Exercise Physiology: Gas Exchange and Transport

Gas Exchange Overview

  • Gas Exchange Laws
    • Partial Pressures of Gases
    • PAO2: Partial pressure of oxygen in alveolar chambers
    • PACO2: Partial pressure of carbon dioxide in alveolar chambers
    • PaO2: Partial pressure of oxygen in arterial blood
    • PVO2: Partial pressure of oxygen in venous blood
    • SaO2%: Percent saturation of arterial blood with oxygen
    • SvO2%: Percent saturation of venous blood with oxygen
    • PaCO2: Partial pressure of carbon dioxide in arterial blood
    • PvCO2: Partial pressure of carbon dioxide in venous blood
    • a-vO2 diff: Difference in oxygen content between arterial and venous blood (arteriovenous oxygen difference)
    • Mixed-venous blood: Oxygen difference between arterial blood and mixed-venous blood

Gas Transfer Mechanism

  • Diffusion
    • Gas exchange occurs at lungs and tissues via diffusion down a partial pressure gradient.
    • Water vapor presence decreases proportions of other gases due to decreased partial pressures.
  • Laws of Gas
    • Boyle's Law: Pressure of gas decreases as volume of gas increases (constant temperature).
    • Dalton's Law: Total pressure of a gas mixture is the sum of the partial pressures of individual gases.
    • Henry's Law: Gases dissolve in liquids proportional to their partial pressures.

Partial Pressure Concepts

  • Partial Pressure Difference
    • It is the pressure exerted by a gas, resulting from gas molecules moving against a surface.
    • Creates the driving force for gas diffusion across the pulmonary membrane.
  • Atmospheric Air Composition (at sea level 760 mm Hg):
    • O2 = 20.93%
    • N2 = 79.04%
    • CO2 = 0.03%
  • Partial Pressures in Inspired Air:
    • Inspired Air:
    • O2: 0.2093 x 760 mmHg = 159 mmHg
    • N2: 0.7904 x 760 mmHg = 600.7 mmHg
    • CO2: 0.0003 x 760 mmHg = 0.3 mmHg
  • Partial Pressures in Alveolar Air:
    • O2: 100 mmHg
    • N2: ~573 mmHg
    • CO2: 40 mmHg

Gas Solubility

  • Influencing Factors:
    • Affected by gas solubility (mL of gas per 100 mL of fluid), type of gas, temperature, and its partial pressure.
    • Different gases have varying solubility coefficients in blood:
    • CO2: 57.03 mL/100mL of fluid,(high solubility)
    • O2: 2.26 mL/100mL of fluid
    • N2: 1.30 mL/100mL of fluid

Gas Exchange at Rest and During Exercise

  • Resting Conditions
    • PO2 between lungs and tissues at ~100 mmHg to ~40 mmHg.
    • The net diffusion gradient for O2 is high at rest, allowing efficient transfer.
    • Pulmonary circulation and heart have a pressure gradient facilitating gas exchange.
  • During Exercise
    • Increased gradients result in faster diffusion of gases.
    • Ventilation and metabolic demands are matched to maintain gas composition.

Blood Composition and Gas Transport

  • Functions of Blood
    • Transport, immunity, haemostasis, homeostasis.
  • Components of Blood
    • Plasma, leukocytes, thrombocytes, erythrocytes.
  • Blood parameters include:
    • Hemoglobin (Hgb): 14–18 g/100 mL (male); 12–16 g/100 mL (female)
    • Hematocrit (HCT): % of RBC in blood volume.
    • Males: 40–54%
    • Females: 38–47%

Oxygen Transport Mechanisms

  • O2 transport occurs in two ways:
    1. Dissolved in plasma.
    2. Bound to hemoglobin (197 mL O2 per L of blood).
  • Oxygen capacity:
    • A blood sample can hold 20 mL O2/100 mL blood at full saturation.

Oxyhemoglobin Dissociation Curve

  • Describes the relationship between PO2 and % saturation of hemoglobin with oxygen.
  • Conditions affecting the curve:
    • Bohr Effect:
    • Causes a right shift in the curve with decreased pH, increased temperature, increased PCO2, indicating less oxygen affinity of hemoglobin.
  • At rest, human blood exhibits optimal saturation at a PO2 of 100 mm Hg, with ~20 mL of oxygen carried in 100 mL of blood.