Transport of Gases — Revision Notes

⚡ 30-Second Revision

O\_2 Transport: — 97% by Hb (Oxyhemoglobin), 3% dissolved in plasma.
CO\_2 Transport: — 70% as $HCO\_3\_ -$, 20-25% as Carbaminohemoglobin, 7-10% dissolved in plasma.
Oxygen-Hemoglobin Dissociation Curve (ODC): — Sigmoidal shape due to cooperative binding.
Right Shift (ODC): — $\uparrow PCO\_2$, $\downarrow pH$, $\uparrow \text{Temp}$, $\uparrow 2,3\text{-BPG}$ (O\_2 released).
Left Shift (ODC): — $\downarrow PCO\_2$, $\uparrow pH$, $\downarrow \text{Temp}$, $\downarrow 2,3\text{-BPG}$ (O\_2 bound).
Bohr Effect: — $\uparrow PCO\_2 / \downarrow pH \implies \downarrow \text{Hb-O\_2 affinity}$ (Right shift).
Haldane Effect: — $\uparrow O\_2 \text{ binding to Hb} \implies \downarrow \text{Hb-CO\_2/H\_ + affinity}$.
Carbonic Anhydrase (CA): — Enzyme in RBCs, catalyzes $CO\_2 + H\_2O \rightleftharpoons H\_2CO\_3$.
Chloride Shift: — $HCO\_3\_ - \text{ (out of RBC)} \leftrightarrow Cl\_ - \text{ (into RBC)}$ in tissues.
Reverse Chloride Shift: — $HCO\_3\_ - \text{ (into RBC)} \leftrightarrow Cl\_ - \text{ (out of RBC)}$ in lungs.
CO Poisoning: — CO has 200-250x higher affinity for Hb than O\_2, forms Carboxyhemoglobin (HbCO), causes left shift.

2-Minute Revision

Gas transport is the crucial movement of oxygen (O\_2) from lungs to tissues and carbon dioxide (CO\_2) from tissues to lungs via blood. Oxygen is primarily transported (97%) bound to hemoglobin in red blood cells, forming oxyhemoglobin.

Its binding and release are depicted by the S-shaped oxygen-hemoglobin dissociation curve (ODC). Factors like increased PCO\_2, decreased pH, increased temperature, and increased 2,3-BPG shift the ODC to the right (Bohr effect), promoting O\_2 release in active tissues.

Carbon dioxide is mainly transported (70%) as bicarbonate ions, formed inside red blood cells by the enzyme carbonic anhydrase, followed by the chloride shift where bicarbonate moves out and chloride moves in.

A smaller portion (20-25%) travels as carbaminohemoglobin, and 7-10% is dissolved in plasma. The Haldane effect states that oxygenation of hemoglobin reduces its affinity for CO\_2 and H\_ +, facilitating CO\_2 release in the lungs.

These mechanisms ensure efficient gas exchange and maintain the body's acid-base balance.

5-Minute Revision

The transport of respiratory gases, oxygen (O\_2) and carbon dioxide (CO\_2), is a sophisticated process vital for cellular respiration. Oxygen, absorbed in the lungs, is predominantly transported (approximately 97%) bound to the iron atom of the heme groups in hemoglobin molecules within red blood cells, forming oxyhemoglobin ($HbO\_2$).

The remaining 3% dissolves directly in the blood plasma. The affinity of hemoglobin for oxygen is not constant but is influenced by several factors, which are graphically represented by the oxygen-hemoglobin dissociation curve (ODC).

This curve is sigmoidal due to the cooperative binding of oxygen to hemoglobin.

Factors that decrease hemoglobin's affinity for oxygen, causing the ODC to shift to the right (known as the Bohr effect), include an increase in the partial pressure of carbon dioxide ($PCO\_2$), a decrease in pH (increased $H\_ +$ concentration), an increase in temperature, and an increase in the concentration of 2,3-bisphosphoglycerate (2,3-BPG).

These conditions are characteristic of metabolically active tissues, ensuring efficient oxygen unloading where it's most needed. Conversely, a left shift of the ODC (increased affinity) occurs under opposite conditions, favoring oxygen loading in the lungs.

Carbon dioxide, a metabolic waste product, is transported from tissues to the lungs in three forms. The largest proportion (about 70%) is transported as bicarbonate ions ($HCO\_3\_ -$). This conversion occurs rapidly inside red blood cells, catalyzed by the enzyme carbonic anhydrase, which facilitates the reaction $CO\_2 + H\_2O \rightleftharpoons H\_2CO\_3$.

The carbonic acid then dissociates into $H\_ +$ and $HCO\_3\_ -$. To maintain electrical neutrality, $HCO\_3\_ -$ moves out into the plasma, and chloride ions ($Cl\_ -$) move into the red blood cells, a process called the chloride shift.

Approximately 20-25% of CO\_2 binds to the amino groups of hemoglobin to form carbaminohemoglobin ($HbCO\_2$), and 7-10% dissolves directly in the plasma.

The Haldane effect is another critical aspect of CO\_2 transport: deoxygenated hemoglobin has a higher affinity for CO\_2 and $H\_ +$ than oxygenated hemoglobin. This means that as oxygen is released in the tissues, hemoglobin becomes more effective at picking up CO\_2 and $H\_ +$.

In the lungs, as hemoglobin becomes oxygenated, it releases CO\_2 and $H\_ +$, facilitating their expulsion. These coordinated mechanisms ensure continuous and efficient gas exchange, vital for maintaining cellular function and acid-base balance.

Worked Example: Consider a person exercising vigorously. Their muscle tissues will have: $\uparrow PCO\_2$, $\downarrow pH$ (due to lactic acid), and $\uparrow \text{Temperature}$. According to the Bohr effect, these conditions will shift the ODC to the right, causing hemoglobin to release more oxygen to the active muscles.

Simultaneously, as hemoglobin releases oxygen, it becomes deoxygenated. Due to the Haldane effect, this deoxygenated hemoglobin has a higher capacity to pick up the increased CO\_2 and $H\_ +$ produced by the muscles, efficiently transporting them away.

This interplay optimizes both oxygen delivery and carbon dioxide removal during high metabolic demand.

Prelims Revision Notes

I. Oxygen Transport (O\_2):

Primary Mode (97%): — Bound to Hemoglobin (Hb) in Red Blood Cells (RBCs) as Oxyhemoglobin ($HbO\_2$). Each Hb can carry 4 O\_2 molecules.
Secondary Mode (3%): — Dissolved in blood plasma.
Oxygen-Hemoglobin Dissociation Curve (ODC):

* Shape: Sigmoidal (S-shaped) due to cooperative binding (binding of one O\_2 increases affinity for subsequent O\_2). * Plateau (Lungs): High PO\_2 ($~100\,\text{mmHg}$), high saturation ($~97\%$). Ensures efficient loading. * Steep Portion (Tissues): Lower PO\_2 ($~40\,\text{mmHg}$), significant O\_2 unloading ($~75\%$ saturation in venous blood at rest).

**Factors Affecting ODC (Bohr Effect - Right Shift, $\downarrow$ affinity, $\uparrow$ O\_2 release):**

* $\uparrow PCO\_2$ (Carbon dioxide) * $\downarrow pH$ (Increased acidity, $\uparrow H\_ +$) * $\uparrow \text{Temperature}$ * $\uparrow 2,3\text{-Bisphosphoglycerate (2,3-BPG)}$ (produced in RBCs during glycolysis, stabilizes deoxy-Hb).

Left Shift ($\uparrow$ affinity, $\downarrow$ O\_2 release): — Opposite conditions ($\downarrow PCO\_2$, $\uparrow pH$, $\downarrow \text{Temp}$, $\downarrow 2,3\text{-BPG}$). Favors O\_2 loading in lungs.

II. Carbon Dioxide Transport (CO\_2):

Primary Mode (70%): — As Bicarbonate Ions ($HCO\_3\_ -$).

1. CO\_2 diffuses into RBCs from tissues. 2. Carbonic Anhydrase (CA) enzyme (in RBCs) rapidly catalyzes: $CO\_2 + H\_2O \rightleftharpoons H\_2CO\_3$. 3. $H\_2CO\_3$ dissociates: $H\_2CO\_3 \rightleftharpoons H\_ + + HCO\_3\_ -$. 4. $HCO\_3\_ -$ moves out of RBCs into plasma. 5. Chloride Shift (Hamburger Phenomenon): To maintain electrical neutrality, $Cl\_ -$ moves from plasma into RBCs. 6. $H\_ +$ is buffered by deoxygenated Hb.

Secondary Mode (20-25%): — Bound to amino groups of Hb (not heme) as Carbaminohemoglobin ($HbCO\_2$).
Tertiary Mode (7-10%): — Dissolved in blood plasma (CO\_2 is more soluble than O\_2).
Haldane Effect: — Oxygenation of Hb (in lungs) $\downarrow$ its affinity for CO\_2 and H\_ +, promoting their release. Deoxygenation of Hb (in tissues) $\uparrow$ its affinity for CO\_2 and H\_ +, promoting their uptake. Quantitatively more important for CO\_2 transport than Bohr effect for O\_2 transport.
Reverse Chloride Shift: — In lungs, $HCO\_3\_ -$ re-enters RBCs, $Cl\_ -$ moves out. $H\_ +$ is released from Hb, combines with $HCO\_3\_ -$ to form $H\_2CO\_3$, which then breaks down to $CO\_2 + H\_2O$. CO\_2 diffuses into alveoli.

Vyyuha Quick Recall

CADET, face Right!

This mnemonic helps remember the factors that cause the Oxygen-Hemoglobin Dissociation Curve to shift to the Right (meaning more O\_2 is released to tissues):

C — CO\_2 (Increased PCO\_2)
A — Acid (Decreased pH, increased H\_ +)
D — DPG (Increased 2,3-BPG)
E — Exercise (Increased metabolic activity, leading to all above)
T — Temperature (Increased temperature)

Remember, a 'right shift' is beneficial for active tissues, as it means hemoglobin is 'letting go' of oxygen more easily, exactly where it's needed most.