Question 11

College Answer

This is an applied physiology question. Better answers categorised the changes in some manner and included a measure of the degree of change as applicable (e.g., what increases, what decreases and what may stay the same). The question was to describe the changes so that the detail behind the mechanisms enabling these changes to occur was expected (e.g., neurohumoral, local factors). Marks were also awarded for any regional variation that occurs.

Discussion

• The response to exercise consists of:
  • Regional muscle vasodilation
  • Increase in cardiac output
  • Central coordination of these response
• Regional mucle vasodilation
  • Increased muscle activity results in increased oxygen demand
  • This increased demand is met by increasing blood flow to exercising muscle
  • The increase in blood flow is mediated mainly by a regional decrease in vascular resistance
  • The mechanisms for this vasodilation are:
    • Vasoactive substrates and products of muscle metabolism, eg. CO₂, lactate, hydrogen peroxide and potassium ions
    • Vasoactive mediators released by the endothelium, eg. nitric oxide (NO), ATP, adenosine, prostaglandins
    • β-2 adrenoceptor activation
  • There is also corresponding vasoconstriction of other vascular beds, redirecting blood flow away from viscera and skin
• Increase in cardiac output
  • Cardiac aoutput can increase massively during exercise, up to 30L/min
  • The increase in cardiac output is due to both an increase in heart rate and in stroke volume
  • With increasing workload, the heart rate continues to increase but stroke volume decreases because of diminishing diastolic time
  • As the result the cardiac output plateaus at near-maximal workload
  • Central venous pressure and PCWP also increase
• Changes in haemodynamic parameters
  • Systolic blood pressure and MAP increase with increasing workload
  • Diastolic blood pressure decreases 
  • The pulse pressure widens
• Central coordination of cardiovascular responses to exercise
  • Afferents:
    • Increased muscle activity is sensed by stretch receptors and chemoreceptors in the muscle tissue
    • Decreased peripheral vascular resistance, translating into decreased blood pressure, is sensed by the baroreflex
    • The central nervous system itself can generate the afferent signals for exercise-related cardiovascular responses in anticipation of effort
  • Central processing:
    • The motor cortex can unilaterally activate the autonomic response to exercise in the absence of afferent stimuli, and in anticipation of exercise
  • Efferents:
    • ​​​​​​​Vagus nerve - which increases the heart rate
    • Sympathic nervous system which:
      • Increases the cardiac contractility
      • Releases catecholamines from the adrenal gland
      • Adjusts organ perfusion to redistribute more blood flow to the skeletal muscle

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Munch, G. D. W., et al. "Maximal heart rate does not limit cardiovascular capacity in healthy humans: insight from right atrial pacing during maximal exercise." The Journal of physiology 592.2 (2014): 377-390.

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