Oxygen and Carbon Dioxide Transport in Blood

Oxygen and Carbon Dioxide Transport in Blood

  • Hemoglobin Structure

    • Hemoglobin consists of four subunits:
    • Each subunit contains a globin chain (protein) and an iron-containing heme group.
    • Hemoglobin has four sites for binding oxygen due to its four iron heme groups.

  • Oxygen Binding

    • Oxygen binds specifically to the iron in the heme group.
    • When all four heme groups are bound to oxygen, hemoglobin is fully saturated and termed oxyhemoglobin (abbreviated as HbO₂).
    • If hemoglobin binds fewer than four oxygen molecules, it is termed deoxyhemoglobin (abbreviated as HHB).
    • It is important to note that deoxygenated blood still carries some oxygen (~75% saturated if less than four oxygens are bound).

  • Oxygen Transport Dynamics

    • Oxygen dissociation curve: illustrates how hemoglobin releases oxygen at the tissues and binds oxygen at the lungs.
    • At the tissues, oxygen is released when partial pressure of oxygen (pO₂) drops (e.g., at pO₂ = 40 mmHg, hemoglobin is ~75% saturated).
    • Physiological conditions affecting the oxygen affinity of hemoglobin:
    • Increased temperature, increased CO₂, and increased hydrogen ions (lower pH) decrease hemoglobin's affinity for oxygen.
    • This enables more efficient oxygen release during activities like exercise when metabolism increases.

  • Shifts in the Oxygen Dissociation Curve

    • Rightward shift: indicates decreased affinity for oxygen (e.g., due to increased temperature, CO₂, or H⁺ concentration).
    • Leftward shift: indicates increased affinity for oxygen (e.g., due to decreased temperature, CO₂, or H⁺ concentration).

  • Carbon Dioxide Transport

    • CO₂ is transported in blood through three main mechanisms:
    1. Dissolved in plasma (7-10% of CO₂).
    2. Bound to hemoglobin (about 20%, forming carbaminohemoglobin: HbCO₂).
    3. Bicarbonate ions (HCO₃⁻), making up ~70% of CO₂ transport.
      • Reaction occurs in red blood cells:
      • CO₂ + H₂O ↔ H₂CO₃ (carbonic acid) ⇌ HCO₃⁻ + H⁺
      • Enzyme involved: carbonic anhydrase.
      • Carbonic acid dissociates into bicarbonate and hydrogen ions, which is essential for transport.

  • Chloride Shift

    • As HCO₃⁻ ions exit the red blood cells into the plasma, chloride ions (Cl⁻) enter the red blood cells to maintain electric neutrality.
    • This exchange is facilitated by the chloride shift.

  • Interrelation of O₂ and CO₂ transport

    • Increased CO₂ results in increased H⁺ concentration, prompting oxygen release from hemoglobin (Bohr effect).
    • Le Chatelier's Principle applies: removing products (like H⁺) from a reaction drives the reaction forward, promoting oxygen dissociation.

  • Key Equations

    • Formation of oxyhemoglobin:
      HHB + O2 ⇌ HbO2 + H .
    • Bicarbonate formation:
      CO2 + H2O ↔ H_2CO₃ ⇌ HCO₃⁻ + H⁺ .

  • Physiological Relevance

    • Understanding the transport and dynamics of O₂ and CO₂ is critical for appreciating how the body meets metabolic demands, particularly during exercise and in physiological changes.