Transport of Gases

Oxygen Transport and Unloading

1. From Lungs to Blood:

  • Inhalation: During inhalation, air containing oxygen enters the alveoli, where the partial pressure of oxygen (PO₂) is high (~100 mmHg).
  • Diffusion: Oxygen diffuses from the alveoli into the blood in the pulmonary capillaries due to the concentration gradient (high PO₂ in alveoli, low PO₂ in blood).
  • Binding to Hemoglobin:

    O2​ + Hb → HbO2​
    (Oxygen + Hemoglobin → Oxyhemoglobin)

2. Transport in Blood:

  • Oxyhemoglobin: Most oxygen is transported bound to haemoglobin (Hb), forming oxyhemoglobin (HbO₂).
  • Dissolved Oxygen: A small percentage (~2%) of oxygen is dissolved in the blood plasma.

3. From Blood to Tissues (Unloading of Oxygen):

  • Oxygen Release Mechanism:
    The release of oxygen from haemoglobin is influenced by several factors described by the Bohr effect:
  • a. Partial Pressure of Oxygen (PO₂):
  • In tissues, the PO₂ is lower (~40 mmHg) than in the blood (~100 mmHg). This gradient drives the diffusion of oxygen from blood into tissues.
  • b. Partial Pressure of Carbon Dioxide (PCO₂):
  • In tissues, PCO₂ is higher due to cellular respiration. CO₂ diffuses into the blood, increasing local PCO₂, which promotes oxygen unloading.

    CO2​ + H2​O → H2​CO3​ →. H+ + HCO3−​

    Carbon Dioxide + Water → Carbonic Acid → Hydrogen Ion + Bicarbonate Ion
  • c. pH (Bohr Effect):
  • Increased CO₂ in tissues leads to the production of H⁺ ions (lower pH), reducing haemoglobin’s affinity for oxygen and promoting its release.
    HbO2​ + H+ → HHb + O2​
    (Oxyhemoglobin + Hydrogen Ion → Reduced Hemoglobin + Oxygen)
  • d. Temperature:
  • Active tissues produce heat, increasing local temperature, which decreases haemoglobin’s affinity for oxygen, enhancing oxygen release.
  • e. 2,3-Bisphosphoglycerate (2,3-BPG):
  • 2,3-BPG, a byproduct of glycolysis in red blood cells, binds to deoxygenated haemoglobin, reducing its affinity for oxygen and facilitating oxygen release.

4. Cellular Respiration:

  • Oxygen diffuses into cells and is used in the mitochondria for aerobic respiration to produce ATP:

    C6​H12​O6​ + 6O2​ → 6CO2​ + 6H2​O + Energy (ATP)
    Glucose + Oxygen → Carbon Dioxide + Water + Energy

Carbon Dioxide Transport

1. From Tissues to Blood:

  • Production: CO₂ is produced as a byproduct of cellular metabolism, leading to higher PCO₂ in the tissues (~45 mmHg) compared to the blood (~40 mmHg).
  • Diffusion into Blood: CO₂ diffuses from the tissues into the blood in the systemic capillaries due to the concentration gradient.

2. Transport in Blood:

  • Dissolved in Plasma: About 7-10% of CO₂ is transported dissolved directly in the plasma.
  • Carbaminohaemoglobin:
    CO2​ + Hb → HbCO2​
    (Carbon Dioxide + Hemoglobin → Carbaminohemoglobin)
  • Bicarbonate Ions:
    CO2 ​+ H2​O → H2​CO3​ → H+ + HCO3−​
    (Carbon Dioxide + Water → Carbonic Acid → Hydrogen Ion + Bicarbonate Ion)
  • CO₂ reacts with water inside red blood cells, catalysed by carbonic anhydrase, forming carbonic acid (H₂CO₃), which quickly dissociates into bicarbonate (HCO₃⁻) and hydrogen (H⁺) ions.
  • Bicarbonate ions diffuse out of red blood cells into the plasma in exchange for chloride ions (Cl⁻) (chloride shift).

3. From Blood to Lungs:

  • Return to Lungs: Deoxygenated blood carrying CO₂ returns to the right side of the heart and is then pumped to the lungs via the pulmonary arteries.
  • Reverse Reactions in Lungs:
    H+ +. HCO3−​ →. H2​CO3​ →. CO2​ +. H2​O
    (Hydrogen Ion + Bicarbonate Ion → Carbonic Acid → Carbon Dioxide + Water)
  • Bicarbonate ions re-enter red blood cells and recombine with hydrogen ions to form carbonic acid, which is then converted back into CO₂ and water by carbonic anhydrase.
  • CO₂ is released from carbamino-haemoglobin.

4. Exhalation:

  • Diffusion into Alveoli: CO₂ diffuses from the blood (high PCO₂) into the alveoli (low PCO₂) due to the concentration gradient.
  • Exhalation: CO₂ is expelled from the lungs during exhalation, completing the respiratory cycle.

Summary

  • Oxygen Transport: Inhaled oxygen diffuses into the blood, binds to haemoglobin, and is transported to tissues where it diffuses into cells for cellular respiration.
  • Oxygen Unloading: Facilitated by lower PO₂, higher PCO₂, lower pH (Bohr effect), higher temperature, and the presence of 2,3-BPG.
  • CO₂ Transport: CO₂ produced by cellular respiration diffuses into the blood, is transported dissolved in plasma, bound to haemoglobin, or as bicarbonate ions, and is ultimately exhaled from the lungs.