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Chapter 8

Chapter 8. Cardiorespiratory Responses to Acute Exercise. Chapter 8 Overview. Cardiovascular responses to acute exercise Cardiac responses Vascular responses Integration of exercise responses Respiratory responses to acute exercise Ventilation (normal exercise, irregularities)

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Chapter 8

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  1. Chapter 8 • Cardiorespiratory Responses to Acute Exercise

  2. Chapter 8 Overview • Cardiovascular responses to acute exercise • Cardiac responses • Vascular responses • Integration of exercise responses • Respiratory responses to acute exercise • Ventilation (normal exercise, irregularities) • Ventilation and energy metabolism • Respiratory limitations • Respiratory regulation of acid-base balance

  3. 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

  4. 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 

  5. 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)

  6. 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

  7. Figure 8.1

  8. Figure 8.2

  9. 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

  10. Figure 8.3

  11. Figure 8.4

  12. 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)

  13. 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

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

  15. 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

  16. Figure 8.5

  17. Figure 8.6a

  18. Figure 8.6b

  19. Figure 8.6c

  20. 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

  21. 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

  22. Cardiovascular Responses:Blood Pressure • Rate-pressure product = HR x SBP • Related to myocardial oxygen uptake and myocardial blood flow • Resistance exercise  periodic large increases in MAP • Up to 480/350 mmHg • More common when using Valsalva maneuver

  23. Figure 8.7

  24. 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

  25. 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

  26. Figure 8.8

  27. 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)

  28. Figure 8.9

  29. 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

  30. 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

  31. Figure 8.10

  32. Cardiovascular Responses:Plasma Volume • Capillary fluid movement into and out of tissue • Hydrostatic pressure • Oncotic, osmotic pressures • Upright exercise   plasma volume • Compromises exercise performance –  MAP   capillary hydrostatic pressure • Metabolite buildup   tissue osmotic pressure • Sweating further  plasma volume

  33. Figure 8.11

  34. Cardiovascular Responses:Hemoconcentration •  Plasma volume  hemoconcentration • Fluid percent of blood , cell percent of blood  • Hematocrit increases up to 50% or beyond • Net effects • Red blood cell concentration  • Hemoglobin concentration  • O2-carrying capacity 

  35. 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

  36. Central Cardiovascular Control During Exercise

  37. 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.)

  38. Figure 8.12

  39. 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

  40. 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

  41. Figure 8.13

  42. Respiratory Responses:Breathing Irregularities • Dyspnea (shortness of breath) • Common with poor aerobic fitness • Caused by inability to adjust to high blood PCO2, H+ • Also, fatigue in respiratory muscles despite drive to  ventilation • Hyperventilation (excessive ventilation) • Anticipation or anxiety about exercise –  PCO2 gradient between blood, alveoli –  Blood PCO2   blood pH  drive to breathe

  43. 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

  44. 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

  45. Figure 8.14

  46. 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

  47. Ventilatory Equivalents During Exercise

  48. 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

  49. 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)

  50. 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

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