Physiology · MU Sofia

104. Physiology of exercise. Cardiovascular adaptation. Body fluids and electrolytes during exercise.

Covers the three energy systems used in exercise, the ATP-PC, glycolytic and aerobic pathways, with their timing and the fuels used at low, medium and high intensity. It then explains how cardiac output rises through heart rate and stroke volume, how blood flow is redirected towards working muscles, and how sweat losses of sodium and chloride are managed by aldosterone and ADH. A short list of recap questions closes the note.

Inside: 7 sections · 6 labelled pictures · 21 flashcards · 6 min

Topic 103 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 7 sections, about 820 words (6 min of reading), with 21 flashcards.

In this note

  1. Physiology of exercise. Cardiovascular adaptation. Body fluids and electrolytes during exercise.
  2. Energy Systems During Exercise
  3. Cardiac Output and Its Determinants
  4. Redistribution of Blood Flow
  5. Regulation of Blood Flow:
  6. Body Fluids and Electrolytes During Exercise
  7. Hormonal Regulation of Fluid and Electrolyte Balance:

Physiology of exercise. Cardiovascular adaptation. Body fluids and electrolytes during exercise.

Exercise physiology explores the body's intricate responses and adaptations to physical activity. During exercise, the demand for energy (ATP) rises significantly to fuel muscle contractions and maintain homeostasis. The body relies on stored energy reserves in the form of carbohydrates, fats, and proteins, utilizing both aerobic (with oxygen) and anaerobic (without oxygen) pathways to generate ATP.

Energy Systems During Exercise

The body utilizes different energy systems depending on the intensity and duration of exercise:

  • Anaerobic Systems:
  • ATP-PC System (Phosphagen System): This system provides immediate energy for short-duration, high-intensity activities lasting up to about 10 seconds, like sprinting or weightlifting. It relies on stored ATP and creatine phosphate (PCr), which rapidly replenishes ATP without requiring oxygen. This is the fastest pathway but produces the least energy.
  • Glycolytic System (Anaerobic Glycolysis): This pathway kicks in after the ATP-PC system is depleted, providing energy for moderate-intensity activities lasting up to a couple of minutes. It breaks down glucose (from glycogen or blood sugar) into

pyruvate, producing ATP without oxygen. However, it also generates lactic acid as a byproduct, leading to muscle fatigue.

  • Aerobic System (Oxidative Phosphorylation): This system dominates during prolonged, low to moderate-intensity activities. It utilizes oxygen to completely break down glucose, fatty acids, and amino acids, generating ATP through the Krebs cycle and electron transport chain in the mitochondria. It is the slowest pathway but produces the most

energy, particularly from fats.

The stages of energy generation during exercise are:

  1. 1. Phosphocreatine and stored ATP are utilized in the first few seconds.
  2. 2. Glycolysis starts after about 8-10 seconds.
  3. 3. Aerobic respiration reaches its maximum rate of ATP synthesis after 2-4 minutes.
  4. 4. Repayment of oxygen debt occurs in the recovery period, involving the conversion of lactic acid back to pyruvate, rephosphorylation of creatine, glycogen synthesis, and oxygen rebinding to myoglobin and hemoglobin.

The primary sources of energy also vary based on exercise intensity:

  • Low-intensity exercise primarily relies on free fatty acids from blood plasma.
  • Medium-intensity exercise utilizes a larger proportion of muscle glycogen.
  • High-intensity exercise depends heavily on muscle glycogen and an increased uptake of plasma glucose.

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