Question

What is meant by chloride shift?

Answer 1

Chloride shift is also known as Hamburger effect. A difference of 2-4 mmol/L of chloride between the arterial and venous blood along with a difference in bicarbonate concentration occurs during this process.

Complete answer:
Chloride shift can be defined as the passage of chloride ions into the RBCs to ensure that no build-up of electric change takes place during gas exchange. It occurs in a cardiovascular system.
It is the movement of chloride into RBCs when the buffer effects of deoxygenated haemoglobin increases intracellular bicarbonate concentration, and the bicarbonate is exported from the RBC in exchange for chloride.
Mechanism:

Chloride moves into erythrocytes, and bicarbonate moves out, in venous blood.
$C{O_2}$ diffuses into RBCs and is converted into bicarbonate by carbonic anhydrase.
Bicarbonate is pumped out of the RBCs, and chloride pumped into it.
The reverse events take place in the pulmonary capillaries- Bicarbonate is pumped back into the red cell, and chloride pumped out and carbonic anhydrase converts bicarbonate back into carbon dioxide and water.
Effects of chloride shift:
It mitigates the change in pH which would otherwise occur in the peripheral circulation due to metabolic byproducts.
It increases the $C{O_2}$-carrying capacity of the venous blood
It increases the unloading of oxygen, because of the allosteric modulation of the haemoglobin tetramer by chloride.

Note: The phenomenon of chloride is possible due to the presence of carbonic anhydrase in RBCs. It is seen as a critically important element. Without it, the reaction converting $C{O_2}$ to $HC{O_3}$- would be too slow. The continuous process of carbonic acid dissociation and outflow of bicarbonate ions leads to a change of intracellular electric potential. Inflow of chloride ions maintains electrical neutrality of a cell. Wieth & Brahm had determined that 99% of the chloride shift process completes within about 700 milliseconds.