Transport of Gases

Transport of Gases

The Respiratory System Overview

  • Structure of the respiratory system: nasal cavity, voice box, windpipe, bronchi, lungs.
  • Key terms:
    • Alveoli: site of gas exchange.
    • Capillaries: small blood vessels for nutrient and gas exchange.
    • Diaphragm: muscle aiding in breathing.

Transport of Oxygen (O2) in Blood

Mechanisms of O2 Transport
  1. Dissolved in Plasma:
    • Only ~1.5% of total O2.
  2. Bound to Hemoglobin (Hb):
    • 98.5% of O2 is transported this way.
    • Each Hb molecule can bind to 4 O2 molecules (one for each iron atom).

Transport of Carbon Dioxide (CO2) in Blood

Forms of CO2 Transport
  1. Dissolved in Plasma:
    • ~7% to 10%.
  2. Bound to Hemoglobin and Plasma Proteins:
    • ~20% of CO2 binds to Hb to form Carbaminohemoglobin.
  3. Bicarbonate Ions (HCO3-):
    • About 70% of CO2 transported as bicarbonate.
    • This conversion is represented by the equation:
      • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

Bicarbonate Buffer System

  • Maintains pH of blood through the following mechanisms:
    • Acidosis (increased H+ concentration):
    • Excess H+ combines with HCO3-, shifting the reaction left.
    • Alkalosis (decreased H+ concentration):
    • H2CO3 dissociates, releasing H+ and HCO3-, shifting the reaction right.

Gas Exchange Mechanisms

Internal Respiration
  • CO2 enters the RBCs and combines with water to form carbonic acid (H2CO3), catalyzed by carbonic anhydrase.
  • This dissociates into H+ and HCO3-.
External Respiration
  • HCO3- ions diffuse back into RBCs, bind with H+, and form H2CO3.
  • H2CO3 dissociates into CO2 and H2O; CO2 then diffuses into alveoli.

Chloride Shift

  • Exchange of chloride ions (Cl-) and bicarbonate ions (HCO3-) between RBCs and plasma is crucial for pH maintenance.
  • Occurs at tissue level (HCO3- accumulates in RBCs as CO2 is picked up) and is reversed at the lungs.

Ventilation/Perfusion (V/Q) Mismatch

  • Occurs when ventilation and perfusion are normal but not matching in different lung areas. Conditions include:
    • High Po2 in well-ventilated alveoli cannot compensate for low Po2 in underventilated alveoli.
    • Can lead to hypoxemia.

Oxygen-Hemoglobin Dissociation Curve

  • Illustrates the relationship between % saturation of Hb with O2 and PO2.
  • Haldane Effect:
    • Oxygenation of blood promotes CO2 dissociation from hemoglobin.
  • Bohr Effect:
    • The binding affinity of Hb for O2 decreases with increased CO2 concentration and acidity.

Factors Affecting Oxygen-Hemoglobin Dissociation Curve

  1. Blood PCO2:
    • Increased PCO2 leads to acidosis, lowering Hb-O2 affinity and shifting curve to the right.
  2. pH Changes:
    • Acidosis decreases affinity (shifts curve right); alkalosis increases affinity (shifts curve left).
  3. Temperature:
    • Increased temperature reduces Hb-O2 affinity.
  4. Organic Chemicals (e.g., 2,3-BPG):
    • Stabilizes deoxy-Hb, reducing Hb-O2 binding.

Fetal Hemoglobin vs. Maternal Hemoglobin

  • Fetal Hemoglobin (Hb-F) carries about 30% more O2 and has a higher binding affinity compared to adult Hemoglobin (Hb-A), which is critical for O2 transfer in low PO2 environments present in the placenta.

Clinical Correlation: Hemoglobinopathies

  • Genetic defects leading to abnormal Hb structures affecting O2 capacity (e.g., sickle cell anemia, thalassemia).

Respiratory Patterns

Types of Patterns
  • Eupnea: Normal quiet breathing.
  • Hyperpnea: Increased respiratory rate/volume during exercise.
  • Hyperventilation: Increased breathing without increased metabolism.
  • Hypoventilation: Reduced alveolar ventilation.
  • Tachypnea: Rapid breathing, often increased rate with decreased depth.
  • Dyspnea: Subjective difficulty in breathing.
  • Apnea: Cessation of breathing.
Example Scenario
  • Fast and shallow breathing after exercise may indicate hyperventilation, reflecting changes in blood PO2, PCO2, and pH that affect gas transport.

Learning Outcomes

  • Transport of O2 and CO2 in blood is crucial for respiration.
  • Understanding the oxygen-Hb dissociation curve and its factors is essential for clinical applications.

References

  1. Tortora, G.J. Principles of Anatomy and Physiology, 13th Ed., 2011.
  2. Martini, F.H. Fundamentals of Anatomy & Physiology, 9th Ed., 2012.
  3. Marieb, E.N. Human Anatomy & Physiology, 8th Ed., 2006.