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The properties of blood

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Maturation of white cells


There are three types of granulocytes: neutrophils, basophils, and eosinophils. Multipotential progenitor cells (promyelocytes) in the hematopoietic tissue give rise to three kinds of precursor cells: neutrophilic, basophilic, and eosinophilic myelocytes. These mature further, with condensation of the nucleus and an increase in specific granule content over the next 10 days or so, before appearing in the circulation. During an infection, the rate of granulocyte production (especially of neutrophils) increases considerably.

^ Maturation of monocytes

The committed precursor cell of the monocyte is the monoblast which differentiates further to generate the promonocyte, which is a large (18 /U,m) diameter cell, containing a large nucleus and nucleoli. Promonocytes divide twice more to become monocytes containing a large amount of rough endoplasmic reticulum, Golgi complex, and lysosomes (Chapter 3). After entering the blood, mature monocytes circulate for about 8 hours before entering the connective tissues where they mature into macrophages.


  1. Mature blood cells are renewed continuously by hematopoiesis. All
    the cell types are generated ultimately from a common population of
    pluripotent stem cells in the bone marrow. These form two distinct
    cell lines—myeloid and lymphoid cells. The myeloid cells remain in
    the marrow and form red cells and leukocytes other than lympho­
    cytes. Lymphoid stem cells migrate to the lymph nodes, spleen, and
    thymus where they develop into lymphocytes.

  2. The stem cells divide to form committed precursor cells which differ­
    entiate, via a series of cell divisions, into one of the mature cell types.
    The precursor cells for erythrocyte production, for example, are called
    erythroblasts. Through successive divisions these start to synthesize
    hemoglobin before losing their nuclei to become reticulocytes, in
    which form they are released into the circulation.

  3. Erythropoiesis is closely matched with the requirement for red cells
    in the circulation. This is controlled by erythropoietin, a hormone
    secreted by the kidneys. After about 120 days in the circulation, red
    cells are destroyed by macrophages in the spleen, liver, or lymph

  4. Granulocytes, monocytes, and lymphocytes mature from precursor
    cells in a fashion similar to the red cells.

  5. Platelets bud off from giant cells (megakaryocytes) which are them­
    selves derived from the pluripotent stem cells in the bone marrow.

13.5 Iron metabolism

^ Maturation of lymphocytes

Circulating lymphocytes originate mainly in the thymus and peripheral lymphoid organs (spleen, lymph nodes, tonsils, etc.). The first identifiable progenitor is the lymphoblast. These cells divide several times to become smaller prolymphocytes. These sub­sequently synthesize the cell-surface receptors that distinguish them as T or В lymphocytes (Chapter 14).

Production of platelets

The precursor cells which give rise ultimately to the platelets are known as megakaryoblasts. These cells are 15-20 /iim in dia­meter and possess a large ovoid or kidney-shaped nucleus. Differentiation of the megakaryoblasts gives rise to the megakary-ocytes, which are giant cells (35—150/xm in diameter) whose cytoplasm contains numerous mitochondria, rough endoplasmic reticulum, and Golgi complex. As these cells mature within the bone marrow, invaginations of the plasma membrane become evident, eventually branching throughout the entire cytoplasm. These form the so-called demarcation membranes which define the areas that will be shed as platelets. The factors that control the rate of production of platelets are poorly understood, although it is known that the maturation of megakaryocytes and the budding off of their platelets is stimulated by a hormone called thrombopoietin.

Iron is an essential component of hemoglobin and myoglobin, as well as of certain pigments and enzymes. The body of an adult man contains, in total, about 4.5 g of iron, of which about 65 per cent is within the hemoglobin of red blood cells. A further 5 per cent or so is contained within myoglobin and enzymes, while the remainder is stored in the form of ferritin, largely by the liver, but to a lesser extent also by the spleen and intestine.

When red blood cells become senescent they are removed by phagocytes in the liver and spleen. Much of the iron derived from hemoglobin is recycled by the body, as illustrated in Fig. 13.3. The iron from the digested hemoglobin is either returned to the plasma where ir binds to transferrin (an iron-carrying protein), or is stored in the liver as ferritin. The iron which is bound to transferrin in the plasma travels to the ery-thropoietic tissue in the bone marrow and is either used immedi­ately in the production of hemoglobin for developing red cells, or is stored within the marrow itself. If blood loss occurs, iron contained within the stores of the marrow is utilized and there is also an increase in the rate of uptake of iron from the circulation. A few days later, the balance is restored by an increased rate of iron absorption in the gut.

Because the recycling of iron is so efficient, the need for dietary iron in adults arises mainly from loss by bleeding and rhe death of intestinal cells. It therefore follows that the dietary requirement for iron is greater in menstruating women than in men, being about 1 mg day"1 in men and 2 mg day"1 in women

242 13 The properties of blood

Fig. 13.3 The principal stages in the recycling of iron from red cell hemoglobin.

Fig. 13.4 An outline of the mechanism by which iron is absorbed by the intestine.

ported across the basolateral membrane or it becomes bound to specific cytoplasmic proteins, the best known of which is apofer-ritin. The path taken is determined by the body's demand for iron. When demand is high, for example following hemorrhage, iron is preferentially absorbed into the blood. When demand is low, iron is preferentially stored bound to ferritin. The processes that determine the fate of absorbed iron are, however, poorly understood.

of child-bearing age. Furthermore, children and pregnant women need relatively more iron because of their expanding circulatory volume. Dietary sources of iron include meat (specifically the myoglobin of the muscle), vegetables, and fruits.

How is iron absorbed in the intestine?

Ionized iron can exist in two oxidation states, Fe2+ (ferrous) and Fe3+ (ferric). The low pH of the stomach lumen caused by the secretion of HCl by the gastric mucosa solubilizes iron salts and both Fe2+ and Fe3+ are absorbed as complexes with various con­stituents of the diet, including sugars and amino acids. In addi­tion, ascorbate (vitamin C) reduces Fe3+ to Fe2+ which is less likely to form insoluble complexes with other constituents of the diet (particularly the fiber of cereal grains).

A simple scheme of the mechanism by which the epithelial cells of the upper intestine absorb iron is shown in Fig. 13-4. Iron is taken up by a carrier-mediated process into the epithelial cells. Iron complexed with heme (derived from dietary meat) is absorbed directly by a separate pathway. Within the cell, heme oxygenase liberates Fe2+ from the heme molecule. Once inside the cell, iron can follow one of two pathways: either it is trans-

Iron absorption is regulated in accordance with the body's needs

In iron deficiency, or following hemorrhage, the capacity of the small intestine to absorb iron is increased. After severe blood loss, there is a time lag of 3 or 4 days before absorption is enhanced. This is the time needed for rhe enterocytes to migrate from their sites of origin in the mucosal glands to the tips of the villi, where they are best able to participate in iron absorption. The enterocytes of iron-deficient animals are able to absorb iron from the intestinal lumen more rapidly than normal.

Excess iron absorption is as undesirable as iron deficiency since high levels of iron can be toxic. This can become a problem if the diet is excessively rich in iron or in the genetic disease idiopathic hemochromatosis, in which excessive amounts of iron are absorbed even from a healthy diet. This situation is normally prevented by the binding of iron to ferritin within the cytoplasm of the enterocyte. This binding is almost irreversible, so any iron bound in this way is unavailable for absorption into the plasma. Instead, it is lost in the feces when the intestinal cell desqua­mates. The amount of iron held in the so-called storage pool increases when dietary intake rises, to maintain homeostasis. It is thought that the level of iron in the plasma in some way regulates the synthesis of ferritin.

13.6 The carriage of oxygen and carbon dioxide by the blood



  1. About two-thirds of the total body iron is within the hemoglobin of
    red blood cells, 5 per cent is within myoglobin and enzymes, while
    the rest is stored, mainly in the liver, as ferritin.

  2. When red blood cells are phagocytosed, most of their iron is recycled
    and either reused immediately, or stored as ferritin in the liver or
    within the bone marrow itself.

  1. Most of the iron in the diet is absorbed as ferrous iron (Fe2*).
    Duodenal and jejunal epithelial cells take up iron from the intestinal
    lumen by a carrier-mediated process. Iron is stored within the entero-
    cytes bound to iron-binding proteins, including ferritin. Absorbed
    iron is released into the blood across the basolateral membrane where
    it combines with transferrin in the plasma to be transported to the

  2. Iron absorption appears to be matched with the body's requirement
    for iron. Following a hemorrhage, for example, the capacity of the
    small intestine to absorb iron is enhanced.

13.6 The carriage of oxygen and carbon dioxide by the blood

The blood transports the respiratory gases around the body. Oxygen is carried from the lungs to all the tissues of the body while the carbon dioxide produced by metabolizing cells is transported back to the lungs for removal from the body. The principles governing the exchange of gases in the lungs and the tissues are discussed fully in Chapter 16. Briefly, oxygen passes from the alveoli to the pulmonary capillary blood by diffusion because the partial pressure of oxygen (PO2) in the alveolar air is greater than that of the pulmonary blood. In the peripheral tissues, the PO2 is lower in the cells than in the arterial blood entering the capillaries and so oxygen diffuses out of the blood, through the interstitial spaces, and into the cells. Conversely, the partial pressure of carbon dioxide (PCO2) in metabolizing cells is much higher than that of the capillary blood so that carbon dioxide diffuses into the blood and is transported to the lungs. Here, the PCO2 of the pulmonary capillary blood is greater than that of the alveoli and carbon dioxide diffuses across the capillary and alveolar membranes and is removed from the body during expiration. Standard values for the partial pressures of the blood gases are given in Table 13-3.

Table 13.3 Standard values for the partial pressures of blood gases

Arterial blood Mixed venous blood

Oxygen 13.3 kPa( 100 mmHg) 5.33 kPa(40 mmHg) Carbon dioxide 5.3 3 kPa (40 mmHg) 6 Л 2 kPa (46 mmHg)

Note that the capacity of the blood to carry oxygen will depend on its hemoglobin content. In males there is about 15 g dl"1 (150 g I"1) of hemoglobin while in females the value is usually lower, at about 13.5 g dl"1 (135 g I"1). See text for further details.

Hemoglobin increases the capacity of the blood to transport oxygen: each gram of hemoglobin can bind 1.34 ml of oxygen

At rest, oxygen is consumed by the body at a rate of around 25Omlmin~1 and this must be supplied by the blood. The solubility of oxygen in the plasma water is very low—for each kPa PO2 only 0.225 ml of oxygen are dissolved in every liter of plasma (equivalent to 0.03 ml mmHg"1). At the normal arterial PO2 of 13-3 kPa (100 mmHg), therefore, each liter of plasma will contain just 3 ml of dissolved oxygen. If this were the only means of transporting oxygen to the tissues, the heart would need to pump more than 80 liters of blood each minute to supply the required 25Omlmin~1. In fact, the blood is able to carry far more oxygen than this. At a PO2 of 13.3 kPa (100 mmHg), the oxygen content of whole blood is about 20 ml dl"1 blood (i.e. 200 ml I"1). As a result, the normal resting cardiac output (about 5 1 min"1) is more than sufficient to meet the oxygen requirements of the body at rest.

The vast majority of the oxygen in the blood is carried in chemical combination with hemoglobin, an oxygen-binding protein contained within the red cells. Each hemoglobin mole­cule consists of a protein part (globin) consisting of four polypeptide chains, and four nitrogen-containing pigment mole­cules, called heme groups. Each of the four polypeptide groups is combined with one heme group (Fig. 13.5). In the center of each heme group is one atom of ferrous (Fe2+) iron which can combine loosely wirh one molecule of oxygen. Each molecule of hemo­globin (Hb) can, therefore, combine with four molecules of oxygen, to form oxyhemoglobin (often written as HbO2). The reaction for the binding of oxygen can be expressed as:

When oxyhemoglobin dissociates to release oxygen to the tissues, the hemoglobin is converted to deoxyhemoglobin—also called reduced hemoglobin. Combination of oxygen with hemo­globin to form oxyhemoglobin occurs in the alveolar capillaries of the lungs where the PO2 is high (13-3 kPa or 100 mmHg). Where rhe PO2 is low (as in the capillaries supplying meta-bolically active cells), oxygen is released from oxyhemoglobin and is then able to diffuse down its concentrarion gradient to the cells via the interstitial space.

Hemoglobin that is fully saturated with oxygen is bright red, while hemoglobin that has lost one or more oxygen molecules (deoxyhemoglobin) is darker in appearance. When it has lost most of its oxygen, hemoglobin becomes deep purple in color. As the blood passes through the tissues it gives up its oxygen and the percentage saturation falls. For this reason venous blood is much darker in color than arterial blood. When the quantity of deoxyhemoglobin exceeds 5 g dl~' the skin and mucous mem­branes appear blue—a condition known as cyanosis.

The ease with which hemoglobin accepts an additional mole­cule of oxygen depends on how many of the binding sites are

^ 244

13 The properties of blood

Fig. 13.5 The structure of hemoglobin. The hemoglobin molecule consists of four peptide chains—two a- and two /3-chains. Each peptide chain has a single heme group (shown left) which binds a single molecule of oxygen. Thus one molecule of hemoglobin can carry four molecules of oxygen.

already occupied by oxygen molecules. There is cooperativity between the binding sites such that occupancy of one of the four sites makes it easier for a second oxygen molecule to bind, and so on. As a result, the amount of oxygen bound to hemoglobin increases in an S-shaped (sigmoid) fashion as the PO2 increases (Fig. 13-6). This is known as the oxyhemoglobin dissociation curve {or the oxygen dissociation curve). The sigmoidal nature of the disso­ciation curve is physiologically significant because as PO2 falls from 13.3 kPa (100 mmHg)—the value in arterial blood—to about 8 kPa (60 mmHg) the saturation of the hemoglobin with

Fig. 13.6 The oxyhemoglobin dissociation curve for a Pco2 of 5.33 kPa (40 mmHg) at 37 °C. Under these conditions, the P50 value is 3.46 kPa (26 mmHg). a, the Po2 in arterial blood (97 per cent saturated); v, the Po2 for mixed venous blood (5.33 kPa or 40 mmHg) at which value the hemoglobin is still 75 per cent saturated. Note that as the Po2 falls below 8 kPa (60 mmHg) the curve becomes progressively steeper.

oxygen decreases by only about 10 per cent. As the PO2 falls below 8 kPa, however, the curve becomes relatively steep so that small changes in PO2 cause large changes in the degree of hemoglobin saturation.

The quantity of oxygen in a given volume of blood must be carefully distinguished from the percentage saturation, which only indicates what proportion of the available hemoglobin is saturated. This distinction should be clear from the following definitions:

^ Oxygen content. This is the quantity of oxygen in a given

sample of blood, whether obtained from an artery or a vein. It represents the quantity of oxygen combined with hemoglobin plus that physically dissolved in the plasma.

Oxygen capacity. This is the maximum quantity of oxygen
that can combine with the hemoglobin of a given sample of
blood. It can be determined in two ways: first by equilibrat­
ing a sample of blood at 20 kPa (150 mmHg) at 37 °C and
determining the quantity of oxygen in the sample. This will
provide a value for the amount of oxygen combined with
hemoglobin plus that physically dissolved in the plasma. At
a PO2 of 20 kPa about 0.5 ml O2 is dissolved per deciliter of
blood. This must be subtracted from the total to obtain the
value for the oxygen capacity. Alternatively (and more con­
veniently) the hemoglobin concentration of the blood
sample is first determined. This is normally about 15 gdl"1
(150 g I-1) in males and 13.5 g dl"1 (135gH)in females.
When fully saturated, each gram of hemoglobin will bind
1.34 ml of O2 (at standard temperature and pressure (STP)),
the oxygen capacity is then given by the hemoglobin con­
centration X 1.34 in milliliters O2 per deciliter blood. The
oxygen capacity of a sample of blood depends, therefore, on
the hemoglobin content and is independent of the partial
pressure of oxygen.

1 Ъ.Ь The carriage of oxygen and carbon dioxide by the blood


O2 content - dissolved O,


^ Oxygen saturation. This is the term given to the ratio of the quantity of oxygen combined with hemoglobin in a given sample of blood to the oxygen capacity of that sample. It is expressed as a percentage, thus:

saturation =

O2 capacity

Thus for a normal adult male, when the PO2 is close to 13-3 kPa (100 mmHg), as in the arterial blood, the hemoglo­bin is 97 per cent saturated and the oxygen content of the blood will be 15 x 1.34 X 0.97 = 19.5 ml O2dH bound to hemoglobin plus 13.3 X 0.0225 ml = 0.3 ml O2 in physical solution, giving a total O2 content of 19-8 ml dl"1. In the case of an anemic patient (Section 13.7) with, say, a hemoglo­bin concentration only half of normal (7.5 g dl"1 blood), at a PO2 of 13.3 kPa (100 mmHg) the amount of oxygen bound to hemoglobin will be 7.5 X 1.34 X 0.97 = 9.7 ml plus 0.3 ml O2 in solution, giving a total of only 10 ml—about half the total content of normal arterial blood.

The affinity of hemoglobin for oxygen is

influenced by pH, PCO2, 2,3-DPG, and


So far, the oxyhemoglobin dissociation curve has been considered as though the percentage saturation of hemoglobin remained constant for a given PO2. In reality, the position of the curve varies with temperature, pH, PCO2 and the concentration of certain metabolites, such as 2,3-bisphosphoglycerate (commonly known as 2,3-diphosphoglycerate or 2,3-DPG). In view of this, the dissociation curve is usually given for a pH of 7.4, a PCO2 of 5.3 kPa (40 mmHg) and a temperature of 37 °C. It is worth noting that, under these conditions, hemoglobin in normal red cells is 50 per cent saturated with oxygen at a PO2 of 3.4 kPa (26 mmHg). This is also expressed as a P50 value of 3.4 kPa (26 mmHg).

Both an increase in the PCO2 (above 5.3 kPa or 40 mmHg) and a reduction in pH (i.e. an increase in H+ ion concentration) shift the hemoglobin dissociation curve to the right (Fig- 13.7). This effect is known as the Bohr shift or the Bohr effect. Physiologically this effect is very important as the affinity of hemoglobin for oxygen becomes less as the PCO2 rises. Thus, in the tissues where the PCO2 is relatively high, the affinity of hemoglobin for oxygen is lower than in the lungs (i.e. less oxygen is bound for a given PO2). Consequently, oxygen delivery to actively metabolizing tissues is facilitated. In the lungs, as the PCO2 of the pulmonary capillary blood falls the Bohr shift acts to increase the affinity of hemoglobin for oxygen. In this way the uptake of oxygen is facilitated during the passage of blood through the lungs.

As the temperature increases, the affinity of the hemoglobin for oxygen is also reduced and the dissociation curve for hemo­globin shifts to the right. Consequently, for a given level of PO2,

the percentage saturation of hemoglobin will be less than at 37 °C. This may be of benefit during heavy muscular exercise, for example, since oxygen will be delivered more readily from the blood to the active tissues as body temperature rises.

The affinity of purified hemoglobin for oxygen is much greater than that seen in whole blood—indeed purified hemoglobin has an affinity for oxygen similar to that of myoglobin, which has a P50 of 0.13 kPa (1 mmHg; see below). In normal red cells, however, hemoglobin has a P50 of 3.4 kPa at a PCO2 of 5.3 kPa (i.e. P50 is 26 mmHg at a PCO2 of 40 mmHg). This difference in the affinity of hemoglobin for oxygen is attributable to 2,3-DPG which is synthesized by the red cells during glycolysis. 2,3-DPG binds strongly to hemoglobin and decreases its affinity for oxygen (i.e. it causes the oxyhemoglobin dissociation curve to be shifted to the right). The concentration of 2,3-DPG is about 4 mM in normal red cells but may be increased in anemia, or when living at high altitude where PO2 of the inspired air is significantly reduced.


Myoglobin is an oxygen-binding protein present in cardiac and skeletal muscle that has a much higher affinity for oxygen than the hemoglobin of the red cells. It is half saturated at a PO2 of only 0.13 kPa (1 mmHg). It is probable that myoglobin acts as a store of oxygen for situations where the oxygen supply from the capillaries is insufficient to meet the demands of aerobic metabolism in an exercising muscle. Thus the muscles of diving mammals such as seals contain very large quantities of myo-

Fig. 13-7 The effect of increasing PCO2 on the oxyhemoglobin dissociation curve. As the PCO2 increases the P50 value for the dissociation curve is shifted to the right. This is known as the Bohr shift. The dissociation curve is affected in a similar manner by a fall in pH or an increase in 2,3-DPG or temperature. The effect of the rightward shift is to decrease the affinity of hemoglobin for oxygen. This is shown by the difference in hemoglobin saturation when PO2 is 5.33 kPa (40 mmHg) as PCO2 increases from 5.33 kPa (40 mmHg) (point a) to 10.66 kPa (80 mmHg) (point b).

246 13 The properties of blood

Fig. 13.8 A comparison between the oxygen dissociation curves for myoglobin and hemoglobin. Myoglobin has a P50 value of about 0.13 kPa(l mmHg) while hemoglobin has a P50 of 3.46 kPa (26 mmHg).

globin. The shape of the oxygen dissociation curve for myoglo­bin is shown in Fig. 13.8. Oxygen is not liberated in significant quantities until the PO2 falls below 0.65 kPa (5 mmHg). This situation may arise both in skeletal muscles during heavy exer­cise and during the contraction of the heart when the capillary circulation is temporarily interrupted. During periods of severe tissue hypoxia the oxygen bound by myoglobin can be used to maintain the production of ATP by the mitochondria until the local circulation is restored.

Carbon monoxide binds strongly to hemoglobin

Carbon monoxide is another gas that is able to bind to hemo­globin. Indeed, the affinity of carbon monoxide for hemoglobin is more than 200 times that of oxygen. This would mean that breathing air containing a Pco of only 0.13 kPa (1 mmHg) would quickly result in virtually all the hemoglobin in the blood being bound to carbon monoxide (as carboxyhemoglobin). Moreover, carbon monoxide tends to shift the oxygen—hemoglobin dissocia­tion curve to the left and this impairs the unloading of oxygen from the blood. For these reasons, carbon monoxide is a highly toxic gas. Treating patients suffering from CO poisoning requires a means of overcoming the high affinity of hemoglobin for CO. This is achieved by ventilating the patients with a gas mixture containing 95 per cent O2 to drive the CO from its binding sites on the hemoglobin, and 5 per cent CO2, to stimulate breathing.

Carbon dioxide is carried in the blood in

three different forms: as dissolved gas, as

bicarbonate, and as carbamino


Chemical determination shows that arterial blood contains much more CO2 than O2 (49 ml dl"1 (490 ml I"1) compared to

19.8 ml dl"1 (198 ml I"1) for O2). The CO2 is carried in the blood in several forms:

  1. in physical solution as dissolved CO2;

  2. as bicarbonate ions;

  3. as carbamino compounds—a combination between CO2
    and free amino groups on proteins.

At first sight, this gives the impression that the carriage of carbon dioxide by the blood is far more complex than that of oxygen. In reality, the principles involved are quite straight­forward. In what follows each form of transport will be considered briefly.

As with all gases, the concentration of dissolved CO2 in the blood is determined by its solubility and its partial pressure. For plasma at normal body temperature, the solubility of CO2 is 0.526 ml dHkPa-1 (0.07 ml dH mmHg-1). Therefore, at a normal arterial PCO2 of 5.3 kPa (40 mmHg), the amount of CO2 transported in solution is 5.3 X 0.526 = 2.8mldl~1 (this is equivalent to 1.2 mmoles CO2 per liter of blood). Mixed venous blood has a PCO2 of around 6.12 kPa (46 mmHg) and will therefore contain 6.12 X 0.526 = 3-2 ml CO2dH. Because of its high solubility, between 5 and 7 per cent of total blood carbon dioxide is in physical solution (in normal arterial blood only 1.5 per cent of oxygen is in solution).

to form carbonic acid. This readily dissociates to form hydrogen ion (HT) and bicarbonate ions (HCO9~) as follows:

The carbon dioxide which is produced as a result of tissue metabolism also combines with water in the reaction:

Reaction 13.1 takes place only very slowly in the plasma but in the red cells it is catalyzed by an enzyme called carbonic anhydrase. Consequently, as carbon dioxide diffuses into the red blood cells, carbonic acid is formed which immediately dissociates to yield bicarbonate and hydrogen ions. The latter are buffered mainly by hemoglobin, while much of the bicarbonate moves back out of the cell in exchange for chloride ions. This is known as the chloride shift or Hamburger effect and accounts for the fact that plasma Cl~ is lower in venous blood than in arterial blood. About 90 per cent of the total blood CO2 is transported in the form of bicarbonate ions.

The buffering of the hydrogen ions formed by dissociation of carbonic acid by hemoglobin is extremely important since it allows large amounts of carbon dioxide to be carried in the blood (as bicarbonate) without the pH of the blood altering by more than about 0.05 pH unit (Section 29-3).

Although the majority of the carbon dioxide that enters the red blood cells from the tissues is hydrated to form carbonic acid which dissociates into H+ and HCOj as described above, about a third combines with amino groups on the hemoglobin molecules in the reaction:

13.6 The carriage of oxygen and carbon dioxide by the blood
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