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Slide 1-

Cardiorespiratory Responses to Acute Exercise Chapter 8
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Cardiovascular Responses to 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
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Cardiovascular Responses: 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 
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Cardiovascular Responses: Heart 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)
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Cardiovascular Responses: Heart 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
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Figure 8.1
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Figure 8.2
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Cardiovascular Responses: 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
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Figure 8.3
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Cardiovascular Responses: Factors 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)
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Cardiovascular Responses: Stroke Volume 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
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Cardiac Output and Stroke Volume: Untrained Versus Trained Versus Elite
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Cardiovascular Responses: 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
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Figure 8.5
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Cardiovascular Responses: Fick 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
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Cardiovascular Responses: Blood 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
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Figure 8.7
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Cardiovascular Responses: Blood 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
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Cardiovascular Responses: Blood 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
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Figure 8.8
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Cardiovascular Responses: Cardiovascular Drift Associated with  core temperature and dehydration SV drifts  Skin blood flow  Plasma volume  (sweating) Venous return/preload  HR drifts  to compensate (Q maintained)
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Figure 8.9
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Cardiovascular Responses: Competition 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
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Cardiovascular Responses: Blood 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
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Figure 8.10
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Central Regulation of Cardiovascular 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
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Central Cardiovascular Control During Exercise
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Cardiovascular Responses: Integration 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.)
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Figure 8.12
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Respiratory Responses: Ventilation 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
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Respiratory Responses: Ventilation 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
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Figure 8.13
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Respiratory Responses: Breathing 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
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Respiratory Responses: Ventilation 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
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Figure 8.14
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Respiratory Responses: Estimating 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
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Ventilatory Equivalents During Exercise
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Respiratory Responses: Limitations 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
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Respiratory Responses: Limitations to Performance Exception: elite endurance-trained athletes exercising at high intensities Ventilation may be limiting Ventilation-perfusion mismatch Exercise-induced arterial hypoxemia (EIAH)
Slide 40-

Respiratory Responses: Acid-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
Slide 41-

Figure 8.15
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Respiratory Responses: Acid-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
Slide 43-

Table 8.2
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Figure 8.16