Arteriovenous Oxygen Difference and Fick Equation

Arteriovenous Oxygen Difference (AVO2)

• Definition: The difference in oxygen content between arterial and venous blood.
• Leads to a discussion about the Fick equation.

Hemoglobin and Oxygen

• Oxygen Binding Capacity: One gram of hemoglobin can hold 1.34 mL of oxygen.
• Molecular Capacity: Each hemoglobin molecule can bind up to four oxygen molecules.
• Reversible Reaction:
  Hemoglobin + 4O2 \rightleftharpoons Oxyhemoglobin (Hb4O_8)
• Saturation Variability: Hemoglobin is not always fully saturated; varying degrees of saturation or desaturation occur.

Factors Influencing Hemoglobin-Oxygen Bonds

• Partial Pressure of Oxygen:
  • Determines the quantity of oxygen molecules bound to hemoglobin.
  • Influences the strength of the bond between oxygen and hemoglobin.
• High Oxygen Environment (e.g., Arterial Blood): Oxygen and hemoglobin are loosely bound.
• Low Oxygen Environment (Hypoxic): Hemoglobin binds oxygen tightly.

Oxyhemoglobin Dissociation Curve

• Gases move from high to low partial pressure.
• Arterial Blood:
  • High partial pressure of oxygen (100 mm Hg).
  • 100% saturation of hemoglobin with oxygen.
• Resting Organ/Muscle:
  • Partial pressure of oxygen around 40 mm Hg.
  • 75% oxygen saturation of hemoglobin (three out of four binding sites are full).
• Zero Partial Pressure of Oxygen:
  • 0% saturation of hemoglobin.
  • All four binding sites are desaturated.
• Magic Number: 1.34 mL of oxygen can be bound to every gram of hemoglobin.

Oxygen Carrying Capacity of Arterial Blood

• Expressed in mL of oxygen per 100 mL of blood.
• Calculation:
  • Men typically have ~15 grams of hemoglobin per 100 mL due to testosterone.
  • 15 \frac{g}{100 mL} * 1.34 \frac{mL O2}{g} = 20 \frac{mL O2}{100 mL}
  • Approximately 20 mL of oxygen bound per 100 mL of arterial blood in an average male (all four binding sites full).
• Analogy: Red blood cells are like trucks, picking up oxygen at the alveoli and delivering it throughout the body.

Oxygen Carrying Capacity of Venous Blood

• Partial Pressure at Resting Muscle: 40 mm Hg.
• Oxygen Saturation at 40 mm Hg: 75%.
• Calculation:
  • 15 \frac{g}{100 mL} * 1.34 \frac{mL O2}{g} * 0.75 = 15 \frac{mL O2}{100 mL}
  • Approximately 15 mL of oxygen per 100 mL of venous blood.
• Minute amount of oxygen also bound in the plasma, but ignored for this calculation.

Arteriovenous Oxygen Difference (AVO2) Explained

• Difference between oxygen carrying capacity of arterial and venous blood.
• Example: Arterial blood carries 20 mL O2/100 mL, venous blood carries 15 mL O2/100 mL.
• Difference: 5 mL O2/100 mL.
• Represents the amount of oxygen used by the muscle as fuel.
• AVO2 diff = Arterial Oxygen Content - Venous Oxygen Content
• During exercise, the muscle needs more oxygen, causing the partial pressure of oxygen within the muscle to drop.

Impact of Exercise on AVO2 Difference

• Rest: AVO2 difference is about 5 mL O2/100 mL.
• Exercise:
  • Partial pressure of oxygen in the muscle drops further.
  • More oxygen is liberated into the muscle.
  • Two to three (or even four) oxygen molecules are offloaded into the muscle.
  • Greater AVO2 difference.
• High-Intensity Exercise:
  • Partial pressure of oxygen can drop to 15 mm Hg.
  • More oxygen is liberated.
  • Only 5 mL of oxygen remains bound to hemoglobin.
  • AVO2 difference can reach 10 mL O2/100 mL or higher.
• Vigorous Exercise:
  • Partial pressure of oxygen in the muscle approaches zero.
  • Nearly all oxygen is liberated from hemoglobin.

Significance of AVO2 Difference During Exercise

• Higher intensity exercise leads to a higher AVO2 difference.
• Reducing partial pressure of oxygen in the muscle increases oxygen liberation from hemoglobin.
• The body can liberate more oxygen without increasing blood supply to the muscle.

Automatic Reserve of Oxygen

• The body can quickly liberate more oxygen by using more oxygen in the muscle.
• Signals hemoglobin to release more oxygen without waiting for increased heart rate or blood flow.

Additional Factors Enhancing Oxygen Delivery (Bohr Effect)

• Greater differential of partial pressure of oxygen.
• Higher temperature.
• More acidic environment.

AVO2 Difference in Recreational vs. Elite Athletes

• Recreational Athlete:
  • Arterial blood: 20 mL O2/100 mL.
  • Venous blood: 15 mL O2/100 mL.
• Elite Athlete (Exercise-Trained):
  • Arterial blood: 20 mL O2/100 mL (same as recreational).
  • Venous blood: 13 mL O2/100 mL (lower than recreational).
  • Greater AVO2 difference (17 mL O2/100 mL).
• Exercise training increases the body's ability to extract oxygen from the blood due to:
  • More capillaries.
  • More mitochondria in cells.
  • Increased enzyme production.

Adaptations with Chronic Exercise Training

• Changes in hematocrit (percentage of red blood cells per volume of blood) due to increased plasma volume.
• Lower blood viscosity.
• More efficient sweating.
• Increased total oxygen transport.

VO2 Max Calculation and Significance

• VO2 max: Gold standard measurement of oxygen capacity.
• VO2 = Cardiac Output * AVO2 difference
• Example:
  • Cardiac output: 5 liters/minute (same for both individuals).
  • Recreational athlete: AVO2 difference = 15 mL O2/100 mL.
    • VO2 = 5 L/min * 150 mL O2/L = 750 mL O_2/min
  • Elite athlete: AVO2 difference = 17 mL O2/100 mL.
    • VO2 = 5 L/min * 170 mL O2/L = 850 mL O_2/min
  • The elite athlete can use 100 mL of oxygen more per minute, a significant difference for delivering oxygen to muscles.