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Chemistry

CO2 & O2 Transport

Oxygen and carbon dioxide transport are essential processes for maintaining life. Oxygen, vital for cellular respiration, is primarily transported by hemoglobin within red blood cells, with a small fraction dissolved in plasma. Hemoglobin’s affinity for oxygen is influenced by factors like pH, \(CO_2\), temperature, and 2,3-BPG, as reflected in the oxygen dissociation curve.

Carbon dioxide, a byproduct of metabolism, is transported in three forms: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and predominantly as bicarbonate ions. The conversion of carbon dioxide to bicarbonate is catalyzed by carbonic anhydrase within red blood cells, while the chloride shift maintains electrical neutrality during this process. These intricate mechanisms ensure efficient gas exchange, supporting cellular function and overall homeostasis

Oxygen (\(O_2\)) Transport

  • Partial Pressure of Oxygen (\(PO_2\))
    • Definition: The pressure exerted by oxygen in a mixture of gases
    • Units: mmHg or kPa
    • Normal Range:
      • Arterial Blood (Pa\(O_2\)): 80-100 mmHg
      • Venous Blood (Pv\(O_2\)): 30-40 mmHg
  • Oxygen in the Blood
    • Dissolved Oxygen: A small amount of oxygen is dissolved directly in the plasma
      • Approximately 1.5-3% of total oxygen in the blood
      • Determines the partial pressure of oxygen (Pa\(O_2\))
    • Hemoglobin-Bound Oxygen: Most oxygen is transported bound to hemoglobin (Hb) in red blood cells
      • Each hemoglobin molecule can bind up to four oxygen molecules
      • The binding of oxygen to hemoglobin is reversible
  • Hemoglobin Structure
    • Tetrameric protein consisting of four polypeptide chains (globins): two alpha (α) chains and two beta (β) chains
    • Each globin chain is associated with a heme group, which contains an iron (\(Fe^{2+}\)) atom
    • The iron atom binds oxygen reversibly
  • Oxygen Binding to Hemoglobin
    • Oxyhemoglobin: Hemoglobin bound to oxygen (Hb\(O_2\))
    • Deoxyhemoglobin: Hemoglobin not bound to oxygen (Hb)
    • Cooperative Binding: The binding of the first oxygen molecule to hemoglobin increases the affinity of hemoglobin for subsequent oxygen molecules
    • Sigmoidal Oxygen Dissociation Curve: Reflects the cooperative binding of oxygen to hemoglobin
  • Oxygen Dissociation Curve
    • Definition: A graph that plots the percentage of hemoglobin saturation (Sa\(O_2\)) against the partial pressure of oxygen (Pa\(O_2\))
    • Shape: Sigmoidal (S-shaped)
    • P50: The partial pressure of oxygen at which hemoglobin is 50% saturated
      • Normal P50: Approximately 27 mmHg
      • Shift to the Right: Decreased affinity of hemoglobin for oxygen (increased P50)
        • Caused by increased temperature, increased Pa\(CO_2\), decreased pH, increased 2,3-BPG
      • Shift to the Left: Increased affinity of hemoglobin for oxygen (decreased P50)
        • Caused by decreased temperature, decreased Pa\(CO_2\), increased pH, decreased 2,3-BPG
  • Factors Affecting Oxygen Binding
    • pH (Bohr Effect):
      • Decreased pH (increased acidity) decreases the affinity of hemoglobin for oxygen, shifting the curve to the right
      • Increased pH (decreased acidity) increases the affinity of hemoglobin for oxygen, shifting the curve to the left
    • Carbon Dioxide (\(CO_2\)):
      • Increased Pa\(CO_2\) decreases the affinity of hemoglobin for oxygen, shifting the curve to the right
      • Decreased Pa\(CO_2\) increases the affinity of hemoglobin for oxygen, shifting the curve to the left
    • Temperature:
      • Increased temperature decreases the affinity of hemoglobin for oxygen, shifting the curve to the right
      • Decreased temperature increases the affinity of hemoglobin for oxygen, shifting the curve to the left
    • 2,3-Bisphosphoglycerate (2,3-BPG):
      • Increased 2,3-BPG decreases the affinity of hemoglobin for oxygen, shifting the curve to the right
      • Decreased 2,3-BPG increases the affinity of hemoglobin for oxygen, shifting the curve to the left
      • 2,3-BPG binds to deoxyhemoglobin and stabilizes its conformation, reducing oxygen affinity

Carbon Dioxide (\(CO_2\)) Transport

  • Partial Pressure of Carbon Dioxide (Pa\(CO_2\))
    • Definition: The pressure exerted by carbon dioxide in a mixture of gases
    • Units: mmHg or kPa
    • Normal Range:
      • Arterial Blood (Pa\(CO_2\)): 35-45 mmHg
      • Venous Blood (Pv\(CO_2\)): 40-46 mmHg
  • Carbon Dioxide in the Blood
    • Dissolved Carbon Dioxide: A small amount of carbon dioxide is dissolved directly in the plasma
      • Approximately 5-10% of total carbon dioxide in the blood
    • Carbamino Compounds: Carbon dioxide binds to amino groups in proteins, primarily hemoglobin
      • Carbaminohemoglobin (Hb\(CO_2\)): Hemoglobin bound to carbon dioxide
      • Approximately 5-10% of total carbon dioxide in the blood
    • Bicarbonate (\(HCO_3^-\)): Most carbon dioxide is transported as bicarbonate ions in the plasma
      • Approximately 80-90% of total carbon dioxide in the blood
  • Carbon Dioxide to Bicarbonate Conversion
    • Reaction: \(CO_2\) + \(H_2O\) ↔︎ \(H_2CO_3\) ↔︎ \(H^+\) + \(HCO_3^-\)
    • Catalyzed by Carbonic Anhydrase: An enzyme present in red blood cells that greatly accelerates the conversion of carbon dioxide to bicarbonate
  • Chloride Shift
    • Process: Bicarbonate ions (\(HCO_3^-\)) diffuse out of red blood cells into the plasma, and chloride ions (\(Cl^-\)) move into the red blood cells to maintain electrical neutrality
    • Reversal: In the lungs, the process is reversed: bicarbonate ions move back into the red blood cells, chloride ions move out, and carbon dioxide is released and exhaled

Key Terms

  • Partial Pressure: The pressure exerted by a single gas in a mixture of gases
  • Hemoglobin: A protein in red blood cells that binds and transports oxygen
  • Oxygen Dissociation Curve: A graph that plots the percentage of hemoglobin saturation against the partial pressure of oxygen
  • P50: The partial pressure of oxygen at which hemoglobin is 50% saturated
  • Bohr Effect: The effect of pH and \(CO_2\) on the oxygen-binding affinity of hemoglobin
  • 2,3-Bisphosphoglycerate (2,3-BPG): A molecule that binds to deoxyhemoglobin and reduces its affinity for oxygen
  • Carbonic Anhydrase: The enzyme that catalyzes the interconversion of carbon dioxide and bicarbonate
  • Chloride Shift: The movement of chloride ions into and out of red blood cells to maintain electrical neutrality during carbon dioxide transport
  • Oxyhemoglobin: Hemoglobin bound to oxygen (Hb\(O_2\))
  • Deoxyhemoglobin: Hemoglobin not bound to oxygen (Hb)
  • Carbaminohemoglobin: Hemoglobin bound to carbon dioxide (Hb\(CO_2\))
  • Normal Saline: Normal Saline (or Saline) is a mixture of sodium chloride (salt) in water. It is used for rinsing wounds, nasal passages, and contact lenses. Intravenous normal saline can be used for fluid replacement in a number of conditions, such as dehydration, hypovolemia, and sepsis
  • Hypovolemia: The medical term for a decrease in blood volume which results in decreased oxygen delivery to the body
  • Sepsis: A life-threatening condition that happens when your body’s response to an infection damages its own tissues and organs
  • Red Blood Cell Saturation: Reflects the percentage of hemoglobin molecules in red blood cells that are bound to oxygen. It is also known as oxygen saturation and is commonly abbreviated as Sa\(O_2\)
  • Hypoxia: The deficiency in the amount of oxygen reaching tissues
  • Hypercapnia: The presence of abnormally high levels of carbon dioxide in the blood