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Chapter 8

Chapter 8. Cardiorespiratory Responses to Acute Exercise. 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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Chapter 8

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  1. Chapter 8 • Cardiorespiratory Responses to Acute Exercise

  2. Cardiovascular Responsesto 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

  3. 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 

  4. 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)

  5. 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

  6. Figure 8.1

  7. Figure 8.2

  8. 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

  9. Figure 8.3

  10. 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)

  11. 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

  12. Cardiac Output and Stroke Volume:Untrained Versus Trained Versus Elite

  13. 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

  14. Figure 8.5

  15. 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

  16. 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

  17. Figure 8.7

  18. 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

  19. 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

  20. Figure 8.8

  21. 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)

  22. Figure 8.9

  23. 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

  24. 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

  25. Figure 8.10

  26. 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

  27. Central Cardiovascular Control During Exercise

  28. 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.)

  29. Figure 8.12

  30. 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

  31. 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

  32. Figure 8.13

  33. 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

  34. 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

  35. Figure 8.14

  36. 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

  37. Ventilatory Equivalents During Exercise

  38. 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

  39. 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)

  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

  41. Figure 8.15

  42. 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

  43. Table 8.2

  44. Figure 8.16

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