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The Chemistry of Blood

Key Concepts

Oxygen is transported around the body in blood by the complex molecule haemoglogin (hemoglobin), a globular protein which has a central iron atom.
When haemoglobin (hemoglobin) reacts with oxygen, oxyhaemoglobin (oxyhemoglobin) is formed
The oxygenation of blood is an equilibrium reaction
The pH of blood is maintained at around 7.4 by a series of buffer systems
Acidosis occurs when the pH of blood falls below 7.4
Alkalosis occurs when the pH of blood rises above 7.4
Carbon monoxide, CO, also readily reacts with haemoglobin (hemoglobin) which can result in carbon monoxide poisoning

Oxygen Transport

The equilibrium reaction for the transport of oxygen by haemoglobin (Hb) can be represented as:

Hb(aq) + O₂(aq)

HbO₂(aq)

Hb₄(aq) + 4O₂(aq) Hb₄O₈(aq)

In the lungs where there is a high concentration of oxygen, by Le Chatelier's principle the equilibrium position shifts to the right resulting in the formation of oxyhaemoglobin (oxyhemoglobin).

In tissues the concentration of oxygen is lower, so the equilibrium position shifts to the left, by Le Chatelier's principle. Oxyhaemoglobin releases oxygen to re-form haemoglobin.

Buffering in Blood

Haemoglobin (hemoglobin) is involved in series of equilibrium reactions, the overall net result of these reactions is the equilibrium:

HbH⁺(aq) + O₂(aq)

HbO₂(aq) + H⁺(aq)

Ordinary metabolic reactions result in the formation of several acids. This increases the concentration of H⁺, lowering the pH, and forces the equilibrium position to the left by Le Chatelier's principle. This is known as acidosis. The resulting reduction in the supply of oxygen to the cells causes fatigue and headaches.
Acidosis can occur temporarily during strenuous exercise because the demand for energy can exceed the supply of available oxygen to complete the oxidation of glucose to carbon dioxide. The glucose is converted to the acidic metabolism product lactic acid.

The most important extracellular system in the blood is the bicarbonate system
(H₂CO₃/HCO₃^-) which can be represented by:

CO₂(aq) + 2H₂O(l)

H₂CO₃(aq) + H₂O(l)

H₃O⁺(aq) + HCO₃^-(aq)

In tissue cells there is a higher concentration of carbon dioxide as a result of the respiration reaction so the equilibrium position moves to the right by Le Chatelier's principle. In the lungs there is a lower concentration of carbon dioxide so the equilibrium position moves to the left, by Le Chatelier's principle, and the carbon dioxide is released.

During hyperventilation, which is caused by rapid breathing, there is a reduction in the concentration of carbon dioxide so the equilibrium position shifts to the left by Le Chatelier's principle, decreasing the concentration of hydrogen ions and raising the pH. This is known as alkolisis. By inhaling and exhaling into a paper bag, the concentration of carbon dioxide is increased and the equilibrium position shifts to the right by Le Chatelier's principle.

Carbon monoxide poisoning

Carbon monoxide (CO) reacts with haemoglobin in the same way as oxygen:

Hb(aq) + CO(aq)

HbCO(aq)

Hb₄(aq) + 4CO(aq) Hb₄(CO)₄(aq)

The equilibrium constant for this reaction is very large, about 200 times greater than that for the oxygen reaction, so that there is very little haemoglobin left to react with oxygen. The tissue cells will be starved of oxygen because carbon monoxide rather than oxygen is being transported.

Because the equlibrium constant for the carbon monoxide reaction is so large, low concentrations of carbon monoxide can be harmful. For example, 50ppm CO causes deterioration in motor skills, 100ppm causes headaches and drowsiness, and, 500ppm for an hour can be fatal.

Because the reaction is reversible, carbon monoxide poisoning can be counteracted, if caught earlier enough, just by providing sufficient oxygen.