EP - Respiratory Correlates of Exercise

Question 1

Describe alveolar ventilation:

Answer 1

Alveolar ventilation = Amount of air reaching the alveoli per minute

= (TV - TD) X RR

Alveolar ventilation is significantly different between shallow and deep breathing

Adaptations:

• increased Tidal Volume (V_T): Deeper breaths enhance alveolar ventilation
• Optimized Breathing Frequency: Adjustments in respiratory rate maintain efficient alveolar ventilation

Question 2

Describe the exercise centre (enhanced pulmonary ventilation):

Answer 2

A respiratory exercise centre in the medulla integrates neural and humoral factors to produce a ventilatory pattern entirely appropriate to the metabolic demands of that particular time

Enhanced pulmonary ventilation involves an increase in the rate and depth of breathing to increase air movement into and out of the lungs

Neural activation - central command from the motor cortex and feedback from peripheral chemoreceptors and mechanoreceptors stimulate respiratory centres

Humoral activation - metabolic byproducts (e.g., CO₂, H⁺) activate afferent nerves, increasing ventilation

Question 3

Descrive ventilation perfusion matching in exercise:

Answer 3

Ratio of alveolar ventilation to pulmonary blood flow, essential for optimal gas exchange

Improved V/Q Matching: Enhanced cardiac output and pulmonary perfusion align with increased ventilation

Question 4

Describe the phases of ventilatory response

Answer 4

Phase 1:

• neurogenic phase (immediate increase)
• rapid increase in ventilation at the onset of exercise
• driven by central command (motor cortex) and peripheral mechanoreceptors (muscle and joint receptors
• driven by anticipatory mechanisms and muscle-joint feedback rather than metabolic signals
• Independent of blood gas changes (P_O₂, P_CO₂, pH)

Phase 2:

• exponential phase (gradual increase)
• exponential rise in ventilation
• Primarily neural but influenced by chemoreceptor input
• Central chemoreceptors respond to increased P_CO₂ and decreased pH
• Peripheral chemoreceptors respond to falling P_O₂ and rising P_CO₂
• Ventilation increases in proportion to metabolic demands

Phase 3:

• Steady-State Phase (Fine Tuning)
• ventilation stabilises to meet metabolic demands
• Controlled by:
• Chemoreceptors (adjust to maintain homeostasis).
• Proprioceptive feedback (from muscles).
• Central command (sustains neural drive)

Question 5

Describe the role of central and peripheral chemoreceptors, neural control during ventilatory response

Answer 5

Central chemoreceptors - located in the medulla, sensitive to changes in arterial P_CO₂ and pH

Peripheral chemoreceptors - Found in carotid and aortic bodies, respond to arterial P_O₂, P_CO₂, and pH variations

Neural control:

Central command - motor cortex activation during exercise initiates increased ventilation

Peripheral feedback - Mechanoreceptors and metaboreceptors in muscles and joints provide feedback to respiratory centers

Question 6

Describe the maintenance of blood gases acid-base homeostasis:

Answer 6

Oxygen transport:

• haemoglobin saturation maintained near maximal levels during moderate exercise
• Arterial P_O₂ remains relatively stable due to efficient pulmonary gas
  exchange

CO2 Removal:

• increased ventilation enhances CO₂ exhalation, preventing hypercapnia
• bicarbonate buffering: CO₂ conversion to bicarbonate (HCO₃⁻) aids in pH regulation

Acid-Base Balance:

• Anaerobic metabolism during intense exercise produces lactate and H⁺ ions
• Bicarbonate buffering neutralises excess H⁺, maintaining pH
• renal compensation: Long-term regulation of acid-base balance through H⁺ excretion and HCO₃⁻ reabsorption

Question 7

Describe the anaerobic threshold:

Answer 7

Below this level of exercise, the muscle demands for oxygen are met completely by aerobic means and little lactate is formed.
Above it, the muscles respire anaerobically and produce lactic acid at too great a rate for the body to dispose of.

Hyperventilation and change in RER.
Acidosis
Unmaintainable exercise level.

Question 8

Describe ventilation rise in exercise including ventilatory thresholds:

Answer 8

Moderate exercise - linear relationship as ventilation increases proportionally with oxygen uptake (V̇O₂)

Heavy exercise - non linear relationship because ventilation rises disproportionately due to lactate accumulation and metabolic acidosis

First ventilatory threshold - Point where ventilation starts increasing at a faster rate than V̇O₂

Secondary ventilatory threshold - Further disproportionate increase in ventilation, often coinciding with the anaerobic threshold

Question 9

Describe blood gases during exercise:

Answer 9

With increasing exercise ventilation increases linearly with VO2 until a certain point - the ventilatory threshold

At that point, it breaks linearity due to lactic acid stimulating the chemoreceptors (ventilatory compensation for metabolic acidosis)

bicarb falls mole per mole with lactate rise

Blood gases then actually fall for CO2 and rise for O2

In this way, we can sustain quite an onerous lactate load
Rowers of repute

Question 10

Describe the carriage of blood gases:

Answer 10

Oxygen:

• carried by haemoglobin
• Sigmoidal relationship between P_O₂ and Hb saturation; shifts during exercise to facilitate O₂ unloading
• Bohr effect and temperature affect oxygen dissociation curve

CO2:

• 7% dissolved in plasma
  23% as carbamino compounds (Hb)
  Most important is bicarbonate
  CO2 + H2O <-> H2CO3 <-> H+ + HCO3-