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Chapter 13

Cardiovascular

Responses to

Exercise

After studying the chapter, you should be able to

• Graph and explain the pattern of response for the major cardiovascular variables during short-term, light to moderate submaximal aerobic exercise. • Graph and explain the pattern of response for the major cardiovascular variables during long-term, moderate to heavy submaximal aerobic exercise. • Graph and explain the pattern of response for the major cardiovascular variables during incremental aerobic exercise to maximum. • Graph and explain the pattern of response for the major cardiovascular variables during dynamic resistance exercise. • Graph and explain the pattern of response for the major cardiovascular variables during static exercise. • Compare and contrast the response of the major cardiovascular variables to short-term, light to moderate submaximal aerobic exercise; incremental aerobic exercise to maximum; dynamic resistance exercise; and static exercise. • Discuss the similarities and differences between the sexes in the cardiovascular response to the various classifications of exercise. • Discuss the similarities and differences between young and middle-aged adults in the cardiovascular response to the various classifications of exercise. 351

Cardiovascular-Respiratory System Unit

Introduction

All types of human movement, no matter what the

mode, duration, intensity, or pattern, require an ex- penditure of energy above resting values. Much of this energy will be provided through the use of oxygen. In order to supply the working muscles with the needed oxygen, the cardiovascular and respiratory systems must work together. The response of the respiratory system during exercise was detailed in Chapter 11. This chapter describes the parallel cardiovascular re- sponses to dynamic aerobic activity, static exercise, and dynamic resistance exercise.

Cardiovascular Responses to Aerobic Exercise

Aerobic exercise requires more energy - and, hence, more oxygen (and thus the use of the term aerobic, with oxygen) - than either static or dynamic resistance exercise. How much oxygen is needed depends prima- rily on the intensity at which the activity is performed and secondarily on the duration of the activity. Like the discussion on respiration, this discussion will catego- rize the exercises performed as being short-term (5-10 min), light (30-49% of maximal oxygen con- sumption, VO 2 max) to moderate (50-74% of VO 2 max) submaximal exercise; long-term (greater than 30 min), moderate to heavy submaximal (60- 85% of VO 2 max) exercise; or incremental exercise to maxi- mum, increasing from ?30% to 100% of V?O 2 max. Short-Term, Light to Moderate SubmaximalAerobic Exercise At the onset of short-term, light- to moderate-intensity exercise, there is an initial increase in cardiac output (Q) to a plateau at steady state (see Figure 13.1a). Car- diac output plateaus within the first 2 min of exercise, reflecting the fact that cardiac output is sufficient to transport the oxygen needed to support the metabolic demands (ATP production) of the activity. Cardiac out- put increases owing to an initial increase in both stroke volume (SV) (Figure 13.1b) and heart rate (HR) (Figure 13.1c). Both variables level off within 2 min. During exercise of this intensity the cardiorespi- ratory system is able to meet the metabolic demands

of the body; thus, the term steady stateor steady rateis often used to describe this type of exercise. During

steady state exercise, the exercise is performed at an intensity such that energy expenditure is balanced with the energy required to perform the exercise. The plateau evidenced by the cardiovascular variables (in Figure 13.1) indicates that a steady state has been achieved.

The increase in stroke volume results from an

increase in venous return, which, in turn, increases the left ventricular end-diastolic volume (LVEDV) (preload). The increased preload stretches the myo- cardium and causes it to contract more forcibly in accordance with the Frank-Starling law of the heart described in Chapter 12. Contractility of the myo- cardium is also enhanced by the sympathetic ner- vous system, which is activated during physical ac- tivity. Thus, an increase in the left ventricular end-diastolic volume and a decrease in the left ventricular end-systolic volume (LVESV) account for the increase in stroke volume during light to moder- ate dynamic exercise (Poliner, et al., 1980). Heart rate increases immediately at the onset of activity as a result of parasympathetic withdrawal. As exer- cise continues, further increases in heart rate are due to the action of the sympathetic nervous system (Rowell, 1986).

Systolic blood pressure (SBP) will rise in a pat-

tern very similar to that of cardiac output: There is an initial increase and a plateau once steady state is achieved (Figure 13.1d). The increase in systolic blood pressure is brought about by the increase in cardiac output. Systolic blood pressure would be even higher if not for the fact that resistance decreases, thereby partially offsetting the increase in cardiac output. When blood pressure (BP) is measured intra-arterially, diastolic blood pressure (DBP) does not change. When it is measured by auscultation it either does not change or may go down slightly. Diastolic blood pressure remains rel- atively constant because of peripheral vasodilation, which facilitates blood flow to the working muscles.

The small rise in systolic blood pressure and the

lack of a significant change in diastolic blood pres- sure cause the mean arterial pressure (MAP) to rise only slightly, following the pattern of systolic blood pressure.

Total peripheral resistance (TPR) decreases

owing to vasodilation in the active muscles (Figure

13.1e). The vasodilation of vessels in the active mus-

cles is brought about primarily by the influence of local chemical factors (lactate, K , and so on), which reflect increased metabolism. The decrease in TPR can be calculated using Equation 12.8: TPR ? MAP Q

352Cardiovascular-Respiratory System Unit

Steady StateA condition in which the energy

expenditure provided during exercise is bal- anced with the energy required to perform that exercise and factors responsible for the provision of this energy reach elevated levels of equilibrium.

Example

Calculate TPR by using the following information from

Figures 13.1a and 13.1d:

MAP ?110 mmHgQ?15 L·min

?1

The computation is

TPR ??7.33 (TPR units)

Thus, TPR is 7.33 for light dynamic exercise.

The decrease in total peripheral resistance has

two important implications. First, the vasodilation in the active muscle that causes the decrease in resis- tance has the effect of increasing blood flow to the ac- tive muscle, thereby increasing the availability of oxy- gen and nutrients. Second, the decrease in resistance keeps mean arterial pressure from increasing dra- matically. The increase in mean arterial pressure is

110mmHg

15 Lámin

?1 determined by the relative changes in cardiac output and total peripheral resistance. Since cardiac output increases more than resistance decreases, mean arte- rial pressure increases slightly during dynamic exer- cise. However, the increase in mean arterial pressure would be much greater if resistance did not decrease.

Myocardial oxygen consumption increases during

dynamic aerobic exercise because the heart must do more work to pump an increased cardiac output to the working muscles. The rate-pressure product will increase in relation to increases in heart rate and systolic blood pressure, reflecting the greater myocar- dial oxygen demand of the heart during exercise (Figure 13.1f). The Question of Understanding box on page 354 provides an example of normal responses to exercise. Refer back to it as each category of exercise is discussed and check your answers in Appendix D.

The actual magnitude of the change for each of

the variables shown in Figure 13.1 depends on the

353Chapter 13 Cardiovascular Responses to Exercise

Time (min)

00105
Q (L min ?1 (a) 5

10152025

60
0

Time (min)10

05 HR (b min ?1 (c) 100

140180

220

RPP (units)

Time (min)010

5(f) 0 100
200

300400

Time (min)

20 10 15 5 0 25
100
5

TPR (units)

(e) 0

BP (mmHg)

Time (min)10

05(d) 60

100140180

220

DBPMAPSBP

0(b) 0

Time (min)105

SV (mL)

60100140180

Figure 13.1

Cardiovascular Responses to

Short-Term, Light to Moderate

Aerobic Exercise

workload, environmental conditions, and the genetic makeup and fitness level of the individual.

Blood volume decreases during dynamic aerobic

exercise. Figure 13.2 shows the percent reduction of plasma volume during 30 min of moderate bicycle ex- ercise (60-70% VO 2 max) in a warm environment (Fortney, et al., 1981). The largest changes occur dur- ing the first 5 min of exercise, which is consistent with short-term exercise. Following the initial rapid de- crease, plasma volume stabilizes. This rapid decrease in plasma volume suggests that it is fluid shifts, rather than fluid loss, that accounts for the initial decrease in plasma volume (Wade and Freund, 1990). The magni- tude of the decrease in plasma volume is dependent upon the intensity of exercise, environmental factors, and the hydration status of the individual. Figure 13.3 illustrates the distribution of cardiac output at rest and during light exercise. Notice that cardiac output increases from 5.8 L·min ?1 to

9.4 L·min

?1 in this example (the increase in Qis illus- trated by the increased size of the pie chart). The most dramatic change in cardiac output distribution with light exercise is the increased percentage (47%) and the actual amount of blood flow (4500 mL) that is di- rected to the working muscles. Skin blood flow also increases to meet the thermoregulatory demands of exercise. The absolute amount of blood flow to the coronary muscle also increases although the percent- age of cardiac output remains relatively constant. The absolute amount of cerebral blood flow remains con- stant, which means that the percentage of cardiac

output distributed to the brain decreases. Both renaland splanchnic blood flow are modestly decreased

during light exercise. Long-Term, Moderate to Heavy Submaximal Aerobic Exercise The cardiovascular responses to long-term, moderate to heavy exercise (60-85% of VO 2 max) are shown in Figure 13.4. As for light to moderate workloads, car- diac output increases rapidly during the first minutes of exercise and then plateaus and is maintained at a relatively constant level throughout exercise (Figure

13.4a). Notice, however, that the absolute cardiac out-

put attained is higher during heavy exercise than it was during light to moderate exercise. The initial in- crease in cardiac output is brought about by an in- crease in both stroke volume and heart rate.

Stroke volume exhibits a pattern of initial in-

crease, plateaus, and then displays a negative (down- ward) drift. Stroke volume increases rapidly duringquotesdbs_dbs19.pdfusesText_25