Transport of Carbon dioxide — Revision Notes

⚡ 30-Second Revision

7% $CO_2$ — Dissolved in plasma.
20-25% $CO_2$ — As Carbaminohemoglobin ($CO_2$ binds to globin's amino groups).
70% $CO_2$ — As Bicarbonate ions ($HCO_3^-$).
Carbonic Anhydrase (CA) — Enzyme in RBCs, catalyzes $CO_2 + H_2O \rightleftharpoons H_2CO_3$.
Chloride Shift (Hamburger Phenomenon) — $HCO_3^-$ out of RBC, $Cl^-$ into RBC (in tissues) to maintain electrical neutrality.
Haldane Effect — $O_2$ binding to Hb in lungs decreases Hb's affinity for $CO_2$ and $H^+$, promoting their release. Deoxygenation in tissues increases affinity for $CO_2$ and $H^+$.
Bohr Effect — High $P_{CO_2}$/low pH in tissues promotes $O_2$ release from Hb.
Buffering — Hemoglobin buffers $H^+$ ions produced from $H_2CO_3$ dissociation.

2-Minute Revision

Carbon dioxide, a metabolic waste product, is transported from tissues to lungs via three main routes. A minor fraction (about 7%) travels dissolved in plasma. Another 20-25% binds to the amino groups of hemoglobin, forming carbaminohemoglobin.

The most significant portion (around 70%) is transported as bicarbonate ions. This process begins in red blood cells, where carbonic anhydrase rapidly converts $CO_2$ and water into carbonic acid ($H_2CO_3$).

This unstable acid then dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$). The $H^+$ ions are buffered by hemoglobin, preventing pH changes. The $HCO_3^-$ ions move out into the plasma, and chloride ions ($Cl^-$) move into the red blood cells to maintain electrical balance – this is the chloride shift.

In the lungs, these processes reverse: $HCO_3^-$ re-enters RBCs, combines with $H^+$ (released from hemoglobin due to oxygen binding, known as the Haldane effect), forms $H_2CO_3$, which is then converted back to $CO_2$ by carbonic anhydrase for exhalation.

The Haldane effect is crucial as it ensures efficient $CO_2$ release where $O_2$ is picked up.

5-Minute Revision

The transport of carbon dioxide ($CO_2$) is a vital physiological process ensuring the removal of metabolic waste and maintenance of blood pH. $CO_2$ is transported in three forms:

1
Dissolved in Plasma (7%): — A small amount of $CO_2$ simply dissolves in the plasma. This dissolved $CO_2$ contributes to the $P_{CO_2}$ of the blood.
2
As Carbaminohemoglobin (20-25%): — $CO_2$ binds reversibly to the amino groups of the globin chains of hemoglobin ($Hb$) within red blood cells, forming carbaminohemoglobin ($HbCO_2$). This binding is favored in tissues (high $P_{CO_2}$, low $P_{O_2}$) and reversed in the lungs (low $P_{CO_2}$, high $P_{O_2}$). The Haldane effect plays a key role here: oxygenation of hemoglobin in the lungs reduces its affinity for $CO_2$, promoting $CO_2$ release.
3
As Bicarbonate Ions (70%): — This is the most significant mechanism. In the tissues, $CO_2$ diffuses into red blood cells. Inside, the enzyme carbonic anhydrase (CA) rapidly catalyzes the reaction:

$$CO_2 + H_2O \xrightarrow{CA} H_2CO_3$$

Carbonic acid ($H_2CO_3$) is unstable and immediately dissociates:

$$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$

The **hydrogen ions ($H^+$)** are buffered by deoxygenated hemoglobin, preventing a significant drop in intracellular pH.

The **bicarbonate ions ($HCO_3^-$) then diffuse out of the red blood cell into the plasma. To maintain electrical neutrality, chloride ions ($Cl^-$)** move from the plasma into the red blood cell.

This exchange is known as the chloride shift or Hamburger phenomenon.

In the Lungs: These processes are reversed. As $CO_2$ diffuses into the alveoli (due to low alveolar $P_{CO_2}$), the $P_{CO_2}$ in the red blood cells drops. This causes $HCO_3^-$ to re-enter the red blood cells (reverse chloride shift) and combine with $H^+$ (released from hemoglobin as it binds $O_2$ due to the Haldane effect) to reform $H_2CO_3$. Carbonic anhydrase then converts $H_2CO_3$ back into $CO_2$ and $H_2O$, allowing $CO_2$ to be exhaled.

Example: When $100,\text{mL}$ of deoxygenated blood picks up $CO_2$ from tissues, approximately $7,\text{mL}$ of $CO_2$ is transported. Of this, about $0.5,\text{mL}$ is dissolved, $1.5,\text{mL}$ as carbaminohemoglobin, and $5,\text{mL}$ as bicarbonate.

Prelims Revision Notes

1
**Forms of $CO_2$ Transport:**

* Dissolved in Plasma: ~7% of $CO_2$. Directly contributes to $P_{CO_2}$. * **Carbaminohemoglobin ($HbCO_2$):** ~20-25% of $CO_2$. $CO_2$ binds to amino groups of globin chains (not heme iron). Favored by high $P_{CO_2}$ and low $P_{O_2}$ (tissues). * **Bicarbonate Ions ($HCO_3^-$):** ~70% of $CO_2$. Most significant method.

1
Bicarbonate Formation (in Tissues):

* $CO_2$ diffuses from tissues into RBCs. * Carbonic Anhydrase (CA): Enzyme in RBCs rapidly catalyzes $CO_2 + H_2O \rightleftharpoons H_2CO_3$. * $H_2CO_3$ dissociates: $H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$.

* **Hydrogen Ions ($H^+$):** Buffered by deoxygenated hemoglobin ($Hb$). $Hb + H^+ \rightleftharpoons HHb$. This prevents acidosis. * **Bicarbonate Ions ($HCO_3^-$):** Diffuse out of RBCs into plasma.

* Chloride Shift (Hamburger Phenomenon): To maintain electrical neutrality, $Cl^-$ ions move from plasma into RBCs as $HCO_3^-$ moves out.

1
$CO_2$ Release (in Lungs):**

* Low alveolar $P_{CO_2}$ causes $CO_2$ to diffuse from blood to alveoli. * Reverse Chloride Shift: $HCO_3^-$ re-enters RBCs from plasma (in exchange for $Cl^-$ moving out). * $H^+$ (released from $Hb$ as it binds $O_2$) combines with $HCO_3^-$ to form $H_2CO_3$. * Carbonic Anhydrase converts $H_2CO_3$ back to $CO_2 + H_2O$. * $CO_2$ diffuses out of RBCs, into plasma, then into alveoli for exhalation. * Carbaminohemoglobin dissociates, releasing $CO_2$.

1
Key Effects:

* Haldane Effect: Oxygenation of hemoglobin (in lungs) decreases its affinity for $CO_2$ and $H^+$, promoting their release. Deoxygenation (in tissues) increases affinity for $CO_2$ and $H^+$. * Bohr Effect: High $P_{CO_2}$ and $H^+$ (low pH) in tissues decrease hemoglobin's affinity for $O_2$, promoting $O_2$ release.

1
Quantitative Data: — $100,\text{mL}$ of deoxygenated blood transports approx. $4,\text{mL}$ of $CO_2$ from tissues to lungs.

Vyyuha Quick Recall

Carbon Dioxide Transport: Be Calm, Don't Hurry!

Bicarbonate (70%)
Carbaminohemoglobin (20-25%)
Dissolved in plasma (7%)
Haldane effect (O2 affects CO2)
Hamburger phenomenon (Chloride Shift)