Transport of Oxygen — Core Principles

Core Principles

Oxygen transport is vital for cellular respiration, moving oxygen from the lungs to tissues. In the lungs, high partial pressure of oxygen ($pO_2$) drives oxygen into the blood. The vast majority (97%) of oxygen binds reversibly to hemoglobin within red blood cells, forming oxyhemoglobin.

Each hemoglobin molecule can bind up to four oxygen molecules cooperatively, meaning binding of one oxygen enhances the binding of subsequent ones, leading to the characteristic sigmoidal oxygen-hemoglobin dissociation curve.

In active tissues, lower $pO_2$, higher carbon dioxide ($pCO_2$), increased acidity (lower pH), and elevated temperature cause hemoglobin to release oxygen. This phenomenon, particularly the effect of $pCO_2$ and pH, is known as the Bohr effect, which shifts the curve to the right, favoring oxygen unloading.

Another key factor, 2,3-Bisphosphoglycerate (2,3-BPG), also reduces hemoglobin's oxygen affinity, shifting the curve right, especially in hypoxic conditions. A small fraction (3%) of oxygen is transported dissolved in plasma.

This intricate system ensures precise oxygen delivery to meet varying tissue demands.

Important Differences

vs Transport of Carbon Dioxide

Aspect	This Topic	Transport of Carbon Dioxide
Primary Carrier	Hemoglobin (as oxyhemoglobin)	Bicarbonate ions ($HCO_3^-$), Hemoglobin (as carbaminohemoglobin), Dissolved in plasma
Main Form of Transport	Bound to hemoglobin (97%)	Bicarbonate ions (70%), Carbaminohemoglobin (23%), Dissolved in plasma (7%)
Binding Site on Hemoglobin	Iron ($Fe^{2+}$) in the heme group	Amino groups of globin chains
Factors Favoring Loading (Lungs/Tissues)	High $pO_2$, low $pCO_2$, high pH, low temperature, low 2,3-BPG (Lungs)	High $pCO_2$, low $pO_2$ (Haldane effect) (Tissues)
Factors Favoring Unloading (Tissues/Lungs)	Low $pO_2$, high $pCO_2$, low pH, high temperature, high 2,3-BPG (Tissues - Bohr effect)	Low $pCO_2$, high $pO_2$ (Haldane effect) (Lungs)
Enzyme Involvement	None directly for binding/unbinding to Hb	Carbonic anhydrase (for $CO_2 \rightarrow HCO_3^-$ conversion)

While both oxygen and carbon dioxide are transported by blood, their mechanisms differ significantly. Oxygen is predominantly carried by hemoglobin, binding to the iron in heme groups, with its affinity modulated by factors like $pO_2$, $pCO_2$, pH, and temperature (Bohr effect). Carbon dioxide, conversely, is mainly transported as bicarbonate ions in plasma, with smaller portions carried as carbaminohemoglobin (binding to globin chains) and dissolved in plasma. The transport of each gas influences the other, notably through the Bohr effect for oxygen and the Haldane effect for carbon dioxide, ensuring efficient gas exchange at both pulmonary and tissue levels.