chapter 8

Cardiorespiratory Responses to Acute

Exercise

Cardiovascular ResponsesCardiovascular Responsesto Acute Exerciseto Acute Exercise

• Increases blood flow to working muscle

• Involves altered heart function, peripheral circulatory adaptations– Heart rate– Stroke volume– Cardiac output– Blood pressure– Blood flow– Blood

Cardiovascular Responses:Cardiovascular Responses:Resting Heart Rate (RHR)Resting Heart Rate (RHR)

• Normal ranges– Untrained RHR: 60 to 80 beats/min– Trained RHR: as low as 30 to 40 beats/min– Affected by neural tone, temperature, altitude

• Anticipatory response: HR above RHR just before start of exercise– Vagal tone – Norepinephrine, epinephrine

Cardiovascular Responses:Cardiovascular Responses:Heart Rate During ExerciseHeart Rate During Exercise

• Directly proportional to exercise intensity

• Maximum HR (HRmax): highest HR achieved in all-out effort to volitional fatigue– Highly reproducible– Declines slightly with age– Estimated HRmax = 220 – age in years

– Better estimated HRmax = 208 – (0.7 x age in years)

Cardiovascular Responses:Cardiovascular Responses:Heart Rate During ExerciseHeart Rate During Exercise

• Steady-state HR: point of plateau, optimal HR for meeting circulatory demands at a given submaximal intensity– If intensity , so does steady-state HR– Adjustment to new intensity takes 2 to 3 min

• Steady-state HR basis for simple exercise tests that estimate aerobic fitness and HRmax

Figure 8.1Figure 8.1

Figure 8.2Figure 8.2

Cardiovascular Responses:Cardiovascular Responses:Stroke Volume (SV)Stroke Volume (SV)

• With intensity up to 40 to 60% VO2max – Beyond this, SV plateaus to exhaustion– Possible exception: elite endurance athletes

• SV during maximal exercise ≈ double standing SV

• But, SV during maximal exercise only slightly higher than supine SV– Supine SV much higher versus standing– Supine EDV > standing EDV

Figure 8.3Figure 8.3

Cardiovascular Responses:Cardiovascular Responses:Factors That Increase Stroke VolumeFactors That Increase Stroke Volume

• Preload: end-diastolic ventricular stretch– Stretch (i.e., EDV) contraction strength– Frank-Starling mechanism

• Contractility: inherent ventricle property– Norepinephrine or epinephrine contractility– Independent of EDV ( ejection fraction instead)

• Afterload: aortic resistance (R)

Cardiovascular Responses: Stroke Cardiovascular Responses: Stroke Volume Changes During ExerciseVolume Changes During Exercise

• Preload at lower intensities SV– Venous return EDV preload– Muscle and respiratory pumps, venous reserves

• Increase in HR filling time slight in EDV SV

• Contractility at higher intensities SV

• Afterload via vasodilation SV

Cardiac Output and Stroke Volume:Cardiac Output and Stroke Volume:Untrained Versus Trained Versus EliteUntrained Versus Trained Versus Elite

Cardiovascular Responses:Cardiovascular Responses:Cardiac Output (Q)Cardiac Output (Q)

• Q = HR x SV

• With intensity, plateaus near VO2max

• Normal values– Resting Q ~5 L/min– Untrained Qmax ~20 L/min

– Trained Qmax 40 L/min

• Qmax a function of body size and aerobic fitness

Figure 8.5Figure 8.5

Cardiovascular Responses:Cardiovascular Responses:Fick PrincipleFick Principle

• Calculation of tissue O2 consumption depends on blood flow, O2 extraction

• VO2 = Q x (a-v)O2 difference

• VO2 = HR x SV x (a-v)O2 difference

Cardiovascular Responses:Cardiovascular Responses:Blood PressureBlood Pressure

• During endurance exercise, mean arterial pressure (MAP) increases– Systolic BP proportional to exercise intensity– Diastolic BP slight or slight (at max exercise)

• MAP = Q x total peripheral resistance (TPR)– Q , TPR slightly– Muscle vasodilation versus sympatholysis

Figure 8.7Figure 8.7

Cardiovascular Responses:Cardiovascular Responses:Blood Flow RedistributionBlood Flow Redistribution

• Cardiac output available blood flow

• Must redirect blood flow to areas with greatest metabolic need (exercising muscle)

• Sympathetic vasoconstriction shunts blood away from less-active regions– Splanchnic circulation (liver, pancreas, GI)– Kidneys

Cardiovascular Responses:Cardiovascular Responses:Blood Flow RedistributionBlood Flow Redistribution

• Local vasodilation permits additional blood flow in exercising muscle– Local VD triggered by metabolic, endothelial products– Sympathetic vasoconstriction in muscle offset by

sympatholysis– Local VD > neural VC

• As temperature rises, skin VD also occurs– Sympathetic VC, sympathetic VD– Permits heat loss through skin

Figure 8.8Figure 8.8

Cardiovascular Responses:Cardiovascular Responses:Cardiovascular DriftCardiovascular Drift

• Associated with core temperature and dehydration

• SV drifts – Skin blood flow – Plasma volume (sweating)– Venous return/preload

• HR drifts to compensate (Q maintained)

Figure 8.9Figure 8.9

Cardiovascular Responses:Cardiovascular Responses:Competition for Blood SupplyCompetition for Blood Supply

• Exercise + other demands for blood flow = competition for limited Q. Examples: – Exercise (muscles) + eating (splanchnic blood flow)– Exercise (muscles) + heat (skin)

• Multiple demands may muscle blood flow

Cardiovascular Responses:Cardiovascular Responses:Blood Oxygen ContentBlood Oxygen Content

• (a-v)O2 difference (mL O2/100 mL blood)– Arterial O2 content – mixed venous O2 content

– Resting: ~6 mL O2/100 mL blood

– Max exercise: ~16 to 17 mL O2/100 mL blood

• Mixed venous O2 ≥4 mL O2/100 mL blood– Venous O2 from active muscle ~0 mL

– Venous O2 from inactive tissue > active muscle

– Increases mixed venous O2 content

Figure 8.10Figure 8.10

Central Regulation of Central Regulation of Cardiovascular ResponsesCardiovascular Responses

• What stimulates rapid changes in HR, Q, and blood pressure during exercise?– Precede metabolite buildup in muscle– HR increases within 1 s of onset of exercise

• Central command– Higher brain centers– Coactivates motor and cardiovascular centers

Central Cardiovascular Control Central Cardiovascular Control During ExerciseDuring Exercise

Cardiovascular Responses:Cardiovascular Responses:Integration of Exercise ResponseIntegration of Exercise Response

• Cardiovascular responses to exercise complex, fast, and finely tuned

• First priority: maintenance of blood pressure– Blood flow can be maintained only as long as BP

remains stable– Prioritized before other needs (exercise,

thermoregulatory, etc.)

Figure 8.12Figure 8.12

Respiratory Responses:Respiratory Responses:Ventilation During ExerciseVentilation During Exercise

• Immediate in ventilation– Begins before muscle contractions– Anticipatory response from central command

• Gradual second phase of in ventilation– Driven by chemical changes in arterial blood– CO2, H+ sensed by chemoreceptors– Right atrial stretch receptors

Respiratory Responses:Respiratory Responses:Ventilation During ExerciseVentilation During Exercise

• Ventilation increase proportional to metabolic needs of muscle– At low-exercise intensity, only tidal volume – At high-exercise intensity, rate also

• Ventilation recovery after exercise delayed– Recovery takes several minutes– May be regulated by blood pH, PCO2, temperature

Figure 8.13Figure 8.13

Respiratory Responses:Respiratory Responses:Breathing IrregularitiesBreathing Irregularities

• Valsalva maneuver: potentially dangerous but accompanies certain types of exercise– Close glottis– Intra-abdominal P (bearing down)– Intrathoracic P (contracting breathing muscles)

• High pressures collapse great veins venous return Q arterial blood pressure

Respiratory Responses:Respiratory Responses:Ventilation and Energy MetabolismVentilation and Energy Metabolism

• Ventilation matches metabolic rate

• Ventilatory equivalent for O2

– VE/VO2 (L air breathed/L O2 consumed/min)– Index of how well control of breathing matched to

body’s demand for oxygen

• Ventilatory threshold– Point where L air breathed > L O2 consumed

– Associated with lactate threshold and PCO2

Figure 8.14Figure 8.14

Respiratory Responses:Respiratory Responses:Estimating Lactate ThresholdEstimating Lactate Threshold

• Ventilatory threshold as surrogate measure?– Excess lactic acid + sodium bicarbonate– Result: excess sodium lactate, H2O, CO2

– Lactic acid, CO2 accumulate simultaneously

• Refined to better estimate lactate threshold– Anaerobic threshold– Monitor both VE/VO2, VE/VCO2

Ventilatory Equivalents Ventilatory Equivalents During ExerciseDuring Exercise

Respiratory Responses:Respiratory Responses:Limitations to PerformanceLimitations to Performance

• Ventilation normally not limiting factor– Respiratory muscles account for 10% of VO2, 15%

of Q during heavy exercise– Respiratory muscles very fatigue resistant

• Airway resistance and gas diffusion normally not limiting factors at sea level

• Restrictive or obstructive respiratory disorders can be limiting

Respiratory Responses:Respiratory Responses:Limitations to PerformanceLimitations to Performance

• Exception: elite endurance-trained athletes exercising at high intensities– Ventilation may be limiting– Ventilation-perfusion mismatch– Exercise-induced arterial hypoxemia (EIAH)

Respiratory Responses:Respiratory Responses:Acid-Base BalanceAcid-Base Balance

• Metabolic processes produce H+ pH

• H+ + buffer H-buffer

• At rest, body slightly alkaline – 7.1 to 7.4– Higher pH = Alkalosis

• During exercise, body slightly acidic– 6.6 to 6.9– Lower pH = Acidosis

Figure 8.15Figure 8.15

Respiratory Responses:Respiratory Responses:Acid-Base BalanceAcid-Base Balance

• Physiological mechanisms to control pH– Chemical buffers: bicarbonate, phosphates,

proteins, hemoglobin– Ventilation helps H+ bind to bicarbonate– Kidneys remove H+ from buffers, excrete H+

• Active recovery facilitates pH recovery– Passive recovery: 60 to 120 min– Active recovery: 30 to 60 min

Table 8.2Table 8.2

Figure 8.16Figure 8.16