Medical Physiology, 3rd Edition

Blood Composition

Whole blood is a suspension of cellular elements in plasma

If you spin down a sample of blood containing an anticoagulant for ~5 minutes at 10,000 g, the bottom fraction contains formed elementsRBCs (or erythrocytes), WBCs (leukocytes, which include granulocytes, lymphocytes, and monocytes), and platelets (thrombocytes); the top fraction is blood plasma (Fig. 18-1). The RBCs having the highest density are at the bottom of the tube, whereas most of the WBCs and platelets form a whitish gray layer—the buffy coat—between the RBCs and plasma. Only a small amount of WBCs, platelets, and plasma is trapped in the bottom column of RBCs.

image

FIGURE 18-1 Determination of the hematocrit.

The hematocrit (see p. 102) is the fraction of the total column occupied by RBCs. The normal hematocrit is ~40% for adult women and ~45% for adult men. The hematocrit in the newborn is ~55% and falls to ~35% at 2 months of age, from which time it rises during development to reach adult values at puberty. The hematocrit is a measure of concentration of RBCs, not of total body red cell mass. Expansion of plasma volume in a pregnant woman reduces the hematocrit, whereas her total red cell volume also increases but less than plasma volume (see p. 1142). Immediately after hemorrhage, the hematocrit may be normal despite the loss of blood volume (see pp. 585–586). Total RBC volume is ~28 mL/kg body weight in the adult woman and ~36 mL/kg body weight in the adult man.

Plasma is a pale-white watery solution of electrolytes, plasma proteins, carbohydrates, and lipids. Pink-colored plasma suggests the presence of hemoglobin caused by hemolysis (lysis of RBCs) and release of hemoglobin into the plasma. A brown-green color may reflect elevated bilirubin levels (see Box 46-1). Plasma can also be cloudy in cryoglobulinemias (see pp. 438–439). The electrolyte composition of plasma differs only slightly from that of interstitial fluid on account of the volume occupied by proteins and their electrical charge (see Table 5-2).

Plasma proteins at a normal concentration of ~7.0 g/dL account for a colloid osmotic pressure or oncotic pressure of ~25 mm Hg (see p. 470). Principal plasma proteins are albumin, fibrinogen, globulins, and other coagulation factors. The molecular weights of plasma proteins range up to 970 kDa (Table 18-1). The plasma concentration of albumin ranges from 3.5 to 5.5 g/dL, which provides the body with a total plasma albumin pool of ~135 g. Albumin is synthesized by the liver at a rate of ~120 mg/kg body weight per day and, due to catabolism, has a half-life in the circulation of ~20 days. Urinary losses of albumin are normally negligible (<20 mg/day; see p. 470). Plasma concentration of albumin is typically decreased in hepatic cirrhosis (see p. 569). Hepatic synthesis of albumin is strongly enhanced by a low plasma colloid osmotic pressure.

TABLE 18-1

Major Plasma Proteins*

PROTEIN FRACTIONS OF HUMAN PLASMA

MOLECULAR WEIGHT (kDa)

FUNCTION

Transthyretin

62

Binds T3 and T4
Binds vitamin A

Albumin

69

Oncotic pressure
Binds steroids, T3, bilirubin, bile salts, fatty acids

α1-antitrypsin (α1AT)

54

Protease inhibitor
Deficiency causes emphysema

α2-macroglobulin

725

Broad-spectrum protease inhibitor
Synthesized by liver

Haptoglobin

100

Binds hemoglobin

β-lipoprotein = low-density lipoprotein (LDL)

380

Binds lipid

Transferrin

80

Binds iron

Complement C3

185

Third component of complement system

Fibrinogen

340

Clotting protein
Precursor of fibrin

Immunoglobulin A (IgA)

160

Mucosal immunity
Synthesized by plasma cells in exocrine glands

Immunoglobulin D (IgD)

170

Synthesized by B lymphocytes

Immunoglobulin E (IgE)

190

Synthesized by B lymphocytes
Binds to mast cells or basophils

Immunoglobulin G (IgG)

150

Humoral immunity
Synthesized by plasma cells

Immunoglobulin M (IgM)

970

Humoral immunity
Synthesized by B lymphocytes

*The proteins are listed in the approximate order of decreasing electrophoretic mobility.

Many plasma proteins are involved in blood coagulation through coagulation cascades, the end point of which is the cleavage of fibrinogen into fibrin monomers that further assemble into a fibrin polymer. The fibrinogen molecule is a dimer of identical heterotrimers, each composed of Aα-, Bβ-, and γ chains. Fibrinogen is synthesized only by the liver (see Table 46-3) and circulates in plasma at concentrations of 150 to 300 mg/dL. The acute-phase response (Box 18-1) greatly enhances fibrinogen synthesis. During clotting, the cross-linked polymers of fibrin form strands that trap red and white cells, platelets, and plasma inside the thrombus (i.e., blood clot). Subsequent interaction of myosin and actin in the platelets of the clot allows the clot to shrink to a plug that expels a slightly yellow-tinged fluid. This residual fluid is serum, which differs principally from plasma by the absence of fibrinogen and other coagulation factor. However, serum still contains albumin, antibodies, and other proteins. Note that plasma can also form a clot, but a plasma clot does not retract because it lacks platelets.

Box 18-1

Erythrocyte Sedimentation Rate

Almost any acute stress to the body (trauma, infection, disease) induces a reaction called the acute-phase response. During the course of several hours, in response to inflammatory cytokines the liver rapidly synthesizes and secretes into the circulatory system a number of proteins that aid in the host response to the threat. Among these proteins is fibrinogen (see p. 429), which causes RBCs to cluster and increases their effective density. When anticoagulated blood from a patient with hyperfibrinogenemia is placed in a glass tube, the RBCs fall more quickly under the influence of gravity than when the blood is from a healthy subject. After 1 hour, this sedimentation leaves a layer of clear plasma on the top of the tube (≤15 mm thick for normal blood and often >40 mm thick in certain inflammatory disorders). This rate of fall is called the erythrocyte sedimentation rate (ESR). Although it is nonspecific because so many different conditions can cause it to increase, the ESR is still widely used by clinicians to assess the presence and severity of inflammation. It is a simple technique, easily performed in a physician's office. As an example of its utility, a patient with an inflammatory process that naturally waxes and wanes, such as lupus erythematosus, may present with nonspecific complaints such as fatigue, weakness, and achiness. An elevated ESR would suggest that these complaints are due to the reactivation of the disease and not just to a poor night's sleep or depression.

Subtraction of the albumin and fibrinogen moiety from total protein concentration yields the concentration of all the proteins grouped as globulins. Electrophoresis can be used to fractionate plasma proteins. The electrophoretic mobility of a protein depends on its molecular weight (size and shape) as well as its electrical charge. Plasma proteins comprise the following in decreasing order of electrophoretic mobility (Fig. 18-2A): albumin, α1-globulins, α2-globulins, β-globulins, fibrinogen, and γ-globulins. The three most abundant peaks are albumin, fibrinogen, and γ-globulins. The γ-globulins include the immunoglobulins or antibodies, which can be separated into IgA, IgD, IgE, IgG, and IgM. Immunoglobulins are synthesized by B lymphocytes and plasma cells.

image

FIGURE 18-2 Electrophoretic pattern of human plasma and serum proteins. Normal concentration ranges are as follows: total protein, 6 to 8 g/dL; albumin, 3.1 to 5.4 g/dL; α1-globulins, 0.1 to 0.4 g/dL; α2-globulins, 0.4 to 1.1 g/dL; β-globulins, 0.5 to 1.2 g/dL; γ-globulins, 0.7 to 1.7 g/dL.

Clinical laboratories most often perform electrophoresis of blood proteins on serum instead of plasma (see Fig. 18-2B). Table 18-1 shows the major protein components that are readily resolved by electrophoresis. Proteins present in plasma at low concentrations are identified by immunological techniques, such as radioimmunoassay (see p. 976) or enzyme-linked immunosorbent assay. Not listed in Table 18-1 are several important carrier proteins present in plasma: ceruloplasmin (see p. 970), transcobalamin (see p. 937), corticosteroid-binding globulin (CBG; see p. 1021), insulin-like growth factor (IGF)–binding proteins (see p. 996), sex hormone–binding globulin (SHBG or TeBG; see pp. 1119–1120), thyroid-binding globulin (see pp. 1008–1009), and vitamin D–binding protein (see p. 1064). The liver synthesizes most of the globulins and coagulation factors. imageN18-1

N18-1

Plasma Proteins

Contributed by Emile Boulpaep

Protein

Conventional Units

International Units

Protein, total
Electrophoresis

6.4–8.3 g/dL

64.0–83.0 g/L

Albumin: 3.5–5.0 g/dL

35–50 g/L

α1-globulin: 0.1–0.3 g/dL

1–3 g/L

α2-globulin: 0.6–1.0 g/dL

6–10 g/L

β-globulin: 0.7–1.1 g/dL

7–11 g/L

γ-globulin: 0.8–1.6 g/dL

8–16 g/L

Acid phosphatase

 

M: 2.5–11.7 U/L

F: 0.3–9.2 U/L

Alanine aminotransferase (ALT, SGPT)

M: 10–40 U/L

0.17–0.68 µkat/L

F: 7–35 U/L

0.12–0.60 µkat/L

Albumin

3.4–4.8 g/dL

34–48 g/L

Alkaline phosphatase

25–100 U/L

Adult (>20 yr) 0.43–1.70 µkat/L

Amylase

27–131 U/L

0.46–2.23 µkat/L

Angiotensin I

<25 pg/mL

<25 ng/L

Angiotensin II

10–60 pg/mL

10–60 ng/L

α1-antitrypsin

78–200 mg/dL

0.78–2 g/L

Aspartate aminotransferase (AST)

10–30 U/L

0.17–0.51 µkat/L

Ceruloplasmin

18–45 mg/dL

180–450 mg/L

Chorionic gonadotropin, β-subunit (β-HCG)

M and nonpregnant F: <5.0 mIU/mL

<5.0 IU/L

C-peptide

0.78–1.89 ng/mL

0.26–0.62 nmol/L

C-reactive protein

68–8200 ng/mL

68–8200 µg/L

Creatine kinase (CK)

 

M: 38–174 U/L

F: 26–140 U/L

Erythropoietin

 

5–36 U/L

Ferritin

M: 20–250 ng/mL

20–250 µg/L

F: 10–120 ng/mL

10–120 µg/L

α1-fetoprotein

<10 ng/mL

<10 µg/L

Fibrin degradation products

<10 µg/mL

<10 mg/L

Fibrinogen

200–400 mg/dL

2.00–4.00 g/L

Follicle-stimulating hormone (FSH)

M: 4–25 mIU/mL

4–25 IU/L

F: Follicular phase: 1–9 mIU/L

1–9 IU/L

Ovulatory peak: 6–26 mIU/mL

6–26 IU/L

Luteal phase: 1–9 mIU/mL

1–9 IU/L

Postmenopausal: 30–118 mIU/mL

30–118 IU/L

Gastrin

25–90 pg/mL

25–90 ng/L

γ-glutamyl transferase (GGT)

M: 2–30 U/L

0.03–0.51 µkat/L

Growth hormone or somatotropin (hGH)

Adult, M: 0–4 ng/mL

0–4 µg/L

Adult, F: 0–18 ng/mL

0–18 µg/L

>60 yr, M: 1–9 ng/mL

1–9 µg/L

>60 yr, F: 1–16 ng/mL

1–16 µg/L

Immunoglobulin A (IgA)

40–350 mg/dL

400–3500 mg/L

Immunoglobulin D (IgD)

0–8 mg/dL

0–80 mg/L

Immunoglobulin E (IgE)

0–380 IU/mL

0–380 kIU/L

Immunoglobulin G (IgG)

650–1600 mg/dL

6.5–16 g/L

Immunoglobulin M (IgM)

55–300 mg/dL

550–3000 mg/L

Insulin (12-hr fasting), immunoreactive

0.7–9.0 µIU/mL

5–63 pmol/L

Lactate dehydrogenase (LDH)

 

208–378 U/L

Lipase

31–186 U/L

0.5–3.2 µkat/L

Luteinizing hormone (LH)

M: 1–8 mU/mL

1–8 U/L

F: Follicular phase: 1–2 mU/mL

1–12 U/L

Midcycle: 16–104 mU/mL

16–104 U/L

Luteal: 1–12 mU/mL

1–12 U/L

Postmenopausal: 16–66 mU/mL

16–66 U/L

Lysozyme

0.4–1.3 mg/dL

4–13 mg/L

Myoglobin

 

M: 19–92 µg/L

F: 12–76 µg/L

Parathyroid hormone (PTH)

(Varies with laboratory)
N-terminal, 8–24 pg/mL

8–24 ng/L

C-terminal, 50–330 pg/mL

50–330 ng/L

Intact, 10–65 pg/mL

10–65 ng/L

Prostate-specific antigen (PSA)

M: <4 ng/mL

<4 µg/L

Renin (normal diet)

Supine: 0.2–1.6 ng/(mL hr)

0.2–1.6 µg/(L hr)

Standing: 0.7–3.3 ng/(mL hr)

0.7–3.3 µg/(L hr)

Thyroglobulin (Tg)

3–42 ng/mL

3–42 µg/L

Thyrotropin (hTSH)

0.4–4.2 µU/mL

0.4–4.2 mU/L

Thyrotropin-releasing hormone (TRH)

5–60 pg/mL

5–60 ng/L

Thyroxine-binding globulin or thyroid-binding globulin (TBG)

15.0–34.0 µg/mL

15.0–34.0 mg/L

Transcortin or corticosteroid-binding globulin (CBG)

M: 18.8–25.2 mg/L

323–433 nmol/L

F: 14.9–22.9 mg/L

256–393 nmol/L

Transferrin

200–400 mg/dL

2.0–4.0 g/L

>60 yr: 180–380 mg/dL

1.80–3.80 g/L

Transthyretin (thyroxine-binding prealbumin)

10–40 mg/dL

100–400 mg/L

Troponin I

 

<10 µg/L

Troponin T

 

0–0.1 µg/L

F, females; Kat, katal; M, males.

Adapted from Goldman L, Behrman RE: Cecil Textbook of Medicine, 22nd ed. Philadelphia, Saunders, 2003, Table 478–2.

Bone marrow is the source of most blood cells

If you spread a drop of anticoagulated blood thinly on a glass slide, you can detect under the microscope the cellular elements of blood. In such a peripheral blood smear, the following mature cell types are easily recognized: erythrocytes; granulocytes divided in neutrophils, eosinophils, and basophils; lymphocytes; monocytes; and platelets (Fig. 18-3).

image

FIGURE 18-3 Cellular elements in a peripheral blood smear. Erythrocytes (average diameter, ~7.5 µm) are present in all panels. The cellular elements are not represented according to their abundance. (From Goldman L, Ausiello D: Cecil's textbook of medicine, ed 22, Philadelphia, 2004, WB Saunders.)

Hematopoiesis is the process of generation of all the cell types present in blood. Because of the diversity of cell types generated, hematopoiesis serves multiple roles ranging from the carriage of gases to immune responses and hemostasis. Pluripotent long-term hematopoietic stem cells (LT-HSCs) constitute a population of adult stem cells found in bone marrow that are multipotent and able to self-renew. The short-term hematopoietic stem cells (ST-HSCs) give rise to committed stem cells or progenitors, which after proliferation are able to differentiate into lineages that in turn give rise to burst-forming units (BFUs) or colony-forming units (CFUs), each of which ultimately will produce one or a limited number of mature cell types: erythrocytes, the megakaryocytes that give rise to platelets, eosinophils, basophils, neutrophils, monocytes-macrophages/dendritic cells, and B or T lymphocytes and natural killer cells (Fig. 18-4). Soluble factors known as cytokines guide the development of each lineage. The research of Donald Metcalf demonstrated the importance of a family of hematopoietic cytokines that stimulate colony formation by progenitor cells, the colony-stimulating factors. The main colony-stimulating factors are granulocyte-macrophage colony-stimulating factor (GM-CSF; see p. 70), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), interleukin-3 (IL-3) and IL-5 (see p. 70), thrombopoietin (TPO), and erythropoietin (EPO; see pp. 431–433).

image

FIGURE 18-4 Hematopoietic lineages. CFU, colony forming unit—with suffixes -Baso (basophilic), -E (erythroid), -Eo (eosinophil), -G (granulocyte), -GEMM (granulocyte, erythrocyte, megakaryocyte, macrophage), -GM (granulocyte/macrophage), -M (macrophage), -Meg (megakaryocyte), -MegE (megakaryocyte/erythroid); CMP, common myeloid progenitor; CLP, common lymphoid progenitor; GMP, granulocyte-macrophage progenitor; MEP, megakaryocyte-erythroid progenitor.

GM-CSF is a glycoprotein that stimulates proliferation of a common myeloid progenitor and promotes the production of neutrophils, eosinophils, and monocytes-macrophages. Recombinant GM-CSF (sargramostim [Leukine]) is used clinically after bone marrow transplantation and in certain acute leukemias.

G-CSF and M-CSF are glycoproteins that guide the ultimate development of granulocytes and monocytes-macrophages/dendritic cells, respectively. Recombinant G-CSF (filgrastim [Neupogen]) is used therapeutically in neutropenia (e.g., after chemotherapy). M-CSF is also required for osteoclast development (see p. 1057 and Fig. 52-4).

IL-3 (also known as multi-CSF) has a broad effect on multiple lineages. The liver and the kidney constitutively produce this glycoprotein. IL-5 (colony-stimulating factor, eosinophil), a homodimeric glycoprotein, sustains the terminal differentiation of eosinophilic precursors.

TPO binds to a TPO receptor called c-Mpl, which is the cellular homolog of the viral oncogene v-mpl (murine myeloproliferative leukemia virus). On stimulation by TPO, the Mpl receptor induces an increase in the number and size of megakaryocytes—the cells that produce platelets—which thereby greatly augments the number of circulating platelets.

EPO,imageN18-2 which is homologous to TPO, is produced by the kidney and to a lesser extent by the liver. This cytokine supports erythropoiesis or red cell development (Fig. 18-5). As described on pages 93–94, hypoxia increases the abundance of the α subunit of hypoxia-inducible factor 1 (HIF-1α), which enhances production of EPO messenger RNA. Although EPO is not absolutely required for early commitment of progenitor cells to the erythroid lineage, it is essential for the differentiation of burst-forming unit–erythroid cells (BFU-Es) to colony-forming unit–erythroid cells (CFU-Es) or proerythroblasts (also known as pronormoblasts), which still lack hemoglobin. The further maturation of cells downstream of proerythroblasts does not require EPO. Recombinant EPO has proved effective in the treatment of anemia. Hemoglobin first appears at the stage of polychromatic erythroblasts and is clearly evident in orthochromatic erythroblasts. The subsequent exocytosis of the nucleus produces reticulocytes (see Fig. 18-5), whereas the loss of ribosomes and mitochondria yields mature erythrocytes, which enter the circulation. The mature erythrocyte has a life span of ~120 days. Immature reticulocytes may also appear in the circulation when erythropoiesis is heavily activated.

image

FIGURE 18-5 Erythropoiesis. CMP, common myeloid progenitor; MEP, megakaryocyte-erythroid progenitor.

N18-2

Erythropoietin

Contributed by Walter Boron

Erythropoietin (EPO) is a ~34-kDa glycoprotein made mainly in the kidney by fibroblast-like type I interstitial cells in the cortex and outer medulla (see p. 730). EPO is a growth factor related to other cytokines, and it acts through a tyrosine kinase–associated receptor (see pp. 70–71) to stimulate the production of proerythroblasts in the bone marrow as well as the development of red cells from their progenitor cells. In fetal life, the liver rather than the kidney produces EPO. Even in the adult, Kupffer cells in the liver produce some EPO.

Four lines of evidence indicate that the stimulus for EPO synthesis is a decrease in local image. First, EPO synthesis increases with anemia. Second, EPO production increases with lowered renal blood flow. Third, EPO synthesis increases with central hypoxia (i.e., low arterial image), such as may occur with pulmonary disease or with living at high altitude (see p. 1231). In all three of these cases, local image falls as tissues respond to a decrease in O2 delivery by extracting more O2 from each volume of blood that passes through the kidney. Finally, EPO production increases when hemoglobin has a high O2 affinity. Here, the renal cells must lower image substantially before O2 dissociates from hemoglobin. Thus, mutant hemoglobins with high O2 affinities, stored blood (which has low 2,3-DPG levels), and alkaline blood all lead to increased EPO production.

Besides local hypoxia, several hormones and other agents stimulate EPO production. For example, prostaglandin E2 (PGE2) and adenosine appear to stimulate EPO synthesis by increasing intracellular levels of cAMP. Norepinephrine and thyroid hormone also stimulate EPO release. Finally, androgens stimulate—whereas estrogens inhibit—EPO synthesis, which explains at least in part why women in their childbearing years have lower hematocrit levels than men do.

Because the kidneys are the major source of EPO, renal failure leads to reduced EPO levels and anemia. The development of recombinant EPO has had a major impact in ameliorating the anemia of chronic renal failure.

RBCs are mainly composed of hemoglobin

As evidenced by the magnitude of the hematocrit, RBCs are the most abundant elements in blood. RBCs are non-nucleated biconcave cells with a diameter of ~7.5 µm and a volume of ~90 fL (90 × 10−15 L). Maintaining the shape of the RBC is a cytoskeleton that is anchored to the plasma membrane by glycophorin and the “band 3” Cl-HCO3 exchanger (see Fig. 2-9). The distinctive shape of the RBC provides a much larger surface-to-volume ratio than that of a spherical cell; this maximizes diffusion area and minimizes intracellular diffusion distances for gas exchange. The RBC performs three major tasks: (1) carrying O2from the lungs to the systemic tissues, (2) carrying CO2 from tissues to the lungs, and (3) assisting in the buffering of acids and bases.

Table 18-2 lists the properties of RBCs that are routinely determined in the clinical laboratory. The most important constituent of the RBC is hemoglobin. Globin synthesis begins in the proerythroblast (see Fig. 18-5). By the end of the orthochromatic-erythroblast stage, the cell has synthesized all the hemoglobin it will carry. Normal blood hemoglobin content is ~14.0 g/dL in the adult female and ~15.5 g/dL in the adult male. The hemoglobin concentration in red cell cytosol is extremely high, ~5.5mM. The mean cell hemoglobin concentration is ~35 g/dL RBCs, or about five times the concentration of proteins in plasma. Enclosure of hemoglobin in red cells has the advantage of minimizing the loss of hemoglobin from the plasma through filtration across the blood capillary walls. The structure and various chemical forms of hemoglobin, as well as the carriage of O2 and CO2 by hemoglobin, are discussed in Chapter 29.

TABLE 18-2

Typical Blood Cell Parameters

RBC count (106/µL blood)

4.0 (female); 4.5 (male)

Hematocrit (%)

40 (female); 45 (male)

Hemoglobin (g/dL blood)

14.0 (female); 15.5 (male)

Mean red cell volume, MCV (fL/cell)

90

Mean red cell hemoglobin, MCH (pg/cell)

30

Mean cell hemoglobin concentration, MCHC (g/dL RBCs)

35

Red cell distribution width, RDW (%)

13

WBC count (103/µL blood)

8

Platelet count (103/µL blood)

300

Because the mature RBCs contain no nucleus or other organelles, they can neither synthesize proteins nor engage in oxidative metabolism. The RBC can engage in two metabolic pathways: glycolysis, which consumes 90% of glucose uptake; and the pentose shunt (see Fig. 58-1), which consumes the remaining 10% of glucose. The cell generates its ATP exclusively by glycolysis. An important constituent of the RBC is 2,3-diphosphoglycerate (2,3-DPG). RBCs use DPG mutase to convert 1,3-diphosphoglycerate (1,3-DPG), part of the normal glycolytic pathway (see Fig. 58-6A), into 2,3-DPG. In RBCs, the cytosolic concentration of 2,3-DPG is normally 4 to 5 mM, about the same as the concentration of hemoglobin. 2,3-DPG acts on hemoglobin by reducing the O2 affinity of hemoglobin (see pp. 654–655).

Erythrocytes contain glutathione at ~2 mM, more than any other cell of the body outside the hepatocyte (see p. 955). A high ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) protects the RBC against oxidant damage. Glutathione reductase regenerates GSH from GSSG in a reaction that consumes reduced nicotinamide adenine dinucleotide phosphate (NADPH). The RBC generates all its NADPH from the glycolytic intermediate glucose-6-phosphate through the pentose-shunt pathway (see Fig. 58-1).

RBCs carry two cytoplasmic isoforms of carbonic anhydrase (see p. 656), imageN18-3 CA I and CA II. These enzymes, which rapidly interconvert CO2 and image, play a critical role in carrying metabolically produced CO2 from the systemic tissues to the pulmonary capillaries for elimination in the exhaled air (see pp. 655–657). CA II has one of the fastest known enzymatic turnover rates.

N18-3

Carbonic Anhydrases

Contributed by Walter Boron

Overview

The carbonic anhydrases (CAs) are a family of zinc-containing enzymes with at least 16 members among mammals; eTable 18-1 lists some of these isoforms. Physiologically, the CAs catalyze the interconversion of CO2 and image although they also can cleave aliphatic and aromatic ester linkages. CA I is present mainly in the cytoplasm of erythrocytes. CA II is a ubiquitous cytoplasmic enzyme. CA IV is a GPI-linked enzyme (see p. 13) found, for example, on the outer surface of the apical membrane of the renal proximal tubule (see pp. 828–829). A hallmark of many CAs is their inhibition by sulfonamides (e.g., acetazolamide).

Reaction Catalyzed by CAs

Before we consider the action of CA, it is instructive to examine the interconversion of CO2 and image in the absence of enzyme. When [CO2] increases,

image (NE 18-1)

CO2 can also form image by directly combining with OH, a reaction that becomes important at high pH values, when [OH] is also high:

image

(NE 18-2)

Because the dissociation of H2O replenishes the consumed OH, the two mechanistically distinct pathways for image formation are functionally equivalent. Of course, both reaction sequences are reversible. However, in the absence of CA, the overall speed of the interconversion between CO2 and image is slow at physiological pH. In fact, it is possible to exploit this slowness experimentally to generate image solutions that are temporarily “out of equilibrium” imageN28-4. Unlike normal (i.e., equilibrated) solutions, such out-of-equilibrium solutions can have virtually any combination of [CO2], [image], and pH in the physiological range.

Structural biologists have solved the crystal structures of several CAs. At the reaction site, three histidines coordinate a zinc atom that, along with a threonine, plays a critical role in binding CO2 and image. In CA II, the fastest of the CAs, a fourth histidine acts as a proton acceptor/donor. Extensive site-directed mutagenesis studies have provided considerable insight into the mechanism of the CA reaction. The CAs have the effect of catalyzing the slow CO2 hydration in Equation NE 18-1. Actually, these enzymes catalyze the top reaction in Equation NE 18-2, the direct combination of CO2 with OH to form image. CA II catalyzes both reactions in Equation NE 18-2:

image (NE 18-3)

CA II has one of the highest turnover numbers of any known enzyme: Each second, one CA II molecule can convert >1 million CO2 molecules to image ions. In the erythrocyte, this rapid reaction is important for the carriage of CO2 from the peripheral blood vessels to the lungs (see pp. 655–657). In the average cell, CAs are important for allowing the rapid buffering of H+ by the image buffer pair.

Another role of CAs may be in minimizing the pH changes that occur on membrane surfaces that contain transporters that move H+ or image. For example, the extrusion of H+ from the cell by an Na-H exchanger would generate a low pH at the outer surface of the cell. CA IV, with its extracellular catalytic domain, would consume much of the extruded H+

image (NE 18-4)

and thereby minimize the fall in surface pH.

Preliminary data suggest that the electrogenic Na/HCO3 cotransporters (NBCe1 and NBCe2; see p. 122), and perhaps also the other Na+-coupled “image” transporters (the electroneutral NBCs NBCn1 and NBCn2, the Na+-driven Cl-HCO3 exchanger NDCBE; see p. 124), actually move image. As image enters the cell, the following reaction would not only replenish the image but would also generate H+ on the outer surface of the cell:

image (NE 18-5)

The consequence would be a buildup of H+ on the extracellular surface, just as in the case of an Na-H exchanger. An extracellular CA like CA IV would consume this newly generated H+:

image (NE 18-6)

The net reaction (summing Equation NE 18-5 and Equation NE 18-6) would be as follows:

image (NE 18-7)

Thus, when a transporter like NBCe1 appears to transport 2 image into the cell, what is really happening is that 1 image is being transported by the NBCe1, and 1 CO2 and 1 H2O are entering the cell by another mechanism. CA enzymes on both the extra- and intracellular surfaces of the cell minimize the pH changes (by consuming or generating H+, as necessary) generated by NBCs and H+ transporters.

CA Deficiency in Humans

For a discussion of genetic deficiencies of CAs in humans, see Box 39-1. The homozygous absence of normal CA II causes CA II deficiency syndrome, characterized by osteopetrosis, renal tubular acidosis, and cerebral calcification. At least seven different mutations can cause genetic defects. The mutation that is common in patients of Arabic descent causes mental retardation but less severe osteopetrosis. Other patients may carry two different mutations. Indeed, the first three patients described with CA II deficiency syndrome, sisters in the same family, were compound heterozygotes, having received one mutation from their mother and a second from their father.

Although CA I deficiency exists, the homozygous condition has no obvious consequences because CA I and CA II normally contribute about equally to the CA activity in RBCs.

Nontraditional CAs

Three soluble CAs—VIII, X, and XI—are unusual in that they lack one or more of the three homologous histidine residues that enzymatically active CAs use to coordinate Zn2+. In addition, two receptor tyrosine phosphatases—RPTPβ and RPTPγ—have nontraditional CA domains at the location of the ligand-binding site. It is likely that these RPTPs are CO2 and/or image sensors.

eTABLE 18-1

Some Human CAs*

ISOFORM

MOLECULAR MASS (kDa)

CELLULAR LOCATION

TISSUE DISTRIBUTION

RELATIVE ACTIVITY

SENSITIVITY TO SULFONAMIDES (e.g., ACZ)

I

29

Cytosol

RBCs and GI tract

15%

High

II

29

Cytosol

Nearly ubiquitous

100%

High

III

29

Cytosol

8% of soluble protein in slow-twitch (type I) muscle

1%

Low

IV

35

Extracellular surface of membrane (GPI linked)

Widely distributed, including acid-transporting epithelia

~100%

Moderate

VA, VB

35, 36

Mitochondria (soluble)

Mitochondria

~20%, ~70%

Moderate

VI

39-42

Secreted

Saliva, milk

~25%

High

VII

29

Cytosol

Cytosol

~70%

Very high

IX

54/58

Catalytic domain on extracellular surface

Certain cancers

~100%

High

XII

44

Catalytic domain on extracellular surface

Certain cancers

~30%

(Binds ACZ with unknown affinity)

CAXIII

29

Cytosol

Widely distributed

~10%

High

XIV

54

Catalytic domain on extracellular surface

Kidney, heart, skeletal muscle, brain, retinal pigment epithelium, certain cancers

~100%

Moderate

*Several additional CAs have been cloned. In some cases they have not been functionally characterized.

Integral membrane protein with one membrane-spanning segment.

ACZ, acetazolamide; GI, gastrointestinal.

References

Purkerson JM, Schwartz GJ. The role of carbonic anhydrases in renal physiology. Kidney Int. 2007;71:103–115.

Sly WS, Hu PY. Human carbonic anhydrases and carbonic anhydrase deficiency. Annu Rev Biochem. 1995;64:375–401.

Wykoff CC, Beasley N, Watson PH, et al. Expression of the hypoxia-inducible and tumor-associated carbonic anhydrases in ductal carcinoma in situ of the breast. Am J Pathol. 2001;158:1011–1019.

 https://en.wikipedia.org/wiki/Carbonic_anhydrase [(last accessed August 25, 2015).].

CO2 carriage also depends critically on the Cl-HCO3 exchanger AE1 (see pp. 124–125 and 656) in the RBC membrane. The transporter was originally known as band 3 protein because of its position on a sodium dodecyl sulfate–polyacrylamide gel of RBC membrane proteins. AE1 is the most abundant membrane protein in RBCs, with ~1 million copies per cell. One AE1 molecule can transport as many as 50,000 ions per second—it is one of the fastest known transporters. AE1 and most other members of the SLC4 family of HCO3 transporters are blocked by a disulfonic stilbene known as DIDS.

The water channel AQP1 (see p. 110) is the second most abundant membrane protein in RBCs, with ~200,000 copies per cell. AQP1 appears to contribute more than half of the CO2 permeability of the RBC membrane (see pp. 655–657).

Leukocytes defend against infections

Table 18-3 summarizes the relative abundance of various leukocytes in the blood. These WBCs are in two major groups: the granulocytes, on the one hand, and the lymphocytes and monocytes, on the other. Granulocytes are so named because of their cytoplasmic granules, which on a blood smear stained with Giemsa stain or Wright stain appear red (eosinophils), blue (basophils), or intermediate (neutrophils). The nuclear material is irregularly shaped in the form of the letter S or Z. The name polymorphonuclear leukocytes applies to all three types of granulocytes, but often is used to refer specifically to neutrophils. The average diameter of a neutrophil is ~12 µm, smaller than that of a monocyte (14 to 20 µm) or eosinophil (~13 µm), somewhat larger than that of a basophil (~11 µm), and much larger than that of a lymphocyte (6 to 10 µm). Granulocytes have a brief life span in the blood (<12 hours), but on activation can migrate into the tissues.

TABLE 18-3

Leukocytes*

 

COUNT (103/µL BLOOD)

DIFFERENTIAL COUNT (%)

Total leukocytes

7.4

100

Neutrophils

4.4

59 (56 segmented, 3 band)

Lymphocytes

2.5

34

Monocytes

0.3

4

Eosinophils

0.2

3

Basophils

0.04

0.5

*Listed in order of abundance.

Neutrophils

The most abundant leukocytes, neutrophils are identified on the basis of the shape of the nucleus as mature segmented neutrophils (56% of leukocytes) and immature band neutrophils (3% of leukocytes). Segmented neutrophils have at least two lobes separated by a thin filament, whereas band neutrophils have a nucleus of more uniform thickness. Neutrophils have two types of granules (specific and azurophilic) that contain lysosomal enzymes, peroxidase, collagenase, and other enzymes capable of digesting foreign material. In the presence of a chemotactic attractant, neutrophils approach foreign substances, such as bacteria, to phagocytose them within a phagocytic vacuole. By a process known as degranulation, granules merge with the vacuole and empty their contents into the vacuole. Bacteria are destroyed within the vacuole by the action of hydrogen peroxide (H2O2) and the superoxide anion radical (image; see p. 1238).

Eosinophils

The granules of eosinophils contain major basic protein (MBP), which is toxic to parasites, as well as other enzymes. These cells are important in the response to parasites and viruses. Eosinophils also play a role in allergic reactions.

Basophils

Basophils, the least common granulocytes, are a major source of the cytokine IL-4, which in turn stimulates B lymphocytes to produce IgE antibodies. The granules—which nearly obscure the nucleus—contain histamine, heparin, and peroxidase. Like eosinophils, basophils also play a role in allergic reactions.

Lymphocytes

Like the monocytes discussed next, lymphocytes do not have granules. Lymphocytes come in different classes, although they cannot be distinguished in a blood smear. The lymphocyte precursors originate in the bone marrow where lineage commitment occurs.

T lymphocytes or T cells, which represent 70% to 80% of peripheral lymphocytes in blood, undergo maturation primarily in the thymus. T lymphocytes are responsible for cell-mediated immunity.

B lymphocytes or B cells, which represent 10% to 15% of peripheral lymphocytes in blood, undergo maturation in bone marrow and peripheral lymphoid tissue. When B cells interact with antigen in the presence of T cells and macrophages, B cells can transform into plasma cells, which abundantly make and secrete antibodies that are directed against specific antigens. Thus, B cells are responsible for humoral immunity.

The remaining lymphocytes in blood include a variety of classes, such as the natural killer cells.

Monocytes

Because they migrate from the bone marrow to peripheral tissues, monocytes are not abundant in blood. Rather, monocytes spend most of their long life in peripheral tissues, where they develop into larger macrophages (20 to 40 µm in diameter). The macrophage (from the Greek macros [large] + phagein [eat]) serves two functions: (1) the phagocytosis of pathogens or cellular debris, and (2) the presentation of antigens to lymphocytes.

Platelets are nucleus-free fragments

Platelets form in the bone marrow by budding off from large cells called megakaryocytes, the maturation of which depends on TPO and IL-3. Each megakaryocyte can produce up to a few thousand platelets. Normal blood contains 150,000 to 450,000 platelets per microliter. A feedback mechanism operates between platelets and megakaryocytes, controlling platelet production. Platelets carry receptors for TPO that are able to bind and remove TPO from the plasma. Thus, a hypoplastic marrow generating few megakaryocytes leads to thrombocytopenia (platelet shortage) and thereby little removal of TPO, which in turn stimulates megakaryocyte production and corrects the lack of platelets. Conversely, a hyperplastic marrow creating many megakaryocytes leads to thrombocytosis (platelet excess) and thereby greater TPO removal, which ultimately turns off megakaryocyte production.

The life span of platelets is about 10 days. In their unactivated state, these nucleus-free fragments are disk shaped and 2 to 3 µm in diameter (Fig. 18-6). The external coat is rich in platelet receptors, which are glycoproteins. A circumferential band of microtubules composed of tubulin provides an inner skeleton. Actin and myosin contractile filaments are present in the platelet interior. In addition to mitochondria, lysosomes, and peroxisomes, platelets have two types of special organelles: α granules and, less abundantly, dense-core granules. The α granules store von Willebrand factor, platelet fibrinogen, and clotting factor V. The fibrinogen inside the platelets actually originates in the liver, which secretes it into the blood plasma, where megakaryocytes and platelets then endocytose the fibrinogen. The dense-core granules store ATP, ADP, serotonin, and Ca2+. As discussed below, platelets are essential for hemostasis (see p. 439).

image

FIGURE 18-6 Discoid platelet.