Oxygen and Carbon Dioxide Transport

O2 Carriage: Basics - The O2 Taxi Service

• $O_2$ transported in blood in two forms:
  • Dissolved $O_2$ (~2%): Directly in plasma; $PaO_2 \times \mathbf{0.003}$. Contributes to partial pressure.
  • Bound to Hemoglobin (Hb) (~98%): Main transport mechanism.
• Hemoglobin (Hb):
  • Protein in Red Blood Cells (RBCs); 4 heme groups, each binds one $O_2$ molecule.
  • Normal Hb: M: 13-17 g/dL, F: 12-15 g/dL.
  • $1g \text{ Hb binds } \mathbf{1.34 mL} O_2$.
• $O_2$ Carrying Capacity: Maximum $O_2$ blood can carry if Hb is 100% saturated.
• $O_2$ Content ($CaO_2$): Total $O_2$ in arterial blood.
  • Formula: $CaO_2 = (Hb \times \mathbf{1.34} \times SaO_2/100) + (PaO_2 \times \mathbf{0.003})$.
  • $SaO_2$: % of Hb saturated with $O_2$.
  • $PaO_2$: Partial pressure of $O_2$ in arterial blood.

⭐ Each Hb molecule can bind up to four $O_2$ molecules, exhibiting cooperative binding. The binding of one $O_2$ molecule increases the affinity of Hb for subsequent $O_2$ molecules, facilitating efficient oxygen uptake in the lungs.

O2 Dissociation Curve - Letting Go Gracefully

The Oxygen-Hemoglobin Dissociation Curve (OHDC) is a sigmoid-shaped graph crucial for understanding $O_2$ transport. It plots hemoglobin's $O_2$ saturation against the partial pressure of oxygen ($PO_2$), detailing Hb's $O_2$ binding in lungs and release in tissues.

• P50 Value: The specific $PO_2$ at which hemoglobin is 50% saturated with $O_2$. This value inversely reflects Hb's affinity for oxygen.
  • Normal adult P50: 26.6 mmHg.
  • An ↑P50 indicates ↓ $O_2$ affinity (right shift of the curve), enhancing $O_2$ delivery to tissues.
  • A ↓P50 indicates ↑ $O_2$ affinity (left shift of the curve), promoting $O_2$ retention by Hb.

The mermaid diagram below details factors causing these shifts:

⭐ The OHDC's sigmoid shape is vital: its steep portion allows for substantial $O_2$ unloading to tissues with minor $PO_2$ drops, while the flat upper portion ensures high $O_2$ saturation in lungs despite $PO_2$ fluctuations.

CO2 Transport Mechanisms - The Waste Disposal Unit

• CO2 is transported in blood in three forms:

  • As Bicarbonate ions ($HCO_3^-$): ~70%
    • $CO_2 + H_2O \xrightarrow{\text{Carbonic Anhydrase (in RBC)}} H_2CO_3 \leftrightarrow H^+ + HCO_3^-$
    • $HCO_3^-$ moves from RBC to plasma.
  • As Carbaminohemoglobin: ~23%
    • $CO_2$ binds to globin part of Hb: $CO_2 + Hb \cdot NH_2 \leftrightarrow Hb \cdot NH \cdot COOH$
    • Favored by deoxygenated Hb.
  • Dissolved $CO_2$: ~7%
    • Directly in plasma, contributes to $PCO_2$.

• Chloride Shift (Hamburger's Phenomenon) 📌

  • To maintain electrical neutrality, $Cl^-$ enters RBC as $HCO_3^-$ exits.
  • Occurs in tissues; reverse process in lungs.

• Haldane Effect
  • Deoxygenated Hb has a higher affinity for $CO_2$ and is a better buffer for $H^+$.
  • Oxygenation of Hb in lungs (↑ $PO_2$) promotes $CO_2$ release.

  ⭐ Haldane Effect: Deoxygenation of hemoglobin (e.g., in tissues) ↑ its ability to carry $CO_2$; conversely, oxygenated Hb (e.g., in lungs) has a ↓ affinity for $CO_2$ and releases it.

High‑Yield Points - ⚡ Biggest Takeaways

• Oxygen primarily binds hemoglobin (97%); 3% dissolved.
• Oxygen-Hb curve right shift (↑O₂ release): ↑PCO₂, ↑H⁺ (Bohr effect), ↑2,3-DPG, ↑Temp.
• CO₂ transport: Mainly bicarbonate (70%), carbaminoHb (23%), dissolved CO₂ (7%).
• Haldane effect: Deoxy-Hb has ↑affinity for CO₂ & H⁺, aiding CO₂ tissue loading.
• Chloride shift: HCO₃⁻ exits RBCs in tissues; Cl⁻ influx maintains electroneutrality.
• P₅₀ (~26.6 mmHg) reflects Hb-O₂ affinity; ↑P₅₀ means ↓affinity.