Chloride Shift

17.3.3.1 At Tissues

When arterial blood is delivered by systemic capillaries to the tissues, it is exposed to:

1. Oxygen at very low tension (1-40 mm Hg).

2. Carbon dioxide at high tensions (50-60 mm Hg).

Tidal CO₂ diffuses into plasma with water then into RBCs via its semi-permeable membrane, where it is buffered (pre­viously mentioned).

The continuous accumulation of KHCO₃ in red cells upsets the ionic equilibrium, so HCO₃^- diffuses into plasma (K+ cannot accompany HCO₃^-, as it is retained inside the RBCs by negative charges e.g., proteins) where it combines with Na⁺ → NaHCO₃. On the other hand, Cl^- ions diffuse into red cells forming KCl to restore ionic equilibrium (Chloride shift; Hamburger’s phenomenon).

Water Shift: formation of KHCO₂ and KCl inside RBCs → increases the osmotic pressure inside RBCs → diffusion of water into RBCs → swelling of RBCs

17.3.3.2 At Lungs

When venous blood flows through the pulmonary capillar­ies of the lungs it is exposed to:

1. Oxygen at high tension (98 mm Hg).

2. Carbon dioxide at low tensions (40 mm Hg).

a) O₂ diffuses from alveolar air into plasma (O₂ in

solution) then into RBCs, where it combines with reduced Hb (H. Hb; very weak acid) → Oxy Hb (HbO2) + H⁺

b) HbO₂ (more acidic than Hb and holds more K⁺

and less CO₂), so recovers the K⁺ that is lost

HbO₂ + KCl → K HbO₂ + Cl

The continuous accumulation of Cl^- in red cells upsets the ionic equilibrium, so Cl^- diffuses into the plasma where it combines with Na⁺ to form NaCl. On the other hand, HCO₃^- ions diffuse into red cells, where they react with H⁺ (from reduced Hb).

* CO₂ diffuses into plasma, then to alveolar air →

atmosphere.

* H₂O shifts from RBCs to plasma.

Net results of chloride shift at:

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Alkali reserve (CO₂ content/ 100 ml arterial plasma)

It is the volume of CO₂ present in chemical combination (e.g., NaHCO₃) in 100 ml arterial plasma (not whole blood) = 55-70 ml CO₂/ 100 ml arterial plasma.

17.3.3.3 Measurement

• Venous blood is centrifuged and plasma is intro­duced into a tonometer.

• Plasma is exposed to CO₂ tension of 40 mm Hg (= CO₂ tension of alveolar air at the last part of deep expiration) for 20 minutes → venous blood is arterialised.

• Plasma is then analysed for CO₂ content per 100 ml.

Why is the measurement of alkali reserve done on plasma but not on whole blood?

• HCO₂ concentration in plasma is three times greater than that of RBCs.

• Haematocrit value varies under different con­ditions, even in the same person. For example, in anaemia plasma volume increases while Haematocrit value decreases. If whole blood is used in this case, the alkali reserve will be falsely increased.

17.3.3.4 Importance of Alkali Reserve

1) CO₂ carriage: most of CO₂ in the blood is carried as NaHCO₃ in plasma.

2) Acid-base balance: neutralise acid metabolites (e.g., lactic acid) formed by tissues and added to the blood.

3) Gives information about:

Acidosis

= Alkali reserve:

Decrease below 55%

If accompanied by: Decrease blood pH !

Acidaemia

Alkalosis

Increase above 70%

Increase blood pH !

Alkalaemia

17.3