Carbon dioxide is produced by the tissues and diffuses into the plasma. In the plasma:
- a small proportion becomes bound to plasma proteins forming carbamino compounds
- a small proportion reacts slowly to form carbonic acid which dissociates to form a hydrogen ion and bicarbonate
- largest proportion enters the red cell where it rapidly forms carbonic acid:
- this reaction is catalysed by carbonic anhydrase
- within the red cell, the carbonic acid dissociates and the hydrogen ion combines with the reduced haemoglobin and the bicarbonate diffuses out of the cell
- the loss of anionic bicarbonate from the cell is compensated (maintaining electrical neutrality) by an influx of chloride ions (anions) - the so-called 'chloride shift'
- 5-10% of carbon dioxide is transported via direct binding of carbon dioxide to haemoglobin (carbaminohaemoglobin)
- production of this compound is increased in conditions of hypoxia and reduced when the haemoglobin is oxygenated
The chloride shift may facilitate oxygen loading and unloading to/from the hemoglobin from the brown bear (Ursus arctos L.).
Brix O, Thomsen B, Nuutinen M, Hakala A, Pudas J, Giardina B.
1. The oxygen binding properties of hemolyzed bear blood were studied in 0.1 M Tris and 0.1 M Hepes buffer with respect to the possible effects of temperature, pH, pCO2, 2,3-DPG, and chloride ions.
2. There was a significant Bohr shift with a Bohr factor (delta log P50/delta pH) of the magnitude of -0.5. The temperature sensitivity expressed by the apparent heat of oxygenation minus the heat of oxygen in solution was about -8.1 kcal/mol at pH 7.4.
3. Chloride ions decreased the oxygen affinity in the concentration range 50-200 mM, and there was a marked increase in the co-operativity of oxygenation up to a chloride concentration of about 200 mM.
4. There were no effects of pCO2 and 2,3-DPG in the presence of 200 mM Cl-, while in the absence of Cl-, 2,3-DPG had the same effect as 200 mM Cl- at 37 degrees C and pH 7.4.
5. Our results suggests at least two different binding sites for the chloride ion, one high affinity site which may also bind 2,3-DPG in the absence of chloride, and one or more low affinity sites, which only binds chloride.
6. The results further show, that a chloride shift of about 33 mM may account for as much as a 40% increase in O2 unloading, without taking into account the additional effect of the Bohr shift.
• Production of bicarbonate occurs in RBCs
• The H+ combines with Hb
• H+ + Hb ® HbH
• HCO3 - diffuses out & Cl- are attracted in
• In the lungs we get a reversed chloride shift
is formed it diffuses out of the red cell. Cl- diffuses into the red cell to maintain electroneutrality. This is the Chloride Shift or Hamburger Shift.
The chloride shift is rapid and is complete before the cells exit the capillary.
The osmotic effect of the extra
and Cl- in venous red cells causes the venous RBC volume to increase slightly. For this reason venous hematocrit slightly exceeds arterial hematocrit.
Quantitative summary of CO2 transport.
90% of arterial CO2 stores are carried as
with 5% of the stores carried as dissolved CO2, and 5% of the stores as carbamino compounds.
Of the CO2 added in systemic capillaries, 60% is added as
, 30% is added as carbamino compounds and 10% is added as dissolved CO2.
Medical Sciences Program,
The chloride shift is the movement of Cl(-) from the plasma into erythrocytes as blood moves from the arterial to the venous end of systemic capillaries. The traditional explanation for the chloride shift emphasizes the causative roles of the rise in Pco(2) and the exclusive presence of carbonic anhydrase within the red blood cell. The purpose of this article is, first, to reexamine two aspects of the chloride shift that we feel are traditionally underemphasized. They are the role of hemoglobin in causing the chloride shift and the affect of the chloride shift on the acid-base status of the blood. Second, we wish to reconcile more recent work with the traditional understanding of the chloride shift. The chloride shift has never been modeled from the perspective of the Stewart strong ion approach. Similarly, the traditional understanding has generally treated Cl(-) as a passive participant in the chloride shift whose role was simply to replace the lost negative charge of the outward moving HCO-3. More recent work has suggested that the ingoing Cl(-) is important for both O(2) unloading and acid-base balance of the blood. We conclude this article with a model of the chloride shift that uses the Stewart approach and, though harmonious with the traditional understanding, highlights the importance of hemoglobin and Cl(-) in the chloride shift.
In animals, blood flowing through the capillaries of the body tissues gains carbon dioxide. This carbon dioxide is transported to the lungs in one of the three forms: as carbon dioxide dissolved in plasma, as part of a compound formed by bonding to hemoglobin, or as part of a bicarbonate ion. Unlike oxygen, which combines with the iron atoms of hemoglobin molecules, carbon dioxide bonds with the amino groups of these molecules. Consequently, oxygen and carbon dioxide do not directly compete for binding sites -- a hemoglobin molecule can transport both gases at the same time. Carbon dioxide combining with hemoglobin forms a loosely bound compound called carbaminohemoglobin. Only about 15% to 25% of the total carbon dioxide is carried this way. The most important carbon dioxide transport mechanism involves the formation of bicarbonate ions (HCO3-). Carbon dioxide reacts with water to form carbonic acid. This reaction occurs slowly in the blood plasma, but much of the carbon dioxide diffuses into the red blood cells. These cells contain an enzyme, carbonic anhydrase, which speeds the reaction between carbon dioxide and water. The resulting carbonic acid dissociates almost immediately, releasing hydrogen ions and bicarbonate ions. As the bicarbonate ions leave the red blood cells and enter the plasma, chloride ions, which also have negative charges, are repelled electrically, and they move from the plasma into the red blood cells. This reversible exchange of chloride and bicarbonate ions between erythrocytes and plasma to effect transport of carbon dioxide and maintain ionic equilibrium during respiration is known as the chloride shift in animal physiology.
In plants, along with manganese, chloride is required for the oxygen evolving reaction of photosynthesis. Cl- is a highly mobile anion with two principal functions: it is both a major counterion, maintaining electrical neutrality across membranes, and one of the principal osmotically solutes in the vacuole. Chloride ion also appears to be required for cell division in both leaves and shoots. Since chloride ion (Cl-) is ubiquitous in nature and highly soluble, it is readily taken up and most plants accumulate chloride ion far in excess of their minimal requirements. It is thus rarely, if ever, deficient. Deficiencies can normally be shown only in very carefully controlled solution culture experiments. Plants deprived of chloride tend to exhibit reduced growth, wilting of the leaf tips, and a general chlorosis.