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Understanding Cardiovascular Responses to Exercise

Learn about the intricate cardiorespiratory adaptations to acute exercise, including heart rate, stroke volume, blood flow, and vascular responses. Explore how the body adjusts during physical activity to meet increasing demands.

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Understanding Cardiovascular Responses to Exercise

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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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