Gas Exchange: Fick's Law and Partial Pressure

Gas Exchange: Fick's Law

• Diffusion of CO2 & O2 is influenced by pressure gradient, solubility, temperature, and time
• Exercise can affect these factors and alter diffusion.

Partial Pressure

• Defined as the pressure exerted by each gas in a mixture, if it alone occupied the total volume.
• Dalton's Law states: Partial Pressure of a gas = [Gas] x Total Pressure of gas mixture

Pressure Gradients for O2 and CO2: Alveolus to Blood

• Alveolar PO2 (PAO2) is approximately 100 mmHg, lower than in atmosphere (~159 mmHg) due to water evaporation and CO2 diffusion.
• Oxygen diffuses from alveoli into blood, driven by this pressure gradient.

Oxygen Transport in Blood

• Arterial oxygen content is influenced by inspired oxygen, ventilation, gas exchange, hemoglobin concentration & affinity.
• Content (CaO2) measured in mL O2/100mL blood, while saturation (SaO2) reflects % of hemoglobin binding sites occupied.
• Oxygen-hemoglobin dissociation curve illustrates relationship between SaO2 and PO2.

Arterial Desaturation during Exercise

• Can occur in various scenarios like COPD with impaired gas exchange and elite athletes with high oxygen offloading.
• Reasons for desaturation:
  • Shorter transit time, high vascularization in athletes.
  • Ventilation-perfusion mismatch at high intensities.
  • Increased oxygen delivery and high pressure gradient, leading to rapid offloading.

Arteriovenous Oxygen Difference (a-vO2 diff)

• Represents the difference in oxygen content between arterial and venous blood.
• Varies significantly at rest versus intense exercise, especially in active muscles.

Limiting Factors for Exercise Performance

• Pulmonary factors can limit exercise:
  • Desaturation of O2.
  • High airway resistance (like in COPD).
  • Pulmonary pressure effects on cardiac output.
  • High work of breathing, leading to respiratory muscle fatigue & increased sympathetic outflow.

CO2 Transport

• Transported in three forms:
  • 5% dissolved in plasma, tightly regulated, drives ventilation at rest.
  • 20% bound to hemoglobin.
  • 60-80% as bicarbonate, which acts as a buffer to maintain pH.

Acid-Base Buffering

• Buffers resist changes in pH.
• Bicarbonate accepts H+ ions, releasing CO2, which is expelled by the lungs.
• Three buffering mechanisms: Chemical, pulmonary ventilation and renal function.

Exercise at Altitude

• Above 2100m, exercise performance is significantly impacted.
• While % oxygen remains constant, atmospheric pressure decreases with altitude, affecting PO2.
• PO2 drops linearly with altitude.

Altitude Responses

• Responses are proportional to altitude level.
• Acute responses are cardiorespiratory, including increased HR and ventilation.
• Adaptive responses include peripheral changes: muscle oxidative changes, increased RBC production.

Early Responses to Altitude

• Increased ventilation to drive more O2 into the blood.
• Respiratory alkalosis due to decreased buffering capacity.
• Increased HR, but HRmax may decrease.
• Increased a-vO2 difference to offload more O2.
• Increased 2,3-DPG to facilitate oxygen offloading.
• Decreased plasma volume, leading to increased oxygen concentration.
• Increased EPO production, potentially leading to increased Hb and red cell volume.

Exercise Performance at Altitude: Physiology

• Decreased PO2, leading to a smaller pressure gradient for gas exchange.
• Shift in oxygen dissociation curve to the right to enhance oxygen offloading.
• Decreased performance due to:
  • Increased HR and VE.
  • Impaired buffering capacity, resulting in respiratory alkalosis.
  • Reduced cardiac output.
• Overall, less oxygen delivery to skeletal muscle.

Summary

• Pulmonary factors like desaturation, airway resistance, and work of breathing can limit exercise performance.
• O2 and CO2 transport differ significantly, with CO2 diffusing and dissolving more readily than O2.
• Altitude significantly impacts exercise performance by affecting PO2, resulting in a smaller pressure gradient and reduced oxygen delivery to skeletal muscle.