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Systemic Responses to Exercise Notes

Systemic Responses to Exercise

  • Exercise requires collaboration among systems to maintain homeostasis.
  • Focus on metabolic, respiratory, and cardiovascular systems.
  • This lecture series serves as a review of energy, respiratory, and cardiovascular systems.

Short Term, Light to Moderate Aerobic Exercise

  • Variables: Lactate, oxygen consumption (VO_2), AVO2 difference, minute ventilation, cardiac output, and blood pressure.
  • Lactate: Subtle change in blood lactate due to the transition from rest to exercise.
  • Regardless of activity, lactate will be produced via both anaerobic & aerobic pathways.
  • Pyruvate breakdown stimulates lactate dehydrogenase (LDH) activity.
  • Oxygen Consumption: Oxygen deficit and Excess Post-exercise Oxygen Consumption (EPOC).
  • Small oxygen deficit and smaller EPOC in short-term, light exercise below maximal VO_2.
  • Metabolic inertia contributes to oxygen deficit; temperature contributes to EPOC.
  • Minute Ventilation: Short, sharp spike, small plateau, and a slight linear increase into plateau.
  • Transition from rest to exercise increases tidal volume and frequency of breathing.
  • AVO2 Difference: Slight increase, indicating greater oxygen utilization.
  • Steady state is achieved relatively quickly (around 3 minutes).
  • Cardiac Output: Steep initial increase due to increased heart rate and stroke volume, followed by a plateau.
  • Blood Pressure: Increase in systolic blood pressure mirroring cardiac output; no change in diastolic blood pressure; modest increase in mean arterial pressure.
  • Increased demand for ATP leads to increased oxygen consumption, driving up minute ventilation and cardiac output.
  • Increased sympathetic nervous system activity and withdrawal of parasympathetic nervous system.
  • Blood flow distribution shows skeletal muscle receiving about half of the blood.
  • Brain blood flow remains relatively constant; heart and skin receive slightly more.
  • Increased sympathetic activity and parasympathetic withdrawal facilitate maintenance of steady state.
  • Effects after exercise are small, with a minor EPOC.
  • Blood supply to working tissue increases over threefold (from 20% to 50%).
  • No substantial change in blood supply to the heart or brain.

Long Term Moderate to Heavy Aerobic Exercise

  • Lactate: Increase from rest, but lactate steady state is maintained, indicating efficient lactate clearance.
  • VO_2 Max: Increased oxygen consumption with a plateau, followed by a drift.
  • Drift is observed across energy and respiratory systems but not cardiac output.
  • Observed in oxygen consumption, minute ventilation, and AVO2 difference.
  • Drift occurs despite no change in workload due to competing interests, particularly thermoregulation.
  • Blood is shunted to the periphery for heat dissipation via subcutaneous vasculature.
  • Minute ventilation increases due to changes in tidal volume and respiration rate.
  • AVO2 difference shows a mismatch between oxygen delivery and perfusion, causing an uptick.
  • Cardiac Output: No drift observed because a decrease in stroke volume is offset by an increase in heart rate.
  • Stroke volume decreases due to blood shift away from working muscle to the periphery.
  • Slight negative drift in systolic blood pressure indicates increased vasodilation to the periphery and decreased total peripheral resistance.
  • No change in diastolic blood pressure.
  • Thermoregulatory response to increased oxygen consumption drives ventilation and cardiac output.
  • Lactate levels suggest aerobic state, with efficient clearance.
  • Oxygen Dissociation Curve: Shifts to the right, favoring the unloading of oxygen at the working tissue.
  • Skeletal muscle receives about 70% of blood flow.
  • Heart and brain blood flow remains relatively constant (around 750 ml).
  • Skin receives increased blood flow (close to 2 liters) for heat dissipation.
  • Physiological drift is observed across major systems.
  • Significant redistribution of blood to working tissue.
  • The brain continues to receive enough oxygen and glucose.
  • The shift in the oxygen dissociation curve favors oxygen unloading at the working tissue.
  • Lactate is able to maintain steady state, indicating dominant aerobic metabolic pathways.

Incremental to Maximum Aerobic Exercise

  • Lactate and ventilatory thresholds are achieved.
  • Cardiac output plateaus, AVO2 difference shows a plateau, and blood pressure increases linearly.
  • Linear increases are due to the incremental nature of the exercise.
  • Ventilatory threshold mirrors metabolic processes reflected by lactate threshold.
  • Minute ventilation indicates the ability to consume enough oxygen for aerobic metabolism.
  • If oxygen consumption requirements are not met, the body shifts to anaerobic metabolism.
  • AVO2 difference plateaus because oxygen delivery cannot keep up with requirements, but oxygen is still adequately utilized.
  • Cardiac output increase is initially due to increased stroke volume (up to 40-50% max work).
  • After 50% max, increased heart rate drives cardiac output.
  • Blood pressure increases linearly to maintain perfusion throughout systemic circulation.
  • Total peripheral resistance decreases due to vasodilatory response.
  • Diastolic blood pressure should not change noticeably.

Maximal Exercise Changes

  • Skeletal muscle receives close to 90% of blood flow.
  • Thermoregulatory capacity diminishes as blood is shunted to working muscle.
  • Cerebral blood flow increases slightly due to increased cognitive load.
  • There is fourfold increase in blood flow to coronary muscles.
  • Ventilatory and metabolic responses are strongly correlated, indicating a shift to anaerobic metabolism.
  • Oxygen delivery is met by oxygen extraction (AVO2 difference).
  • Increased systolic blood pressure maintains oxygen delivery.
  • A substantial shift in blood goes to working muscle (22 liters), facilitated by a large increase in cardiac output (25-26 liters per minute).
  • Maximal oxygen consumption is a multifactorial concept dependent on cardiac output and AVO2 difference.
  • AVO2 difference reaches its maximum around 50% maximal capacity.
  • Cardiac output is a product of stroke volume and heart rate.
  • Systemic contributions to VO2max are primarily limited by cardiac output, specifically the ability to deliver oxygen to working muscles, which is limited by the ability to fill the heat and pump blood out to muscles.
  • Early cardiac output is driven by stroke volume, while later increases are driven by heart rate.
  • Maximal capacity is limited by inadequate blood flow to working muscle.

Other Systemic Responses

  • Neural Changes: Elevated brain serotonin levels, increased brain-derived neurotrophic factor (BDNF) expression, enhanced sympathetic response.
  • Hormonal Responses: Increased plasma levels of cortisol, epinephrine, norepinephrine, and dopamine.
  • Cortisol mobilizes free fatty acids, maximizing aerobic metabolism and limiting glycogen depletion.
  • Epinephrine and norepinephrine have similar responses.
  • Dopamine provides a feel-good effect.
  • Growth hormone is released by the pituitary gland.
  • Insulin sensitivity is improved, potentially offsetting diabetes risk.
  • Increase in testosterone production.
  • Immune System: Transient increases in natural killer cells, neutrophils, leukocytes, and lymphocytes.
  • Intensity-dependent responses: There is a sweet spot, with intensities too low or too high not eliciting beneficial changes. The right amount of intensity gives acute and chronic adaptations, and high intensities lead to detrimental effect.

Physiological Redundancy and Systemic vs Remote Responses to Exercise

  • Exercise elicits systemic responses, and systems don't work in isolation to maintain homeostasis.
  • Physiological redundancy refers to multiple mechanisms responsible for generating physiological change or response.
  • Muscle energy turnover increases significantly during exercise.
  • Exercise induces systemic responses depending on the level (cellular vs. whole body) and the system examined.
  • The human body has multiple mechanisms responsible for initiating physiological responses to exercise.
    • Metabolic: ATP generation aerobically and anaerobically.
    • Respiratory: Multiple receptors regulate respiratory rate (command center, cerebral cortex, hypothalamus, chemoreceptors, mechanoreceptors).
    • Cardiovascular: Blood pressure regulated by baroreceptors and fluid balance.
    • Thermoregulation: Sweating and vasodilation regulate core temperature.
    • These overlapping mechanisms provide safety and elasticity in unpredictable environments.
  • From an integrated perspective, exercise begins with skeletal muscle contraction, which the central nervous system coordinates through afferent and efferent motor neurons.
  • Muscle contractions trigger systemic responses to preserve homeostasis.
  • Feedforward mechanisms initiate rapid responses, while feedback mechanisms fine-tune the response to meet exercise demands.
    • Feed forward initiates rapid responses to stressors, and feedback mechanisms fine tune responses. Neural and hormonal signals drive feed forward mechanisms. And feedback mechanisms send different signals back to the central nervous system to be precise.
  • Systemic responses are changes throughout the entire body (e.g., heart rate, blood pressure, circulating hormones).
  • Remote responses are changes in areas not directly involved in the immediate exercise environment (e.g., myokines, exosomes, progenitor cells).

Exercise and Risk Reduction

  • Traditional measures of risk reduction include regulating blood lipids, blood pressure, blood glucose, and body weight.
  • These contribute to about 50% of the protective effects of exercise and are more beneficial than risk-factor-modulating drugs.
  • Exercise plays a role in arterial remodeling, improving endothelial function, and fibrinolysis.
    • Arterial Remodeling helps limit the risk for myocardial infarction.
  • Improved vagal tone protects against fatal ventricular arrhythmias.
  • Cellular activity is improved through myokines, adipokines, exosomes, and progenitor cells that offer regenerative abilities and disease reduction effects.
  • Enhanced endothelial function limits plaque buildup and decreases clot formation.

Key Implications

  • The effects of exercise can be dramatic at almost every level of integration.
  • Key physiological responses to exercise are redundant, with multiple overlapping mechanisms.
  • Skeletal muscle contraction is associated with systemic responses and remote effects on other tissues.
  • A large portion of the risk reduction and improved health associated with exercise is not explained by commonly measured risk factors.