Page 306 - Textbook of Pathology, 6th Edition

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SECTION II
Figure 12.7 Schematic diagram of haemoglobin synthesis in the developing red cell.

chains, the pH, and the concentration of 2,3- 2. CO transport. Another important function of the red cells
2
biphosphoglycerate (2,3-BPG) as follows (Fig 12.8, B): is the CO transport. In the tissue capillaries, the pCO is
2
2
Normal adult haemoglobin (HbA) has lower affinity for high so that CO enters the erythrocytes where much of it is
2
oxygen than foetal haemoglobin and, therefore, releases converted into bicarbonate ions which diffuse back into the
greater amount of bound oxygen at pO of tissue capillaries. plasma. In the pulmonary capillaries, the process is reversed
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A fall in the pH (acidic pH) lowers affinity of and bicarbonate ions are converted back into CO . Some of
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oxyhaemoglobin for oxygen, so called the Bohr effect, thereby the CO produced by tissues is bound to deoxyhaemoglobin
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causing enhanced release of oxygen from erythrocytes at the forming carbamino-haemoglobin. This compound dissociates
lower pH in tissue capillaries. in the pulmonary capillaries to release CO .
2
A rise in red cell concentration of 2,3-BPG, an intermediate RED CELL DESTRUCTION. Red cells have a mean lifespan
product of Embden-Meyerhof pathway, as occurs in anaemia of 120 days, after which red cell metabolism gradually
and hypoxia, causes decreased affinity of HbA for oxygen. deteriorates as the enzymes are not replaced. The destroyed
This, in turn, results in enhanced supply of oxygen to the red cells are removed mainly by the macrophages of the
tissue. reticuloendothelial (RE) system of the marrow, and to some

Haematology and Lymphoreticular Tissues

Figure 12.8 A, Normal adult haemoglobin molecule (HbA) consisting of α 2 β 2 globin chains, each with its own haem group in oxy and deoxy
state. The haemoglobin tetramer can bind up to four molecules of oxygen in the iron containing sites of the haem molecules. As oxygen is bound,
salt bridges are broken, and 2,3-BPG and CO 2 are expelled. B, Hb-dissociation curve. On dissociation of oxygen from Hb molecule i.e. on release
of oxygen to the tissues, salt bridges are formed again, and 2,3-BPG and CO 2 , are bound. The shift of the curve to higher oxygen delivery is affected
by acidic pH, increased 2,3-BPG and HbA molecule while oxygen delivery is less with high pH, low 2,3-BPG and HbF.