Transport of Gases — Revision Notes

⚡ 30-Second Revision

Oxygen Transport: — \~97% as Oxyhaemoglobin ($HbO_2$), \~3% dissolved in plasma.
Haemoglobin: — 4 heme groups, each binds 1 $O_2$. Cooperative binding.
ODC (Oxygen Dissociation Curve): — Sigmoid shape. Right shift = \downarrow O_2 affinity (favors O_2 release).
Factors for Right Shift (Bohr Effect): — \uparrow P_{CO_2}, \uparrow H^+ (\downarrow pH), \uparrow Temp, \uparrow 2,3-BPG.
Carbon Dioxide Transport: — \~70% as Bicarbonate ions ($HCO_3^-$), \~20-25% as Carbamino-haemoglobin ($HbCO_2$), \~7-10% dissolved in plasma.
Carbonic Anhydrase: — Enzyme in RBCs: $CO_2 + H_2O \rightleftharpoons H_2CO_3$.
Chloride Shift: — $HCO_3^-$ out of RBCs, $Cl^-$ into RBCs (at tissues) to maintain electrical neutrality.
Haldane Effect: — Deoxygenated Hb has \uparrow affinity for $CO_2$ and $H^+$ (favors $CO_2$ uptake).

2-Minute Revision

Efficient transport of oxygen and carbon dioxide is vital for cellular function. Oxygen is primarily transported from the lungs to tissues as oxyhaemoglobin, formed by oxygen binding to haemoglobin in red blood cells.

A small amount dissolves in plasma. The oxygen-haemoglobin dissociation curve, which is S-shaped, illustrates this binding. Factors like high $P_{CO_2}$, low pH, and high temperature (conditions in active tissues) shift this curve to the right (Bohr effect), decreasing haemoglobin's oxygen affinity and promoting oxygen release where it's needed.

Carbon dioxide, a metabolic waste, is mainly transported from tissues to lungs as bicarbonate ions (about 70%). This conversion occurs rapidly in red blood cells, catalyzed by carbonic anhydrase. Bicarbonate ions then move into the plasma, with chloride ions entering RBCs to maintain electrical balance (Chloride Shift).

Smaller amounts of CO\_2 are transported as carbamino-haemoglobin and dissolved in plasma. The Haldane effect ensures that deoxygenated haemoglobin in tissues has a higher capacity to pick up CO\_2, while oxygenated haemoglobin in the lungs releases CO\_2 more readily.

These coordinated mechanisms ensure continuous gas exchange.

5-Minute Revision

The transport of respiratory gases, oxygen (O\_2) and carbon dioxide (CO\_2), is a sophisticated physiological process ensuring cellular respiration and waste removal. Oxygen is transported predominantly (about 97%) bound to haemoglobin within red blood cells, forming oxyhaemoglobin ($HbO_2$).

Each haemoglobin molecule can bind four oxygen molecules cooperatively, meaning binding of one O\_2 enhances the affinity for subsequent O\_2 molecules. The remaining 3% of oxygen is transported dissolved in plasma.

The relationship between $P_{O_2}$ and haemoglobin saturation is depicted by the sigmoid-shaped oxygen-haemoglobin dissociation curve. At the lungs, high $P_{O_2}$ ensures nearly complete saturation. In the tissues, lower $P_{O_2}$ facilitates O\_2 release.

Haemoglobin's affinity for oxygen is modulated by several factors, collectively influencing the ODC's shift. The Bohr effect describes how increased $P_{CO_2}$, increased H$^+$ concentration (decreased pH), and increased temperature shift the ODC to the right, decreasing O\_2 affinity and promoting its release in active tissues.

Conversely, decreased $P_{CO_2}$, decreased H$^+$, and decreased temperature shift the curve left, increasing O\_2 affinity, which is beneficial for O\_2 loading in the lungs. 2,3-BPG also reduces O\_2 affinity, causing a right shift.

Carbon dioxide is transported in three forms: about 7-10% dissolved in plasma, 20-25% as carbamino-haemoglobin ($HbCO_2$) by binding to the globin chains of Hb, and the major portion (about 70%) as bicarbonate ions ($HCO_3^-$).

In red blood cells, CO\_2 rapidly reacts with water to form carbonic acid ($H_2CO_3$), catalyzed by the enzyme carbonic anhydrase. $H_2CO_3$ then dissociates into H$^+$ and $HCO_3^-$. The H$^+$ ions are buffered by haemoglobin, preventing significant pH changes.

The $HCO_3^-$ ions diffuse out into the plasma, and to maintain electrical neutrality, chloride ions ($Cl^-$) move into the red blood cells from the plasma; this is the Chloride Shift. In the lungs, these processes reverse, allowing CO\_2 to be reformed and exhaled.

The Haldane effect states that deoxygenated haemoglobin has a higher affinity for CO\_2 and H$^+$, thus enhancing CO\_2 uptake in tissues and release in the lungs. These effects are crucial for efficient and coordinated gas exchange.

Prelims Revision Notes

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Oxygen Transport:

* Major Form (97%): Oxyhaemoglobin ($HbO_2$) - O\_2 binds to Fe$^{2+}$ of heme in red blood cells. * Minor Form (3%): Dissolved in plasma. * Haemoglobin: Tetramer, 4 O\_2 molecules per Hb.

Cooperative binding (sigmoid ODC). * Oxygen-Haemoglobin Dissociation Curve (ODC): S-shaped graph of $P_{O_2}$ vs. % Hb saturation. * Right Shift (\downarrow O\_2 affinity, \uparrow O\_2 release): Caused by \uparrow $P_{CO_2}$, \uparrow H$^+$ (\downarrow pH), \uparrow Temp, \uparrow 2,3-BPG.

(Bohr Effect) * Left Shift (\uparrow O\_2 affinity, \downarrow O\_2 release): Caused by \downarrow $P_{CO_2}$, \downarrow H$^+$ (\uparrow pH), \downarrow Temp, \downarrow 2,3-BPG. * Physiological Significance: Right shift in tissues (active, acidic, hot) ensures O\_2 delivery; Left shift in lungs (alkaline, cool) ensures O\_2 loading.

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Carbon Dioxide Transport:

* Major Form (70%): Bicarbonate ions ($HCO_3^-$) in plasma. * Intermediate Form (20-25%): Carbamino-haemoglobin ($HbCO_2$) - CO\_2 binds to amino groups of globin chains. * Minor Form (7-10%): Dissolved in plasma (CO\_2 is more soluble than O\_2).

* Key Enzyme: Carbonic Anhydrase (in RBCs) rapidly catalyzes $CO_2 + H_2O \rightleftharpoons H_2CO_3$. * Bicarbonate Formation: $H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$. * **H$^+$ Buffering:** H$^+$ ions are buffered by haemoglobin (prevents pH drop).

* Chloride Shift (Hamburger Phenomenon): At tissues, $HCO_3^-$ moves out of RBCs into plasma; $Cl^-$ moves into RBCs to maintain electrical neutrality. * Reverse Chloride Shift: At lungs, $HCO_3^-$ moves into RBCs; $Cl^-$ moves out.

* Haldane Effect: Deoxygenated haemoglobin has a higher affinity for CO\_2 and H$^+$. This facilitates CO\_2 uptake in tissues and its release in lungs (as O\_2 binds to Hb).

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Coordination: — Bohr and Haldane effects are reciprocal and synergistic, optimizing O\_2 unloading and CO\_2 loading in tissues, and vice versa in lungs.

Vyyuha Quick Recall

Bright Curve To Help People Deliver Oxygen: Bohr effect causes Right Curve shift due to Temperature (increase), Hydrogen ions (increase), PCO2 (increase), DPG (increase) to Deliver Oxygen to tissues.