The Bohr effect

The Bohr effect Learning outcome: To describe and explain the effects of raised carbon dioxide concentrations on the haemoglobin dissociation curve. To learn how carbon dioxide is transported in blood.

What determines the loading and unloading of oxygen by haemoglobin? The amount of oxygen that haemoglobin carries is affected by: The partial pressure of oxygen and The partial pressure of carbon dioxide High pC02 Haemo-globin releases oxygen The presence of a high partial pressure of carbon dioxide causes haemoglobin to release oxygen. This is called the Bohr effect

The Bohr effect * Red cell During respiration, CO2 is produced. This diffuses into the blood plasma and into the red blood cells. Inside the red blood cells are many molecules of an enzyme called carbonic anhydrase *. It catalyses the reaction between CO2 and H2O. plasma H2O * CO2 H2CO3 HCO3- + H+. CO2 + H2O ® H2CO3 carbon dioxide   water carbonic acid HCO3- The resulting carbonic acid then dissociates into HCO3- + H+. (Both reactions are reversible).

The Bohr effect (continued) Haemoglobin very readily combines with hydrogen ions forming haemoglobinic acid. As a consequence haemoglobin releases some of the oxygen it is carrying. By removing hydrogen ions from the solution, haemoglobin helps to maintain the pH of the blood close to neutral. It is acting as a buffer.

The Bohr effect Three Oxygen Dissociation curves illustrating the Bohr Effect. Increased carbon dioxide in the blood causes a right-shift in the curves, such that the haemoglobin more easily unloads the oxygen it is carrying.

Why is the Bohr effect useful? High concentrations of carbon dioxide are found in actively respiring tissues, which need oxygen. Due to the Bohr effect, these high carbon dioxide concentrations cause haemoglobin to release its oxygen even more readily than it would do otherwise.

How is carbon dioxide transported? Carbon dioxide is mostly carried as hydrogencarbonate ions in blood plasma, but also in combination with haemoglobin in red blood cells (carbamino-haemoglobin) and dissolved as carbon dioxide molecules in blood plasma.

Carbon dioxide transport About 5% of the CO2 produced simply dissolves in the blood plasma. Some CO2 diffuses into the red blood cells but instead of forming carbonic acid, attaches directly onto the haemoglobin molecules to form carbaminohaemoglobin. Since the CO2 doesn’t bind to the haem groups the Haemoglobin is still able to pick up O2.   About 85% of the CO2 produced by respiration diffuses into the red blood cells and forms carbonic acid under the control of carbonic anhydrase. The carbonic acid dissociates to produce hydrogencarbonate ions (HCO3-) The HCO3- diffuses out of the red blood cell into the plasma