Transport of Carbon Dioxide from Tissue (e.g., Pig’s Big Toe) to Lungs

Context & Example Tissue

• Instructor uses the pig’s big toe as an example because it is one of the tissues farthest from the lungs (long transport path).
  • Clarifies that the big toe is not the most metabolically active tissue, but it still produces CO$_2$ via cellular respiration just like any other tissue.

Three Principal Forms of CO$_2$ Transport

• Directly dissolved in plasma
  • CO$2$ is more soluble in water than O$2$ but still limited.
  • Accounts for 7\% of total CO$_2$ transport.
• Bound to hemoglobin (Hb)
  • Forms a carbamino-hemoglobin complex.
  • Binds NOT to the heme iron (O$_2$ site) but to a separate amino-terminal site on the globin chains.
  • Represents 23\% of CO$_2$ transport.
• As bicarbonate (HCO$_3^-$) inside plasma
  • The major pathway: 70\% of CO$2$ converted to HCO$3^-$.
  • Requires red-blood-cell (RBC) enzymes & membrane transport (chloride shift).
  • • Overall split once CO$_2$ leaves tissue & enters blood
  • Plasma-dissolved: 7\%
  • CO$2$ entering RBC: 93\% • Of that 93 %, 23 % binds Hb, 70 % converted to HCO$3^-$.

Chemistry Inside the RBC

• Initial reaction
  CO2 + H2O \xrightarrow{\text{carbonic anhydrase}} H2CO3 \rightleftharpoons H^+ + HCO_3^-

  • Catalyzed by carbonic anhydrase present in RBC cytosol.
  • Newly formed H$^+$ & HCO$_3^- $ immediately separate.

• Hemoglobin as a buffer

  • Free H$^+$ would acidify the cell; Hb binds it:
    Hb + H^+ \rightarrow HHb
  • This buffering ability keeps intracellular pH stable.

• Chloride Shift (Hamburger phenomenon)

  • HCO$_3^- $ exits the RBC in exchange for Cl$^-$ entering.
  • Maintains electroneutrality because negative charge leaving is replaced by another negative charge entering.
  • Instructor illustrates plasma as a pink region and RBC as black circles in slides.

CO$_2$ Unloading in the Lungs (Step-by-Step)

1. Plasma-dissolved CO$_2$ (the initial 7\%) diffuses down its partial-pressure gradient into alveoli ➔ exhaled first.
2. Loss of CO$2$ lowers plasma PCO$2$ ➔ CO$_2$ bound to Hb has reduced affinity and dissociates:
   • Orange-coded CO$_2$ in slide detaches from Hb.
   • Moves into plasma, then alveoli, then is exhaled.
3. Once CO$_2$ unbinds, H$^+$ also has lower affinity for Hb; it dissociates.
4. Reverse chloride shift brings HCO$_3^- $ back into the RBC in exchange for Cl$^-$ leaving.
5. HCO$3^- $ + H$^+$ reform H$2$CO$3$, which quickly converts to H$2$O + CO$_2$.
6. Newly formed CO$_2$ again diffuses to plasma ➔ alveoli ➔ exhaled.

Key Numerical Summary (Quick Reference)

• Total CO$2$ leaving tissues = 100\% • Plasma-dissolved: 7\% • RBC uptake: 93\% – Bound to Hb: 23\% of total (≈¼ of RBC-CO$2$)
  – Converted to HCO$3^- $: 70\% of total (≈¾ of RBC-CO$2$)

Ancillary/Connecting Concepts

• Buffers discussed earlier in course
  • Phosphate buffer system.
  • Bicarbonate buffer system (focus today).
  • Hemoglobin itself functions as a protein buffer.
• Practical relevance
  • Explains why arterial vs. venous blood pH differ little despite large CO$_2$ movements.
  • Clinically leveraged in interpreting arterial blood gases (ABGs).
• Physiological principle: Gas transport driven by partial-pressure gradients.
• Transport of O$_2$ (previous lecture) is more straightforward—serves as contrast.

Instructor/Slide Notes & Meta-Comments

• Slide sequence labeled steps 1 → 8, but numbering becomes inconsistent mid-way; final slide realigns them.
• Color-coding (green, orange, etc.) on slides to trace individual CO$_2$ molecules.
• Repetition of statement: “CO$2$ is CO$2$ wherever it is; colors are just visual aids.”