Transport of Gases — Core Principles

Core Principles

The transport of gases in the human body is a vital process ensuring oxygen delivery to tissues and carbon dioxide removal. Oxygen is primarily transported by haemoglobin within red blood cells, forming oxyhaemoglobin (about 97%), with a small amount dissolved in plasma (about 3%).

The binding and release of oxygen by haemoglobin are influenced by factors like partial pressure of oxygen ($P_{O_2}$), partial pressure of carbon dioxide ($P_{CO_2}$), pH, and temperature, as depicted by the oxygen-haemoglobin dissociation curve.

A right shift in this curve (Bohr effect) indicates decreased oxygen affinity, facilitating release in active tissues. Carbon dioxide, a metabolic waste, is transported in three forms: dissolved in plasma (7-10%), bound to haemoglobin as carbamino-haemoglobin (20-25%), and predominantly as bicarbonate ions (70%).

The conversion of CO\_2 to bicarbonate occurs rapidly in red blood cells, catalyzed by carbonic anhydrase, followed by the chloride shift to maintain electrical neutrality. The Haldane effect, where deoxygenated blood has a higher affinity for CO\_2, further optimizes CO\_2 transport.

These coordinated mechanisms ensure efficient gas exchange between the lungs, blood, and tissues.

Important Differences

vs Oxygen Transport vs. Carbon Dioxide Transport

Aspect	This Topic	Oxygen Transport vs. Carbon Dioxide Transport
Primary Form of Transport	Oxygen: Oxyhaemoglobin (97%)	Carbon Dioxide: Bicarbonate ions (70%)
Role of Haemoglobin	Oxygen: Binds to Fe$^{2+}$ of heme group, forming oxyhaemoglobin.	Carbon Dioxide: Binds to amino groups of globin chains, forming carbamino-haemoglobin (20-25%). Also buffers H$^+$ from CO\_2 dissociation.
Solubility in Plasma	Oxygen: Very low (approx. 3%)	Carbon Dioxide: Higher than O\_2 (approx. 7-10%)
Key Enzyme Involved	Oxygen: No specific enzyme for binding/release.	Carbon Dioxide: Carbonic anhydrase (for $CO_2 + H_2O \rightleftharpoons H_2CO_3$)
Effect of pH/PCO2 (Bohr/Haldane)	Oxygen: Bohr effect - high $P_{CO_2}$/low pH decreases Hb-O\_2 affinity (right shift).	Carbon Dioxide: Haldane effect - deoxygenated Hb increases Hb-CO\_2 affinity and H$^+$ buffering.
Ionic Exchange	Oxygen: No direct ionic exchange for transport.	Carbon Dioxide: Chloride Shift (Cl$^-$ into RBCs as $HCO_3^-$ leaves).

While both oxygen and carbon dioxide rely on blood for transport, their primary mechanisms and the physiological factors influencing them differ significantly. Oxygen is predominantly carried by haemoglobin, binding to its iron atoms, with its release regulated by factors like $P_{CO_2}$ and pH (Bohr effect). Carbon dioxide, conversely, is mainly transported as bicarbonate ions, a process heavily dependent on the enzyme carbonic anhydrase and involving the chloride shift. Haemoglobin also plays a role in CO\_2 transport, both by directly binding CO\_2 and by buffering H$^+$ ions, with its affinity for CO\_2 being influenced by oxygenation status (Haldane effect). These distinct yet interconnected mechanisms ensure efficient gas exchange.