Oxygen Dissociation Curve

Oxygen Dissociation Curve

• Shows the percentage of hemoglobin (Hb) bound to oxygen (O2) at different partial pressures of oxygen (PO2)
• S-shaped graph with a steep portion between 10 and 60 mmHg, and a flat portion between 70 and 100 mmHg
• Steep portion: Rapid combination of Hb and O2
• Flat portion: Further increase in PO2 induces a slight change in Hb saturation

Clinical Significance of the Flat Portion

• PO2 can fall from 100 mmHg to 60 mmHg, yet Hb saturation remains around 90%
• This plateau acts as a safety zone for loading O2 in the lungs
• Enhances O2 diffusion between alveolar gas and blood as a significant PO2 gradient remains even after most O2 transfer

Clinical Significance of the Steep Portion

• PO2 reductions below 60 mmHg result in a rapid decrease in O2 bound to Hb
• Decreases O2 delivery to tissues

P50

• Partial pressure of O2 at which Hb is 50% saturated
• Normal value: 27 mmHg
• Increase in P50 = Right shift: Lower Hb affinity for O2
• Decrease in P50 = Left shift: Higher Hb affinity for O2

Factors Affecting the Oxygen Dissociation Curve

• pH:
  • Higher hydrogen ion concentration [H+]= Lower pH = Shift to the right: Improves O2 unloading to tissues
  • Lower [H+] = Higher pH = Shift to the left: Improves O2 loading in the lungs
• Temperature:
  • Higher temperature = Shift to the right
  • Lower temperature = Shift to the left: Example - Cyanosis during cold water swimming (Normal PaO2 but Hb doesn't release O2)
• CO2:
  • Higher CO2 = Lower pH = Shift to the right
  • Lower CO2 = Higher pH = Shift to the left: Improves O2 loading in the lungs
• 2,3-Diphosphoglycerate (2,3-DPG):
  • Found in high amounts in red blood cells (RBCs)
  • Formed during anaerobic glycolysis
  • Higher 2,3-DPG = Shift to the right
  • Increased 2,3-DPG in conditions like hypoxia, anemia, low pH
  • Stored blood has low 2,3-DPG, leading to low tissue PO2
• Fetal Hemoglobin (HbF):
  • Higher affinity for O2 compared to adult Hb (HbA)
  • Enhances O2 transfer from maternal blood to fetal blood during development
  • HbF progressively disappears after the first year of life
• Carbon Monoxide Hemoglobin (COHb):
  • Carbon monoxide (CO) has 210 times higher affinity for Hb than O2
  • Small amounts of CO can bind to Hb, preventing O2 binding
  • Shifts the curve to the left, limiting O2 unloading to tissues

Clinical Application of the Oxygen Dissociation Curve

• Shifts to the right or left do not significantly affect Hb's O2 transport ability in the normal PaO2 range of 80-100 mmHg
• These shifts occur on the flat portion of the curve
• Shifts become clinically significant when they occur on the steep portion of the curve

Oxygen Transport

• O2 is transported to the tissues in two forms:
  • Dissolved in plasma
  • Bound to hemoglobin (Hb)

Dissolved Oxygen

• Measured clinically as PO2
• About 0.003 mL of O2 dissolves in 100 mL of blood for every 1 mmHg of PO2 (Solubility coefficient)
• Normal PO2 of 100 mmHg = 0.3 mL of O2 dissolved in every 100 mL of blood

Oxygen Bound to Hemoglobin

• Hemoglobin (Hb):
  • Each RBC contains approximately 280 million Hb molecules
  • Normal HbA consists of:
    • 4 heme groups: Iron-containing non-protein portion
    • 4 amino acid chains that form a globin (protein)
• Iron in heme can reversibly bind to one O2 molecule, forming oxyHb
• Hb (reduced Hb) + O2 = HbO2
• Hb is 100% saturated when 4 O2 molecules are bound to one Hb molecule
• Globin portion of Hb consists of 2 alpha and 2 beta chains
• Changes in the amino acid sequence can alter Hb structure, like in sickle cell Hb (HbS)
• Normal Hb concentration: 14-16 g/dL for males, 12-15 g/dL for females
• Normal infant Hb concentration: 14-20 g/dL

Oxygen Transport Capacity of Hemoglobin

• Each gram of Hb can carry 1.34 mL of O2
• If Hb concentration is 15 g/dL and 100% saturated = 20 vol%
  • HbO2 = 1.34 mL O2/g Hb x 15 g/dL = 20 vol%
• Normal Hb saturation is around 97%
• Small amounts of desaturated Hb can exist in: Thebesian veins, bronchial veins, and underventilated alveoli

Total Oxygen Content

• Measured as the sum of Hb bound O2 and dissolved O2
• CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
• Normal CaO2 = 20 Vol%

Oxygen Transport (DO2)

• A good indicator of cardiopulmonary function
• DO2 = CO x (CaO2 x 10)
• Normal DO2 = 900-1150 mL O2/min

Right Shifts - Loading O2 in the Lungs

• Example: Asthmatic with acute exacerbation, PaO2 of 60 mmHg
• HbO2 is 90% as it leaves the alveoli
• If the curve shifts to the right due to low pH (7.1), HbO2 will be only 75% with the same PO2 of 60 mmHg
• Total O2 delivery may be lower than indicated by PaO2

Right Shifts - Unloading O2 at the Tissues

• Example: Tissue cells metabolize 5 vol% of O2 with a normal curve
• PO2 must fall from 60 mmHg (HbO2 90%) to 35 mmHg to release 5 vol% O2
• With a right shift (pH 7.1), PO2 only needs to fall from 60 mmHg to 40 mmHg to unload the same 5 vol%
• Right shift facilitates oxygen unloading to tissues

Left Shifts - Loading O2 in the Lungs

• Example: Asthmatic with acute exacerbation, PaO2 of 60 mmHg
• HbO2 is 90% as it leaves the alveoli
• If the curve shifts to the left due to high pH (7.6), HbO2 will be 95% with the same PO2 of 60 mmHg

Left Shifts - Unloading O2 at the Tissues

• Example: Tissue cells metabolize 5 vol% of O2 with a normal curve
• PO2 must fall from 60 mmHg (HbO2 90%) to 35 mmHg to release 5 vol% O2
• With a left shift (pH 7.6), PO2 needs to fall from 60 mmHg to 30 mmHg to unload the same 5 vol%
• Left shift hinders oxygen unloading to tissues

Conclusion

The oxygen dissociation curve is a critical tool for understanding how oxygen is transported in the body. Factors such as pH, temperature, CO2, and 2,3-DPG can shift the curve, affecting the efficiency of oxygen loading and unloading. Clinically, understanding these shifts is important for managing patients with respiratory and circulatory disorders.

Oxygen Transport

• Oxygen is transported in the blood in two ways: dissolved in the plasma and bound to hemoglobin.
• Dissolved oxygen:
  • Measured clinically as partial pressure of oxygen (PO2)
  • Approximately 0.003 mL of oxygen dissolves in 100 mL of blood for every 1 mmHg of PO2.
• Hemoglobin:
  • Each red blood cell contains approximately 280 million hemoglobin molecules.
  • Hemoglobin consists of four heme groups (iron-containing non-protein portion) and four amino acid chains (polypeptide) forming a globin (protein).
  • Iron in heme can bind to one oxygen molecule to form oxyhemoglobin.
  • When four oxygen molecules are bound to one hemoglobin molecule, it is considered 100% saturated.
• Total oxygen content:
  • Determined by the amount of oxygen bound to hemoglobin and dissolved oxygen.
  • Calculated as (Hb x 1.34 x SaO2) + (PaO2 x 0.003).

Oxygen Transport (DO2)

• It reflects cardiopulmonary function based on the equation CO x (CaO2 x 10).
• Normal value is 900-1150 mL O2/min.

Oxygen Dissociation Curve

• A graphical illustration showing the percentage of hemoglobin bound to oxygen at each PO2.
• It is 'S' shaped with a steep slope between 10-60 mmHg and a flat portion between 70-100 mmHg.
• Steep portion: rapid combination of hemoglobin and oxygen.
• Flat portion: little change in hemoglobin saturation despite further increase in PO2.

Clinical Importance of the Flat Portion

• Allows for a safety zone for oxygen loading in the lungs.
• Facilitates diffusion as a significant PO2 gradient still exists between alveolar gas and blood, despite oxygen transfer taking place.
• Increasing PO2 beyond 100 mmHg does not significantly increase oxygen carrying capacity.

Clinical Importance of the Steep Portion

• Reduction in PO2 below 60 mmHg causes a rapid decrease in hemoglobin's oxygen-binding capacity, reducing oxygen delivery to tissues.

P50

• The partial pressure of oxygen at which hemoglobin is 50% saturated.
• Normal P50 is 27 mmHg.
• Increase in P50 (shift to the right): low affinity of hemoglobin to oxygen.
• Decrease in P50 (shift to the left): high affinity of hemoglobin to oxygen.

Factors Affecting the Curve

• pH:

  • Low pH (high H+) shifts the curve to the right, enhancing oxygen unloading to tissues.
  • High pH (low H+) shifts the curve to the left, improving oxygen loading in the lungs.

• Temperature:

  • High temperature shifts the curve to the right.
  • Low temperature shifts the curve to the left.

• CO2:

  • High CO2 (low pH) shifts the curve to the right.
  • Low CO2 (high pH) shifts the curve to the left.

• 2,3-Diphosphoglycerate (2,3-DPG):

  • Found in red blood cells, produced during anaerobic glycolysis.
  • Increased 2,3-DPG shifts the curve to the right.
  • Hypoxia, anemia, and low pH increase 2,3-DPG levels.

• Fetal Hemoglobin (HbF):

  • Has a higher affinity for oxygen than adult hemoglobin (HbA).
  • Facilitates oxygen transfer from maternal blood to fetal blood during fetal development.

• Carbon Monoxide Hemoglobin (COHb):

  • Carbon monoxide binds to hemoglobin with 210 times more affinity than oxygen.
  • Small amounts of CO can significantly reduce oxygen binding.
  • Shifts the curve to the left, limiting oxygen unloading to tissues.

Clinical Applications

• When PaO2 is normal (80-100 mmHg), shifts to the right or left do not drastically affect oxygen transport.
• Shifts in this range occur on the flat portion of the curve.
• The problem arises with shifts occurring on the steep portion of the curve.

Right Shifts

• Asthmatic with acute exacerbation: when PaO2 is 60 mmHg, a shift to the right due to low pH reduces HbO2 saturation to 75% instead of 90%.
• If accompanied by decreased cardiac output or reduced hemoglobin, oxygen transport is further compromised.
• Right shifts reduce the pressure necessary to unload oxygen in the tissues.

Left Shifts

• Asthmatic with acute exacerbation and PaO2 of 60 mmHg, a shift to the left due to high pH increases HbO2 saturation to 95% instead of 90%.
• Left shifts increase the pressure required to unload oxygen in the tissues.

This quiz explores the Oxygen Dissociation Curve, highlighting its shape and significance in relation to hemoglobin saturation and partial pressure of oxygen. It delves into the clinical implications of both the steep and flat portions of the curve, as well as the concept of P50. Test your understanding of oxygen transport and its physiological importance.