Buffer Systems of Blood | Biochemistry

In this article we will discuss about:- 1. Introduction to  Buffer Systems of Blood 2. Hemoglobin Buffers 3. Chloride Shift.

Introduction to  Buffer Systems of Blood:

1. Venous blood carries more CO₂ than arte­rial blood. Hence, the pH of venous blood is more acid than that of arterial blood by 0.01-0.03 units i.e. pH 7.40 and 7.43, re­spectively.

2. The blood buffers consists of the plasma proteins, hemoglobin, oxy-hemoglobin, bicarbonates and inorganic phosphates.

3. When CO₂ enters the venous blood, the small decrease in pH shifts the ratio of acid to salt in all the buffer pairs. When the ratio is shifted to form more of the acid, cations become available to form addi­tional bicarbonates. In this respect, the plasma phosphates and bicarbonates play a minor role.

4. The buffering action of the plasma pro­teins is important because they release sufficient cations for the carriage of about 10% of the total CO₂.

5. The phosphates in the red cells carry 25% of the total CO₂.

6. The most important buffering role is of hemoglobin and oxy-hemoglobin which carry 60% of the CO₂ of the whole blood.

Hemoglobin Buffers:

At the lungs the formation of oxy-hemoglobin from reduced hemoglobin releases hydrogen ions which react with bicarbonate to form carbonic acid. The low CO₂ tension in the lung shifts the equilib­rium towards the production of CO₂ which is con­tinually eliminated in the expired air:

H⁺ + HCO₃ ↔ H₂CO₃ ↔ H₂O + CO₂

In the tissues, the oxygen tension is reduced and hence oxy-hemoglobin dissociates delivering O₂ to the cells and reduced hemoglobin is formed. CO₂ produced by metabolism enters the blood, where it is hydrated to form H₂CO₃ which ionizes to form H⁺ and HCO₃^–.

Reduced hemoglobin acting as an anion accepts the H⁺ ions forming acid-reduced hemoglobin (HHb). Very little change in pH occurs because the newly arrived H’ ions are buffered by formation of very weak acid.

When the blood returns to the lungs, these H⁺ ions are released as a result of the formation of stronger acid (oxy-hemoglobin) and the newly re­leased H⁺ ion is promptly neutralized by HCO₃^–. This reaction is necessary for the liberation of CO₂ in the lungs.

Chloride Shift:

1. CO₂ reacts with water to form carbonic acid (H₂CO₃), mainly inside the red cell by the enzyme carbonic anhydrase present in the red cells.

2. The carbonic acid is then buffered by the intracellular buffers (Phosphate and hemoglobin) combining with potassium.

3. Bicarbonate ion also returns to the plasma and exchanges with chloride which shifts into the cell when the tension of CO₂ in­creases in the blood.

4. When the CO₂ tension is reduced, chlo­ride leaves the cell and enters the plasma.

5. Under normal conditions the red cell is impermeable to sodium or potassium. But it is permeable to hydrogen, bicarbonate and chloride ions and intracellular sources of cation (potassium) are indirectly avail­able to the plasma by chloride (anion) ex­change. This permits the carriage of addi­tional CO₂ (as sodium bicarbonate) by plasma.

6. The CO₂ entering the blood from the tis­sues passes into the red cells where it forms carbonic acid by carbonic anhydrase. Some of the carbonic acid returns to the plasma. The remainder reacts with hemoglobin buffers to form bicarbonate which then returns to the plasma in ex­change of chloride. The chloride is neu­tralized by potassium in the red cells.

7. All of these reactions are reversible. At the lung, when the blood becomes arterial, chloride shifts back into the plasma, thus liberating intracellular potassium to buffer the oxy-hemoglobin and in the plasma, neutralizing the sodium which is liberated by the removal of CO₂ during respiration.