produces a higher oxygen uptake with arm compared with leg
exercise. This difference remains small during light exercise but
becomes progressively larger as intensity of effort increases.

51. ◦ Lower economy of effort in arm-crank exercise probably results from
static muscle actions that do not produce external work but consume extra
oxygen. In addition, the extra musculature activated to stabilize the torso
during most forms of arm exercise adds to the oxygen requirement.
◦ Upper-body exercise also produces greater physiologic strain (heart rate,
blood pressure, pulmonary ventilation, and perception of physical effort)
for any level of oxygen uptake (or percentage of maximal oxygen uptake)
than lower-body leg exercise.

52. ◦ Understanding differences in physiologic response between upper- and
lower-body exercise enables the clinician to formulate prudent exercise
programs using both exercise modes. A standard exercise load (e.g., power
output or oxygen consumption) produces greater physiologic strain with
the arms, so exercise prescriptions based on running and bicycling cannot
be applied to upper-body exercise.
◦ Also, VO2 max for arm exercises does not strongly correlate with leg
exercise VO2 max; thus, one cannot predict accurately one’s aerobic
capacity for arm exercise from a test using the legs and vice versa. This
further substantiates the concept of aerobic fitness specificit

53. Reference
◦ Katch VL, McArdle WD, Katch FI. The Cardiovascular System and Exercise. Essentials of exercise
physiology. Lippincott Williams & Wilkins. 4th ed. 2011; 301-336. ISBN 978-1-60831-267-2.