cardiovascular response to exercise

Cardiovascular Physiology Laboratory Exercise

IntroductionThe purpose of this exercise is to examine the physiology of the cardiovascular system during exercise. Although we do not have the research subjects, equipment, or time to perform the actual experiments of this exercise, we can use experimental data to better understand cardiovascular responses to exercise. In addition, it is an opportunity to practice your problem-solving skills.

Divide up into research teams, with no more than four persons to a team. Each team member should be prepared to present part of the team's results to the class.

As you precede through the exercise, you will be asked to review or summarize your knowledge of cardiovascular physiology. This information is necessary to solve the problems. Make sure that every team member understands the basics. Remember that this is a team effort!

Background InformationThis exercise examines the cardiovascular responses of four males.1. Sedentary Individual. Norman Normal is a typical product of our modern society. He is

healthy, with no physical disorders, but has a sedentary life style that does not include any regular exercise activities.

2. Endurance-trained Individual. Jock Player is in excellent physical condition. His regular training regime includes aerobic exercises such as running and swimming. As a result of his training, his heart has hypertrophied: left ventricular volume and muscle mass have increased.

3. Quadriplegic Individual. Ben Armstrong has a spinal lesion at level C7 - C8. He can move his upper limbs, but has lost fine motor control of his hands. By strapping his hands to an arm cycle, he is capable of performing arm exercises.

4. Heart Transplanted Individual. Moore Heart has a heart transplant. His diseased heart was replaced with a donor heart. There is no nerve supply to the donor heart. He is otherwise a normal, sedentary individual. Please note that Moore Heart is taking cyclosporine, a drug that helps to prevent graft rejection. A side effect of cyclosporine in most cardiac transplant patients is hypertension resulting from increased peripheral resistance.

Each male is 30 years old. All individuals are of the same approximate size and weight, except for the quadriplegic, who has lost weight as a result of muscle atrophy. For all you lawyer types, the donated heart came from a sedentary 30 year old male who was the same size and weight as Norman Normal.

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Before preceding to the experiment, you should determine how the condition of each individual effects their cardiovascular system. Fill out Table 1 using these symbols:

0 = Absent + = Normal- = Below normal ++ = Above normal

TABLE 1 ASSESSMENT OF EXPERIMENTAL SUBJECTS

Parasympathetic Innervation of the Heart

Sympathetic Innervation of the Heart

Adrenal Medullary Mechanism

Muscular Venous Pump and Venous Return to the Heart

Norman Normal (Sedentary)

Jock Player (Athlete)

Ben Armstrong (Quadriplegic)*

-

Moore Heart (Transplant)

Reference page number in text**

694 694 752 756

* The quadriplegic's spinal lesion prevents sympathetic stimulation of the adrenal medulla by the brain (see p. 756 in text). However, during exercise, epinephrine and norepinephrine are released from the adrenal medulla, although more slowly and in smaller amounts than in a normal individual. Consequently, it can take three to four minutes for a quadriplegic to achieve a steady state cardiovascular response during exercise. The mechanism controlling this release of epinephrine and norepinephrine is not presently known.**Reference page numbers are for the 10th edition of Seeley’s Anatomy & Physiology.

The ExperimentEach individual performs an aerobic exercise. The sedentary, athletic, and heart transplanted individuals use a stationary bicycle. The quadriplegic uses an arm cycle. The aerobic exercise is performed at different work load levels. Each work load is determined by measuring steady state oxygen consumption.

1. Explain why steady state oxygen consumption is a suitable measure of the work performed by skeletal muscles.

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Analysis of Heart RatesBefore analyzing the heart rates for the experimental subjects, complete Table 2 by indicating if heart rate decreases or increases.

TABLE 2 FACTORS EFFECTING HEART RATE

Heart Rate Reference page number in text

Decreased parasympathetic stimulation of the heart*

694, 696

Increased sympathetic stimulation of the heart*

694, 696

Increased epinephrine and norepinephrine released from the adrenal medulla

694

* When exercise begins, parasympathetic stimulation of the heart decreases until heart rate reaches about 100 beats/min. Above 100 beats/min sympathetic stimulation of the heart increases.

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Each individual exercised at increasing work loads until they achieved their maximum heart rate.

FIGURE 1 HEART RATE Effect of increasing work load, expressed as oxygen consumption, on heart rate (Redrawn with permission from Patil, R.D., Karve, S.V., and DiCarlo, S.E. Adv. Physiol. Ed. 10(1):S22, 1993, figure 1).

Using Figure 1, determine the resting and maximum heart rates for each of the four individuals. Estimate heart rate to the nearest 5 beats/min. Record the heart rates in Table 3.

TABLE 3 COMPARISON OF RESTING AND MAXIMUM HEART RATES

Resting Heart Rate Maximum Heart Rate

Ben Armstrong (Quadriplegic)




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Comparison of Sedentary and Quadriplegic Individuals2. Compare the resting heart rates of the sedentary individual and the quadriplegic.

Explain these heart rates.

3. Compare the maximum heart rates of the sedentary individual and the quadriplegic. Explain these heart rates.

Comparison of Sedentary and Heart Transplanted Individuals4. Compare the resting heart rates of the sedentary individual and the heart transplanted

individual. Explain these heart rates.

5. Compare the maximum heart rates during exercise of the sedentary individual and the heart transplanted individual. Explain these heart rates.

6. What causes the heart rate of the heart transplanted individual to increase during exercise?

Comparison of Heart Transplanted and Quadriplegic Individuals7. Compare the shape of the curve for the heart transplanted and quadriplegic individuals

(Hint: compare the "angle" of the first half of each curve to its last half). Explain the rate of increase in heart rate in these individuals.

Comparison of Sedentary and Athletic Individuals8. Compare the resting heart rates of the sedentary individual and the athletic individual.

Explain these heart rates.

9. Compare the maximum heart rates during exercise of the sedentary individual and the athletic individual. As a general rule, maximum heart rate (HRmax) can be estimated using the following equation:

HRmax = 220 - Age

The decrease in HRmax with age might be attributed to changes in the heart's conducting system, especially in the SA node. There is also a decrease in beta receptors in the heart, which decreases the heart's response to epinephrine and norepinephrine.

Use this equation to determine the HRmax of the sedentary and athletic individuals. Verify that these are the same HRmax values as in Figure 1.

Note that endurance training does not generally change HRmax. However, in highly conditioned endurance athletes, HRmax can be lower than in sedentary individuals of the same age. The decrease in HRmax is offset by an increase in stroke volume. It is more energy efficient for the heart to contract less often, but more forcefully. Also, the lower heart rate allows more time for ventricular filling.

Comparison of Heart Transplanted and Quadriplegic Individuals Versus Sedentary and Athletic Individuals10. Compare and explain the difference in maximum heart rates of the heart transplanted

and quadriplegic individuals versus the sedentary and athletic individuals. Note that the equation for HRmax in question 6 does not apply to the heart transplanted and quadriplegic individuals.

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Analysis of Stroke VolumeBefore analyzing the stroke volumes for the experimental subjects, complete Table 4 by indicating if stroke volume decreases or increases.

TABLE 4 FACTORS EFFECTING STROKE VOLUME

Stroke Volume Reference pagenumber in text

Changes in parasympathetic stimulation of the heart

Little effect 693

Increased sympathetic stimulation of the heart

693


694

Increased venous return to the heart (Starling's Law)

692

Increased end-diastolic volume

691

Decreased end-systolic volume 691

Increased blood volume* 691, 753

*Endurance training produces an increase in blood volume, resulting from an increase in plasma volume. Although the number of red blood cells also increases with exercise, the plasma volume usually increases even more. Thus, hematocrit is reduced. The increased plasma volume is caused by increased secretion of ADH and aldosterone, which promote water and sodium retention. Increased plasma protein production, especially albumin, also promotes water retention.

Deconditioning from lack of muscular activity results in a decrease in blood volume.

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Each individual exercised at increasing work loads until they achieved their maximum stroke volume.

FIGURE 2 STROKE VOLUME Effect of increasing work load, expressed as oxygen consumption, on stroke volume (Redrawn with permission from Patil, R.D., Karve, S.V., and DiCarlo, S.E. Adv. Physiol. Ed. 10(1):S22, 1993, figure 2).

Note that during exercise, stroke volume increases and levels off near its maximum value at about 40% to 60% of the oxygen uptake at maximum exercise.

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Comparison of Resting Stroke Volumes

Use Figures 1 and 2 to complete Table 5 for resting heart rates and stroke volumes. Estimate heart rate to the nearest 5 beats/min and stroke volume to the nearest 5 ml/beat. Use heart rate and stroke volume values to calculate cardiac outputs.

TABLE 5 COMPARISON OF RESTING HEART RATES, STROKE VOLUMES, AND CARDIAC OUTPUTS

Heart Rate Stroke Volume Cardiac Output





11. Explain the different stroke volumes observed in the sedentary, athletic, and heart transplanted individuals at rest (Hint: consider cardiac output).

12. Explain the stroke volume observed in the quadriplegic individual at rest (Hint: deconditioning and atrophy).

Comparison of Maximum Stroke Volumes13. The maximum stroke volumes during exercise of the sedentary and heart transplanted

individuals are similar (much closer to each other than either is to the stroke volumes of the athletic and quadriplegic individuals). Explain this similarity. Note however, that the sedentary individual's stroke volume is larger than that of the heart transplanted individual. Explain.

14. Compare the maximum stroke volumes during exercise of the sedentary and quadriplegic individual. Explain.

15. Compare the maximum stroke volumes during exercise of the sedentary and athletic individual. Explain.

16. Compare the shape of the curves for the sedentary, athletic, and heart transplanted individuals versus the quadriplegic individual (Hint: draw a straight line between the minimum and maximum stroke volumes for each individual). Explain the rate of increase in stroke volume in these individuals.

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Analysis of Cardiac OutputEach individual exercised at increasing work loads until they achieved their maximum cardiac output.

FIGURE 3 CARDIAC OUTPUT Effect of increasing work load, expressed as oxygen consumption, on cardiac output (Redrawn with permission from Patil, R.D., Karve, S.V., and DiCarlo, S.E. Adv. Physiol. Ed. 10(1):S22, 1993, figure 4).

17. How is cardiac output related to the amount of work performed by each of the four individuals?

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Ranking Stroke Volume, Heart Rate, and Cardiac Output

The greater the cardiac output, the greater the ability to perform work. Cardiac output is equal to heart rate times stoke volume. These four individuals have different cardiac outputs because of differences in their heart rates and stroke volumes.

In Table 6, the maximum stroke volume for each of the individuals has been ranked. A rank of 1 is the smallest value, and a rank of 4 is the largest value. Using Figure 2, verify that the assigned ranks are correct.

Note that ranking date is an easy way to emphasize the differences between the individuals without having to remember the actual values.

In Table 6, the maximum heart rate for each individual has also been ranked. Individuals that have the same values for a variable are assigned the average of their ranks. Because the sedentary and athletic individuals have the same maximum heart rate of 190 beats/min, they are both assigned the rank of 3.5, which is the average of rank values 3 and 4, that is, (3 + 4)/2.

Using Figure 3, rank the maximum cardiac output for each individual.

TABLE 6 RANK VALUES FOR MAXIMUM HEART RATE, STROKE VOLUME, AND CARDIAC OUTPUT

Maximum Stroke Volume Rank Value

Maximum Heart Rate Rank Value

Maximum Cardiac Output Rank Value

Ben Armstrong (Quadriplegic) 1 1

Moore Heart (Transplant) 2 2

Norman Normal (Sedentary) 3 3.5

Jock Player (Athlete) 4 3.5

18. The quadriplegic has the lowest cardiac output because he has the lowest stroke volume and heart rate. Why does the quadriplegic have the lowest stroke volume and heart rate?:

19. The heart transplanted individual has the second largest cardiac output. Compared to the quadriplegic he has an increased stroke volume and heart rate. The increase in stroke volume and heart rate accounts for the increase in his cardiac output. Why does the heart transplanted individual have an increased stroke volume and heart rate compared to the quadriplegic?

20. The sedentary individual has the third largest cardiac output. Compared to the heart transplanted individual, why does the sedentary individual have a larger cardiac output?

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21. The athlete has the largest cardiac output. Compared to the sedentary individual, why does the athlete have a larger cardiac output?

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Analysis of Blood PressureSystolic PressureEach individual exercised at increasing work loads until they achieved their maximum systolic pressure.

FIGURE 4 SYSTOLIC PRESSURE Effect of increasing work load, expressed as oxygen consumption, on systolic pressure (Redrawn with permission from Patil, R.D., Karve, S.V., and DiCarlo, S.E. Adv. Physiol. Ed. 10(1):S22, 1993, figure 8).

Use Figures 3 and 4 to complete Table 7.

TABLE 7 MAXIMUM CARDIAC OUTPUTS AND SYSTOLIC PRESSURES

Maximum CardiacOutput (ml/min)

Maximum SystolicPressure (mm Hg)




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Use the data in Table 7 to make a graph of maximum systolic pressure versus maximum cardiac output.

FIGURE 5 SYSTOLIC PRESSURE Effect of increasing cardiac output on systolic pressure.

22. Based on your graph, what conclusion do you come to regarding the effect of cardiac output on systolic pressure? Explain your conclusion.

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Diastolic PressureDiastolic pressure was measured for each of the individuals as they exercised at increasing work loads until they achieved their maximum work load.

FIGURE 6 DIASTOLIC PRESSURE Effect of increasing work load, expressed as oxygen consumption, on diastolic pressure (Redrawn with permission from Patil, R.D., Karve, S.V., and DiCarlo, S.E. Adv. Physiol. Ed. 10(1):S22, 1993, figure 9).

23. As a generalization, did diastolic pressure change very much during exercise? Based on this observation, what conclusion can you make about the relationship between cardiac output, systolic pressure, and peripheral resistance during exercise? Why would this be advantageous during exercise?

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Mean Arterial PressureMean arterial pressure (MAP) is the average pressure in the aorta throughout one cardiac cycle. It is often called the perfusion pressure, because it is the pressure necessary to make blood flow, or perfuse, through tissues. Sometimes, it is taken to be the average of systolic pressure (SP) and diastolic pressure (DP). However, this is slightly in error because systole lasts about one third as long as diastole. A more accurate calculation is:

MAP = DP + (SP - DP)/3

Pulse pressure (PP) is defined to be the difference between systolic pressure and diastolic pressure (PP = SP - DP). Therefore, MAP can be calculated as

MAP = DP + PP/3

Use this formula to calculate the maximum MAP for the four experimental subjects at their maximum work levels. Enter the results in Table 8.

TABLE 8 MAXIMUM MEAN ARTERIAL PRESSURE

DiastolicPressure

SystolicPressure

PulsePressure

Mean Arterial Pressure


70 120



63 200

Jock Player (Athlete) 39 250

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Compare the values in Table 8 to Figure 7.

FIGURE 7 MEAN ARTERIAL PRESSURE Effect of increasing work load, expressed as oxygen consumption, on mean arterial pressure.

Comparison of Sedentary and Quadriplegic Individuals24. Compared to the sedentary individual, the quadriplegic is able to perform less muscular

work. Explain in terms of cardiac output and mean arterial pressure.

Comparison of Sedentary and Heart Transplanted Individuals25. Compared to the sedentary individual, the heart transplanted individual is able to

perform less muscular work. Explain in terms of cardiac output, mean arterial pressure, and peripheral resistance (Hint: cyclosporine)

Comparison of Sedentary and Athletic Individuals26. Compared to the sedentary individual, the athlete is able to perform more muscular

work. Explain in terms of cardiac output, mean arterial pressure, and peripheral resistance. Be sure to explain how they both can have the same maximum mean arterial pressure (Hint: see Figures 4 and 5).

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Cardiovascular Physiology Laboratory Exercise Answers

TABLE 1 ASSESSMENT OF EXPERIMENTAL SUBJECTS

Parasympathetic Nerve Supply to the Heart

Sympathetic Nerve Supply to the Heart

Sympathetic Nerve Supply to Blood Vessels

Sympathetic Nerve Supply to Adrenal Medulla


+ + + +


+ + + +


+ - - -


- - + +

Reference page number in text

694 694 752 756

Teacher’s note: It has been proposed that the release of epinephrine and norepinephrine following spinal cord injury is mediated through spinal cord reflex centers. Following spinal cord injury, there is a depression of spinal cord reflex centers below the level of the injury because of a lack of stimulation from the brain. This is called spinal shock. Often, within a few weeks to months, reflex centers recover and become active again. In fact, after recovery from spinal shock, spinal cord neurons can become hyperactive because of a lack of inhibition from the brain. For example, in autonomic hyperreflexia (dysreflexia), stimulation of sensory receptors below the level of spinal cord injury activates sympathetic reflexes, resulting in increased constriction of arterioles and arteries, and increased secretion of epinephrine and norepinephrine. Consequently, systolic blood pressure can increase up to 300 mm Hg.

1. Muscle contraction requires the expenditure of ATP. While maintaining a given work load, these ATP molecules are produced through aerobic respiration. Thus, oxygen consumption is directly related to the number of ATP molecules produced. The greater the amount of oxygen consumed, the greater the number of ATP molecules produced, the greater the muscle activity using the ATP molecules, and the greater the work accomplished by the muscles.

TABLE 2 FACTORS EFFECTING HEART RATE

Heart Rate Reference page number in text

Decreased parasympathetic stimulation of the heart

Increases 694, 696

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Increases 694, 696


Increases 694

Teacher’s note: Increased venous return can increase right atrial pressure and activate the Bainbridge reflex. Stretch receptors in the right atrial wall detect the increase in pressure and send action potentials through the vagus nerves to the medulla. The medulla sends action potentials through sympathetic nerves to the heart, resulting in an increase in heart rate. The Bainbridge reflex is of minor importance in humans, at best. According to Rowell (1993) there is no Bainbridge reflex in humans and other primates.

Rowell, L.B. (1993) Human Cardiovascular Control, p. 111, Oxford University press, 500pp.

TABLE 3 COMPARISON OF RESTING AND MAXIMUM HEART RATES

Resting Heart Rate Maximum Heart Rate


85 120


Norman Normal (Sedentary) 70 190


2. A. The quadriplegic does not have normal sympathetic control of his blood vessels, resulting in massive vasodilation and loss of vasomotor tone.

B. The loss of vasomotor tone results in decreased peripheral resistance, which causes a decrease in blood pressure.

C. The decreased blood pressure is detected by the baroreceptors. The baroreceptor reflex cannot affect blood vessels because of the spinal cord injury, but it can increase heart rate by withdrawing parasympathetic stimulation through the vagus nerves. Therefore, heart rate increases.

Teacher’s note: Following recovery from spinal shock, the quadriplegic still has a higher heart rate than a sedentary person. Possible contributing factors include:A. Cardiac sympathetic reflexes that are no longer inhibited by the brain. The reflex

arc includes sensory nerve fibers that extend from the heart through sympathetic nerves to the spinal cord and sympathetic nerve fibers that extend from the spinal cord to the heart. Although little is known of the normal function of these reflexes, evidence indicates that increased cardiac volumes and pressure can result in increased sympathetic stimulation of the heart (Physiol. Rev. 71(3):643-645)

B. Intrinsic heart rate increases with deconditioning.

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C. Heart rate is increased to compensate for decreased stroke volume and blood pressure (see below).

3. The quadriplegic has a lower maximum heart rate than the sedentary individual. The quadriplegic lacks sympathetic stimulation of the heart and has a smaller adrenal medullary response.

4. The heart transplanted individual has a higher heart rate at rest than does the sedentary individual. The heart transplanted individual has a denervated heart. Removal of the parasympathetic "brake" results in a higher heart rate.

5. The heart rate of the sedentary individual increases to a greater maximum heart rate during exercise than does the heart rate of the transplanted individual. In the sedentary individual, heart rate increases to about 100 beats/min as parasympathetic stimulation decreases. Above 100 beats/min increased sympathetic stimulation increases his heart rate. The heart transplanted individual does not have sympathetic innervation of his heart and, therefore, has a lower maximum heart rate.

6. The heart transplanted individual still has a functional adrenal medullary mechanism. Release of epinephrine and norepinephrine from the adrenal medulla causes an increase in heart rate.

Increased venous return can cause stretch of the SA node, resulting in a 10% to 30% increase in heart rate. Venous return increases during exercise.

7. The heart transplanted individual's curve is approximately a straight line, showing a gradual increase in heart rate. The heart transplanted individual has a denervated heart and his increase in heart rate results from increased adrenal medullary secretions and increased venous return stretching the SA node.

The quadriplegic's curve is steep at first then becomes more level, indicating a rapid increase in heart rate followed by a more gradual increase in heart rate. The quadriplegic still has functional parasympathetic innervation of the heart. As exercise begins, decreased parasympathetic stimulation of the heart results in a rapid increase in heart rate. Increased adrenal medullary secretions and increased venous return also cause an increase in heart rate. Compared to the heart transplanted individual, however, the quadriplegic has a reduced adrenal medullary response and a smaller venous return (see stroke volume response below).

8. The athlete has a lower resting heart rate than the sedentary individual. Training has resulted in hypertrophy of the athlete's heart, which has a greater stroke volume. Remember that cardiac output is equal to heart rate times stroke volume. Because the athlete has a larger stroke volume, his heart rate can be lower and he can still maintain a similar cardiac output at rest as the sedentary individual.

It would be reasonable to predict that the lower heart rate results from increased parasympathetic stimulation of the athlete's heart.

9. HRmax of sedentary individual = 220 - 30 = 190

HRmax of athletic individual = 220 - 30 = 190

10. The heart transplanted and quadriplegic individuals have a lower maximum heart rate than the sedentary and athletic individuals because they do not have sympathetic stimulation of the heart.

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Teacher's Note: The heart transplanted individual has a slightly higher maximum heart rate than the quadriplegic. The quadriplegic has decreased skeletal motor function and a decreased skeletal muscle venous pump. He also has a decreased blood volume and lacks normal sympathetic regulation of blood vessels. As a result, the quadriplegic has decreased venous return compared to the heart transplanted individual. The greater venous return in the heart transplanted individual stretches the SA node, resulting in a higher maximum heart rate. These explanations will become clear to students when they consider stroke volume.

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TABLE 4 FACTORS EFFECTING STROKE VOLUME

Stroke Volume Reference pagenumber in text

Changes in parasympathetic stimulation of the heart

Little effect 693


Increases 693


Increases 694

Increased venous return to the heart (Starling's Law)

Increases 692

Increased end-diastolic volume

Increases 691

Decreased end-systolic volume Increases 691

Increased blood volume Increases 691, 753

TABLE 5 COMPARISON OF RESTING HEART RATES, STROKE VOLUMES, AND CARDIAC OUTPUTS

Resting HeartRate (beats/min)

Resting Stroke Volume (ml/min)

Resting Cardiac Output (L/min)


70 70 4 .9


45 110 4 .95


85 45 3 .825


100 50 5 .0

11. The sedentary, athletic, and heart transplanted individuals all have similar cardiac outputs at rest. One would expect their cardiac outputs to be similar because they are of the same weight and sex. They have similar cardiac outputs even though they have different resting heart rates (see previous analysis of heart rates). This occurs because heart rate and stroke volume together determine cardiac output. Compared to the sedentary individual, the athlete has a slower heart rate, but a larger stroke volume. Compared to the sedentary individual the heart transplanted individual has a faster heart rate, but a smaller stroke volume.

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12. The quadriplegic individual has decreased blood volume as a result of deconditioning. The decreased blood volume results in a decreased venous return. According to Starling's law, as venous return decreases, stroke volume decreases.

In the quadriplegic, because of lack of tonic stimulation of skeletal muscles by the brain, skeletal muscle atrophy occurs. Muscles cells decrease in size, die, and are replaced by connective tissue and fat. Consequently, there is a loss of tissue mass, and a partial replacement of muscle tissue with less metabolically active tissues. Therefore, a smaller cardiac output can support these tissues.

13. The sedentary and heart transplanted individuals both have complete motor function, a normal skeletal muscle venous pump, normal regulation of blood vessels, and normal blood volumes. Therefore, they have similar venous returns. According to Starling's law, they should have similar stroke volumes because they have similar end-diastolic volumes. But, the sedentary individual also has sympathetic innervation of the heart and greater left ventricular muscle mass . Consequently, the sedentary individual has stronger contractions, decreased end-systolic volume, and increased stroke volume compared to the heart transplanted individual.

14. Compared to the sedentary individual, the quadriplegic has a greatly reduced functional skeletal muscle mass. Although local control mechanisms regulating blood flow and the skeletal muscle venous pump operate normally in his functional muscles, because the quadriplegic has a smaller functional skeletal muscle mass, his overall venous return is reduced. In other words, fewer precapillary sphincters relax, resulting in less blood flow into muscle tissue and less blood to be pumped away from muscle tissue. In addition, the quadriplegic has a reduced blood volume, which reduces venous return. According to Starling's law, the smaller venous return results in a smaller stroke volume.

The quadriplegic also does not have normal sympathetic stimulation of the heart, has reduced left ventricular muscle mass and volume, and has a reduced adrenal medullary response. Therefore his heart contracts less forcefully and stroke volume is reduced.

15. The athletic individual has a hypertrophied heart as a result of his exercise regime. He has an increased left ventricular volume and muscle mass. His larger ventricular volume results in greater filling of the ventricles during diastole and a greater end-diastolic volume. The increased left ventricular muscle mass can contract more forcefully, resulting in a decreased end-systolic volume. Therefore he has a larger maximum stroke volume than the sedentary individual. Note that the athletic individual has a greater stroke volume at all the work loads measured.

The athlete also has a larger blood volume than the sedentary individual, which results in a greater venous return and stroke volume (Starling's Law).

16. The sedentary, athletic, and heart transplanted individuals' curves are steep at first then becomes more level, indicating a rapid increase in stroke volume followed by a more gradual increase in stroke volume. All of these individuals have about the same functional skeletal muscle mass, and as they begin exercise, venous return rapidly increases, resulting in the rapid increase in stroke volume. Stroke volume increases and levels off near its maximum value at about 40% to 60% of the oxygen uptake at maximum exercise for each individual.

The quadriplegic's curve is approximately a straight line, showing a gradual, small increase in stroke volume. Because the quadriplegic does not have sympathetic innervation of his heart, venous return and adrenal medullary secretions are

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responsible for increasing his stroke volume. Compared to the other individuals, the quadriplegic has a greatly reduced functional skeletal muscle mass and a reduced blood volume, both of which result in a greatly reduced venous return and stroke volume. The quadriplegic illustrates the importance of the relationship between Starling's law and stroke volume. Note that the heart transplanted individual, who also lacks sympathetic innervation of the heart, has an increase in stroke volume comparable to the sedentary individual because they both have similar venous returns.

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17. The greater the amount of work performed (oxygen consumed), the greater the cardiac output.

Teacher's Note: It is tempting to suggest that maximum work performed is a function of cardiac output. That is, once cardiac output is maximal, no further increase in work can occur. However, it is possible that some other system might limit work performance before cardiac output actually reaches its potential maximal value. For example, the respiratory system might not be able to take up adequate oxygen from the air. A few studies suggest this may be the case in highly trained athletes.

In the case of the quadriplegic, reduced ability to ventilate the lungs decreases his oxygen consumption. The intercostal nerves supply the intercostal muscles, which move the ribs. The intercostal nerves originate from T1-T12. They are not functional, because they are inferior to the lesion at C8, and he is not able to expand and compress the thoracic in a normal fashion. Consequently, his ability to move air into and out of the lungs is impaired, and he has reduced oxygen consumption.

The phrenic nerves supply the diaphragm, which is the major muscle changing thoracic volume during respiration. The phrenic nerves arise from levels C3 - C5 of the spinal cord. They are functional in this quadriplegic because his spinal cord lesion is at C7 - C8. Therefore he has normal control of the diaphragm.

TABLE 6 RANK VALUES FOR MAXIMUM HEART RATE, STROKE VOLUME, AND CARDIAC OUTPUT

Maximum Stroke Volume Rank Value

Maximum Heart Rate Rank Value

Maximum Cardiac Output Rank Value

Ben Armstrong (Quadriplegic) 1 1 1

Moore Heart (Transplant) 2 2 2

Norman Normal (Sedentary) 3 3.5 3

Jock Player (Athlete) 4 3.5 4

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18. The quadriplegic has the lowest stroke volume because his reduced functional skeletal muscle mass results in a greatly reduced venous return, and because he has a reduced left ventricular muscle mass and volume. He also has a reduced blood volume, which results in a decreased venous return. He has the lowest heart rate because his spinal cord injury prevents sympathetic stimulation of the heart.

19. The heart transplanted individual has a greater functional skeletal muscle mass than the quadriplegic and a larger blood volume, both of which result in increased venous return and increased stroke volume. Most of the increase in cardiac output observed in the heart transplanted individual, compared to the quadriplegic, results from the increased stroke volume.

The heart transplanted individual and the quadriplegic have similar maximum heart rates because they both lack sympathetic stimulation of the heart. However, the greater venous return in the heart transplanted individual results in stretch of the SA node and a slightly larger maximum heart rate.

20. The sedentary individual has sympathetic stimulation of the heart, which increases both heart rate and stroke volume.

21. The difference in cardiac output cannot be attributed to maximum heart rate, because they both have the same maximum heart rate. The difference must result from differences in stroke volume. The athlete has a hypertrophied heart, which allows greater filling during diastole. The larger end-diastolic volume results in a greater stroke volume and cardiac output. The athlete also has a greater left ventricular muscle mass. Increased force of contraction causes a decreased end-systolic volume, resulting in a greater stroke volume and cardiac output. The athlete also has a larger blood volume, which increases venous return and stroke volume.

TABLE 7 MAXIMUM CARDIAC OUTPUTS AND SYSTOLIC PRESSURES

Maximum CardiacOutput (L/min)

Maximum SystolicPressure (mm Hg)


6 120


Norman Normal (Sedentary) 22 200


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FIGURE 5 SYSTOLIC PRESSURE Effect of increasing cardiac output on systolic pressure.

22. From this graph one can conclude that as cardiac output increases, systolic pressure increases. Cardiac output is equal to stroke volume times heart rate. The greater the amount of blood pumped out of the heart per minute, the greater the systolic pressure.

23. Diastolic pressure does not change very much during exercise despite an increase in cardiac output and systolic pressure. This can only happen if peripheral resistance decreases during exercise. Therefore, as fast as blood is pumped into the aorta, it runs off into the vessels. Decreasing peripheral resistance makes it easier for blood to flow through exercising muscles.

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TABLE 8 MAXIMUM MEAN ARTERIAL PRESSURE

DiastolicPressure

SystolicPressure

PulsePressure

Mean Arterial Pressure


70 120 50 87

Moore Heart (Transplant) 100 170 70 123


63 200 137 109

Jock Player (Athlete) 39 250 211 109

24. The quadriplegic has greatly reduced cardiac output and mean arterial pressure. Consequently, he is unable to maintain the same mean arterial (perfusion) pressure, and his muscles are not able to perform the same level of work.

25. Compared to the sedentary individual, the heart transplanted individual has a lower cardiac output. Based just on cardiac output, one would predict that the heart transplanted individual would have a lower mean arterial pressure. However, he has a greatly increased mean arterial pressure because of greatly increased peripheral resistance caused by the cyclosporine. His heart has to generate a high mean arterial pressure to overcome this peripheral resistance. Thus, despite the increased blood pressure, his muscle tissue receives less blood than does the sedentary individual's muscle tissue.

26. The athlete has a larger cardiac output than the sedentary individual, but has the same maximum mean arterial pressure. One can conclude that the athlete has a lower peripheral resistance. This allows greater flow of blood through his muscle tissues.

Mean arterial pressure is a function of systolic and diastolic pressures. The athlete has increased systolic pressure because of his greater cardiac output. He also has decreased diastolic pressure because of his decreased peripheral resistance. The mean value of his higher systolic and lower diastolic pressures are the same as for the sedentary individual. The advantage to the athlete is that he maintains the same perfusion pressure against a decreased after load. Therefore he can have an increased cardiac output and delivery of blood to his muscles, and he can perform more work.

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cardiovascular response to exercise

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