The Bohr effect describes hemoglobin’s lower affinity for oxygen with respect to an increase in the partial pressure of carbon dioxide and a decrease in blood pH. This lower affinity, in turn, enhances the unloading of oxygen into tissues to meet the oxygen demand of the tissue. To simply state, The Bohr effect describes the decrease in the oxygen affinity of hemoglobin in the presence of low blood pH or high CO2 environment. 

In a very similar yet opposite context, the Haldane effect describes how Oxygen’s concentration in the environment determines hemoglobin’s affinity to Carbon dioxide.

PHENOMENON – BOHR EFFECT

The phenomenon was first described in 1904 by the Danish physiologist Christian Bohr; hence the name Bohr effect. According to Bohr, Hemoglobin has a natural affinity towards binding with oxygen. Its affinity is inversely related both to the acidity and to the concentration of carbon dioxide. This acts as a right-ward shift in the Oxygen and Hemoglobin Dissociation Curve as described in Oxygen Transport.

The Bohr effect refers to the shift in the oxygen dissociation curve. The shift is caused by the changes in the concentration of carbon dioxide or the pH of the environment. Since carbon dioxide reacts with water to form carbonic acid, an increase in carbon dioxide results in a corresponding decrease in blood pH as well. This decrease in pH results in hemoglobin proteins to release their load of oxygen. A decrease in carbon dioxide enhances an increase in pH, which results in hemoglobin picking up more oxygen.

HISTORICAL REFERENCE

In the early 1900s, Christian Bohr was a professor at the University of Copenhagen in Denmark. He is already well known for his work in the field of respiratory physiology. He had spent the last two decades while studying the solubility of oxygen, carbon dioxide, and other gases in various liquids. In the league, he also conducted extensive research work on hemoglobin and its affinity for oxygen.

Furthermore, experimentation with varying levels of the CO2 concentration quickly provided some evidence as well. Thus, it was confirmed that the Bohr effect does exist. The Bohr effect increases the efficiency of oxygen to be transported through the blood.

MECHANISM – BOHR EFFECT

When hemoglobin binds to oxygen in the lungs due to the high oxygen concentrations, the Bohr effect eases its release in the tissues. This is more pronounced in those tissues in which most oxygen is needed.

When the metabolic rate of a tissue increases, its carbon dioxide waste production also increases. When released into the bloodstream, carbon dioxide forms bicarbonate and protons. Although this reaction is seen to have proceeded very slowly. The enzyme carbonic anhydrase speeds up the conversion to bicarbonate and protons. Carbonic anhydrase is present in red blood cells.

BLOOD pH CHANGES

The above conversion causes the pH of the blood to decrease. The decreased pH, in turn, promotes the dissociation of oxygen from hemoglobin. It allows the surrounding tissues to take up enough oxygen to meet their demands. In those areas where oxygen concentration is high, (such as the lungs) binding of oxygen causes hemoglobin to release protons. These protons recombine with bicarbonate to give out carbon dioxide during the exhalation process. These activities of protonation and deprotonation occur in an equilibrium which results in a little overall change in blood pH.

The Bohr effect enables the body to adapt to the changing conditions. It makes it possible to supply extra oxygen to those tissues which need it the most. When muscles undergo any strenuous activity, they require large amounts of oxygen to perform cellular respiration. This generates CO2 and then HCO3− and H+ as byproducts. These waste products lower the blood pH which increases oxygen delivery to the active muscles.

Carbon dioxide is not the only molecule that can cause the Bohr effect to occur. If muscle cells are not receiving enough oxygen for cellular respiration, they form lactic acid. The release of lactic acid as a byproduct increases the acidity of the blood more than CO2 alone could do. Under anaerobic conditions, muscles generate lactic acid very quickly. It is so quick that the pH of the blood passing through the muscles drops to around 7.2.

SIGNIFICANCE – BOHR EFFECT

The Bohr Effect allows increased unloading of oxygen in metabolically active tissues such as exercising muscle. Increased muscle activity results in an increase in the partial pressure of carbon dioxide. This reduces the local blood pH. Due to the Bohr Effect, this results in an increased supply of bound oxygen. Thus it improves oxygen delivery.

The Bohr Effect enhances oxygen delivery in proportion to the metabolic activity of the tissue. As more metabolism takes place, the partial pressure of carbon dioxide increases. Thus, causing much larger reductions in blood pH and in turn allowing for greater oxygen unloading. This is especially true in exercising skeletal muscles which may also release lactic acid. The lactic acid could further reduce blood pH and thus enhance the Bohr Effect.

SPECIAL CASES WITH BOHR EFFECT

A special case of the Bohr effect occurs when Carbon Monoxide (CO) is present. This molecule serves as a competitive inhibitor for oxygen. Carbon monoxide binds with hemoglobin to form carboxyhemoglobin. Haemoglobin’s affinity for carbon monoxide (CO) is about 250 times stronger than for oxygen. This means that once associated, it is very hard to dissociate it. Once CO gets bound with oxygen, it blocks the binding of oxygen to other subunits as well.

Carbon monoxide is structurally very similar to oxygen. It is so similar that it can cause carboxyhemoglobin to raise the oxygen affinity of all the remaining unoccupied subunits. This combination can significantly reduce the delivery of oxygen to the tissues of the body. The above properties make carbon monoxide extremely toxic. This toxicity is reduced slightly with an increase in the strength of the Bohr effect in the presence of carboxyhemoglobin.

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