Figure 17.9 Chloride shift

As bicarbonate ions (HCO₃) diffuse into plasma from RBC's, Hydrogen ions (H+) concentration in RBCs increases. For maintaining ionic balance for each bicarbonate (HCO₃) one chloride ion (Cl) from plasma enters the RBC. This is called "chloride shift". H+ ions in RBC's also bring about dissociation of oxyhemoglobin (HbO₂) into Hb and O₂. The oxygen released diffuses into the tissues while hemoglobin buffers hydrogen to form very dilute hemoglobinic acid.

(ii) About 10% of CO₂ combines with an amino group (NH₂) on hemoglobin to form carbaminohemoglobin. This enters the lungs and dissociates into hemoglobin and CO₂, the latter of which then diffuses into the alveoli.

(iii) Nearly 5% of CO₂ never enters RBC but dissolves in plasma, where it combines with plasma protein to form proteinic acid which is carried to the lings where it dissociates into protein and CO₂. CO₂ further enters alveoli of the lungs.

In the lungs CO₂ tension is less than in the blood stream, hence the reversible reactions take place. Bicarbonate enter RBC and because of the enzyme carbonic anhydrase, it dissociates into H₂O and CO₂. This dissociation is facilitated by acidic oxyhemoglobin present at the respiratory surface. As a result CO₂ diffuses into alveoli and is expelled during respiration.

Table of Contents

17.0 - Introduction
17.1 - Gaseous Exchange and Transport

Chapter 18