Oxygen Transport

Introduction to Oxygen Transportation in the Body

  • Exploration of how oxygen is transported through the blood.

Anatomy of Gas Exchange

  • Description of oxygen presence in alveoli and surrounding capillaries.
  • Oxygen represented by blue dots in the alveoli.
  • Importance of capillary density surrounding alveoli for efficient gas exchange.
  • Oxygen is lipophilic which allows it to move easily across:
    • Alveolar cells
    • Capillary endothelial cells
  • Oxygen diffuses into the plasma.

Oxygen Dissolution in Plasma

  • Limited amount of oxygen dissolved in plasma:
    • About three milliliters of oxygen per liter of blood.
    • Total blood volume in the body is approximately five liters.
    • Total carrying capacity from plasma is therefore about 15 milliliters.
    • Comparison: at rest, the body needs about 250 milliliters of oxygen per minute.

Role of Hemoglobin in Oxygen Transportation

  • Hemoglobin significantly enhances the blood's ability to carry oxygen.
  • Baseline oxygen in plasma remains at three milliliters, but with hemoglobin,
    • Oxygen capacity increases by approximately 200 milliliters per liter of blood.
    • Hence for five liters of blood, the total oxygen carrying capacity escalates to over 1,000 milliliters.
    • More than sufficient to meet metabolic demand of 250 milliliters of oxygen per minute.

Partial Pressure of Oxygen (PO2)

  • PO2 measurement represents oxygen dissolved in plasma only; it does not include oxygen bound to hemoglobin.
  • Emphasized importance of PO2 as the clinical number reflecting oxygen diffusion potential into tissues.

Measurement of PO2 in Various Body Areas

  • PO2 in alveoli is approximately:
    • 100 mmHg in the alveoli.
  • PO2 in tissues is approximately:
    • 40 mmHg.
  • Oxygen gradient facilitates movement from blood (PO2 = 100) to tissues (PO2 = 40).
  • Mechanism of oxygen transport:
    • Oxygen diffuses from alveolus into pulmonary capillaries.
    • Some oxygen dissolves in plasma, while most binds to hemoglobin in red blood cells.
  • Unloading process occurs as oxygen transfers from hemoglobin into plasma then to tissues.

Oxygen Unloading

  • Defined as the process where oxygen moves from blood into tissues.
  • Results in post-tissue use PO2 reduction to about 40 mmHg.
  • Blood returning to lungs reflects this decreased PO2.

Oxygen Hemoglobin Saturation Curve

  • Graphical representation of hemoglobin saturation against partial pressure of oxygen.
    • X-axis: PO2
    • Y-axis: Percentage saturation of hemoglobin.
  • Key insights from the curve:
    • At PO2 of 100 mmHg, hemoglobin reaches nearly 100% saturation.
    • At PO2 of 40 mmHg, hemoglobin saturation drops to approximately 75%.
  • Affinity of hemoglobin for oxygen can be altered, representing the following:
    • A rightward shift indicates decreased affinity for oxygen.
    • A leftward shift indicates increased affinity for oxygen.

Factors Influencing the Oxygen Hemoglobin Saturation Curve

Shift to the Right (Decrease in Affinity)

  • Associated with conditions typical of exercise or exertion:
    • Increased Temperature:
    • Metabolically active tissues produce heat, shifting the curve to the right.
    • Increased Hydrogen Ions/Decreased pH:
    • Promoted by metabolic activity, also shifts the curve to the right.
    • Increased CO2:
    • Produced by metabolic pathways, contributes to rightward shift (termed the Bohr effect).
    • Increased 2,3-BPG (or 2,3-DPG):
    • An intermediate in glycolysis, produced at higher rates during metabolic activity, shifts curve right.

Clinical Implications of the Shift

  • Reduced affinity for oxygen allows for more efficient unloading at tissues during exercise.
  • In arterial blood, saturation remains almost unchanged at PO2 of 100 mmHg.
  • However, upon reaching tissues, saturation may drop from 75% potentially down to 50% or less during active exertion.
  • This shift is advantageous as it ensures that more oxygen is released where it is critically needed (tissues with higher metabolic demands).

Conclusion

  • Understanding the mechanisms and factors influencing oxygen transport is vital for recognizing the efficiency of gas exchange and the metabolic needs of human tissues during varying levels of activity.