1. Cardiorespiratory Responses to Acute Exercise
Chapter 8
Cardiorespiratory Responses to Acute Exercise

2. 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

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