Chapter 3: Blood

Blood circulates within the cardiovascular system to provide a vehicle for the distribution of respiratory gases, nutrients, water, electrolytes, hormones, antibodies, drugs, metabolic waste, and heat throughout the body. Most of the major negative feedback control systems of the body defend normal blood composition (e.g., blood glucose concentration, blood volume, and arterial O₂ partial pressure). Blood is composed of cellular elements (i.e., red blood cells, white blood cells, and platelets), which are suspended in blood plasma. A person who weighs 70 kg has approximately 5 L of blood, of which about 2 L is occupied by the formed cellular elements and 3 L is plasma.

This chapter will discuss mainly the physiologic regulation of the red blood cell mass and the mechanisms that prevent bleeding.

Plasma Composition

Plasma is the part of the extracellular fluid that is contained within the cardiovascular system. It is a dilute solution, which by weight is approximately 92% water, 7% protein, and 1% small dissolved solutes (e.g., ions, urea, glucose, amino acids, and lipids). The normal plasma concentrations of selected ions and small molecules are almost the same as those in the interstitial fluid because of the free exchange of water and small solutes across most blood capillaries (Table 3-1). In contrast, most capillaries are impermeable to plasma proteins. The resulting difference in protein concentration between the plasma and the interstitial fluids creates a colloid osmotic (oncotic) pressure gradient that opposes the filtration of plasma out of the capillaries (Chapter 4). Albumin is the most abundant type of plasma protein and, therefore, is the greatest contributor to the plasma oncotic pressure. The major types of plasma protein and their functions are listed in Table 3-2.

Hypoalbuminemia has many causes, including nephrotic syndrome, liver failure, and severe malnutrition. Regardless of the cause, hypoalbuminemia can result in anasarca (generalized massive edema).

The Cellular Elements of Blood

There are three main cell types in circulating blood (Figure 3-1):

1. Red blood cells (erythrocytes) are essential for the transport of O₂ and CO₂ in blood (Chapter 5). There are approximately 5 × 10¹² red blood cells per L of blood.

2. Platelets (thrombocytes) are small cellular fragments that play a key role in hemostasis by adhering to sites of damage and through participation in the blood clotting system. There are approximately 300 × 10⁹ platelets per L of blood.

3. White blood cells (leukocytes) are the only fully functional nucleated cells in circulating blood. White cells play a defensive role in destroying infecting organisms and in the removal of damaged tissue. The total white cell count is approximately 5 × 10⁹ cells per L of blood. The characteristic features and functions of the five major types of white blood cells are described in Table 3-3.

The formation of blood cells (hematopoiesis) occurs in the bone marrow. In neonates, most of the skeleton contains active bone marrow, but in adults it is usually found only in the vertebrae, ribs, skull, pelvis, and the proximal femurs. Blood cells are derived from stem cells in the bone marrow and are produced after several stages of cell division and differentiation. Blood-forming cells may be divided into four groups, depending on their capacity for self-renewal, cell division, and the ability to form different cell types (see Figure 3-1):

1. Pluripotent hematopoietic stem cells, of which there are very few, are capable of forming any type of blood cell.

2. Multipotential progenitor cells can form a specific but wide range of blood cells. There are two types of multiprogenitor cells. One differentiates into lymphoid cells, and another differentiates into other types of blood cells; the latter is sometimes referred to as the GEMM (granulocyte erythroid monocyte megakaryocyte) progenitor cell.

3. Committed progenitor cells are capable of self-renewal but are able to form only one or two cell types. There is a different type of committed progenitor for each of the mature blood cell types found in circulating blood, with the exception of neutrophils and monocytes, which are both derived from a single type of committed progenitor cell.

4. Maturing cells do not divide again and are undergoing structural differentiation to form a specific type of blood cell.

The process of blood cell formation therefore involves sequences of cell division and differentiation. Erythropoiesis is the process of red blood cell formation; thrombopoiesis is the process of platelet production; and leukopoiesis is a general term describing white blood cell production.

Bone marrow (hematopoietic cell) transplant may be used in the treatment of many different hematologic and immunologic diseases, including acquired conditions such as leukemia, lymphoma, and aplastic anemia, and inherited conditions such as sickle cell anemia and severe combined immunodeficiency. Hematopoietic stem cells have several features that make transplantation clinically feasible:

• A remarkable regenerative capacity.

• An innate ability to “home” to the bone marrow of the recipient.

• An ability to survive cryopreservation.

Because of the tremendous regenerative capacity of a stem cell, only a small percentage of the donor's bone marrow is needed for complete replacement of the recipient's entire lymphohematopoietic system. Mouse models have shown that transplantation of a single stem cell can completely replace the entire lymphohematopoietic system of the adult mouse.

Erythropoiesis

The erythron is a dispersed organ that consists of the whole mass of mature red blood cells and their progenitors in the bone marrow. The production of red blood cells is regulated to provide an adequate O₂-carrying capacity in blood. The hormone erythropoietin (EPO) is the primary regulator of erythropoiesis, and is released from the kidney when O₂ tension in the renal parenchyma is reduced (see Chapter 6). Committed erythroid stem cells in the bone marrow are responsive to EPO, which increases the rate of cell division and the differentiation into mature red cells.

The maturation of red cells in the bone marrow requires several factors; most notably iron, for the synthesis of the O₂-carrying pigment hemoglobin, and the essential nutrients folic acid and vitamin B₁₂. There are several general features of the process of stem cell differentiation into mature red cells:

• Cells decrease in size.

• Hemoglobin is produced, which fills the cytoplasm and causes the characteristic red coloration.

• The number of cell organelles decreases; all organelles eventually disappear from red cells.

• The nucleus gradually condenses and disappears.

In the final stage of differentiation, immature red cells are called reticulocytes, which are characterized by the presence of organelle remnants. There is normally less than 1% of circulating red blood cells in the immature reticulocyte stage. However, the appearance of larger numbers of reticulocytes reflects a rapid increase in the demand for red blood cells (e.g., after acute hemorrhage). The reticulocyte count is therefore a useful index of the rate of red blood cell production.

Thrombopoiesis

Platelets are small disk-shaped cell fragments without a nucleus. The cytoplasm contains an extensive cytoskeleton, which allows the shape of a platelet to change upon activation. There are also many secretory granules containing factors that regulate hemostasis. Platelet production (thrombopoiesis) occurs in the bone marrow by the cytoplasmic fragmentation of megakaryocytes (Figure 3-1). Megakaryocytes are the largest cells in the bone marrow and are formed when committed progenitor cells undergo repeated nuclear division without cell division. The rate of platelet formation is regulated by the cytokine thrombopoietin (TPO) (note: the term “cytokine” is used to describe peptide hormones that regulate cell differentiation or immune functions). TPO is constitutively secreted by the liver and kidneys, although the plasma concentration of TPO is mostly determined by the number of platelets in circulation. Platelets remove TPO from the circulation and so high levels of TPO result when there is a low platelet count (thrombocytopenia). Increased TPO concentrations stimulate megakaryocytes to produce more platelets and thereby restore a normal platelet count.

Leukopoiesis

The characteristics of white blood cell production vary depending on the cell type. The development of lymphoid progenitor cells in bone marrow yields only primitive precursor cells. The final differentiation of lymphocytes occurs in peripheral lymphoid tissues as part of a specific immune response; T lymphocytes differentiate in the thymus gland, and B lymphocytes mainly differentiate in the lymph nodes. Myelopoiesis is the formation of the other (nonlymphoid) white blood cells through immature myeloid cell stages in the bone marrow into fully differentiated cells.

• The granular white blood cells (i.e., neutrophils, eosinophils, and basophils) all have morphologically similar stages of cell differentiation in the bone marrow. Granulocytes are released from the bone marrow as mature cells. In the case of neutrophils, a large pool of cells is maintained in the bone marrow and can be rapidly mobilized in response to an infection.

• Monocytes leave the bone marrow soon after their formation. In contrast to neutrophils, there is no pool of mature monocytes in the bone marrow. Monocytes typically spend 2–3 days in the circulation before entering the tissues to become macrophages.

Differential white blood cell counts include both the total number of white cells and the proportion of each type of white cell present (Table 3-3). The production of white blood cells (leukopoiesis) is under the control of several cytokines, which are collectively known as colony-stimulating factors. An important element in the regulation of leukopoiesis is the secretion of colony-stimulating factors by active white blood cells. For example, bacterial infections are usually associated with an increased proportion of neutrophils and monocytes, whereas viral infections increase the proportion of lymphocytes.

If a malignant transformation occurs in a white blood cell lineage, a person can develop leukemia or lymphoma. Leukemia is a general term used to describe a malignancy that is primarily based in the bone marrow and blood, whereas lymphomas are solid tumors that are primarily based in the lymph nodes. In acute leukemia, the bone marrow becomes infiltrated with malignant blast cells (immature white blood cells). When these blast cells are of myeloid lineage, the condition is called acute myeloid leukemia; when the blast cells are of lymphoid lineage, acute lymphoblastic leukemia occurs. Regardless of cell lineage, the symptoms associated with acute leukemia are typically due to bone marrow failure, which includes suppressed red blood cells (presenting as anemia), suppressed white blood cells (presenting as infection), and suppressed platelet formation (presenting as bleeding tendency). Chronic leukemia, on the other hand, has a much more gradual onset, typified by very high leukocyte counts and by cells that have a mature appearance (e.g., chronic myeloid leukemia and chronic lymphocytic leukemia).

Mature erythrocytes are disk-shaped anucleate cells, approximately 7–8 μm in diameter. The extensive cytoskeleton of red cells causes them to be extremely pliable and allows them to pass through the microcirculation. Red cells can also withstand large osmotic pressure differences, which are encountered when they pass through the renal medullary circulation (see Chapter 6). When red cells leave the bone marrow, they contain several key cytoplasmic proteins:

• Hemoglobin is the major protein present in red blood cells, and is responsible for the large O₂-carrying capacity of blood (see Chapter 5).

• Glycolytic enzymes are needed because red cells have no mitochondria and must synthesize adenosine triphosphate (ATP) via glycolysis.

• Carbonic anhydrase is used to catalyze the following equilibrium reaction, which is essential for CO₂ carriage in blood (see Chapter 5):

  

The normal lifespan of a red blood cell in the circulation is approximately 100–120 days. Erythropoiesis must therefore replace approximately 0.8% to 1.0% of the circulating red cells daily to maintain a stable red cell mass. Aging red blood cells become progressively more fragile and are ultimately removed from the circulation by scavenging macrophages, particularly in the spleen. The end product of hemoglobin breakdown in macrophages is bilirubin, which is conjugated in the liver and excreted in the bile (see Chapter 7).

A critical balance must be maintained between the rate of red cell production and the rate of red cell loss from the circulation. The concept of a balance between the rate of erythropoiesis and red cell loss provides a physiologic basis for understanding disorders when the red cell mass is either decreased (anemia) or increased (polycythemia).

Red Blood Cell Indices

Several indices are routinely reported in a complete blood count (CBC) to characterize the red cell mass (Table 3-4).

• The size of the total red cell mass is described by three variables:

  1. Hemoglobin concentration (Hb) is the amount of hemoglobin in a volume of blood.

  2. Hematocrit (Hct) or packed cell volume (PCV) is the ratio of the volume of red cells to the volume of whole blood. It may be determined by centrifuging a sample of blood and comparing the height of the column of packed red cells at the bottom of the tube to the total height of the column.

  3. Red blood cell count (RBC) is the number of red cells per liter of blood.

• The average size of a red blood cell is described using the mean cell volume (MCV), which is the average volume of a single red cell expressed in femtoliters (fL = 10⁻¹⁵ L). MCV is calculated by dividing the Hct by the red blood cell count. MCV < 80 fL indicates that red cells are small (microcytosis); MCV > 100 fL indicates that red cells are large (macrocytosis).

• Two other variables are derived to describe the adequacy of hemoglobin synthesis:

  1. Mean cell hemoglobin (MCH) is the average amount of hemoglobin in the average red cell, expressed in picograms (pg = 10⁻¹² g). MCH is calculated by dividing the hemoglobin concentration by the red blood cell count.

  2. Mean cell hemoglobin concentration (MCHC) is the average concentration of hemoglobin in red cells, calculated by dividing MCH by MCV.

Example

A 42-year-old woman visits her family doctor complaining of recent heavy menses and a constant feeling of fatigue. A CBC reveals Hb of 8 g/dL, Hct of 24%, and RBC of 2.6 × 10¹²/L. Calculate the MCV, MCH, and MCHC.

This patient has anemia, as defined by her low hemoglobin concentration and low Hct; her red blood cell count is decreased in proportion to the decreased Hct. Therefore, the reduced red cell mass consists of normally sized cells (rather than abnormally sized cells), which contain a normal concentration of hemoglobin.

As a complement to the red cell indices, a blood smear is viewed under the microscope to investigate variations in cell size and in cell shape, which can indicate errors in red cell maturation. A reticulocyte count is determined in patients with anemia to assess the rate of red cell production. Establishing the cause of anemia requires other studies, including measurements of the nutrients required for hemoglobin synthesis (particularly iron, vitamin B₁₂, and folic acid). When defects in red cell production are indicated, an aspirate of the bone marrow may also be studied.

Anemia and Polycythemia

Anemia is defined as a reduced red cell mass and is most often recognized when laboratory test results are abnormal. Anemia is more easily diagnosed if historic data for a patient are available because there is considerable physiologic variation in the Hct (Table 3-4). The symptoms of advanced anemia include fatigue, dizziness, and shortness of breath. A physical examination may indicate signs of compensatory increases in cardiac output, such as strong peripheral pulses and tachycardia. Upon auscultation, flow murmurs may be heard over the branch points of large arteries due to the low viscosity of the blood.

There are many specific causes of anemia, but the physiology of red cell production provides a basis for diagnosis (Figure 3-2). When a reduced hemoglobin concentration and Hct indicate the presence of anemia, a reticulocyte count can be used to determine if the rate of red cell production is normal. A high reticulocyte count suggests an increased rate of erythropoiesis, which reflects physiologic compensation for an increased rate of red cell loss. There are two general causes of decreased red cell survival:

1. Hemorrhage causes immediate loss of red cells. When plasma volume is restored, the reduced red cell mass is revealed by a low Hct.

2. In hemolytic anemia, the rate of red cell breakdown is increased. Hemolytic anemia can be caused by a wide range of conditions, but is uncommon. For example, in hereditary spherocytosis, the cell cytoskeleton is defective, resulting in increased red cell fragility. Hemoglobinopathies (e.g., sickle cell disease) are inherited defects in hemoglobin structure that can result in the deformation and increased destruction of red cells.

In cases of anemia where the reticulocyte count is normal or reduced, a defect in red cell production or maturation is indicated. There are three requirements for normal red cell production:

1. Functional bone marrow. Hypoproliferation of red cells may result if the bone marrow is damaged (e.g., aplastic anemia).

2. Erythropoietin. Secretion of EPO is needed to stimulate the bone marrow. Low EPO levels are found in patients with renal insufficiency.

3. Adequate nutrient supply for hemoglobin synthesis. The most common cause of anemia worldwide is iron deficiency. Other examples in which there is a failure of the normal maturation of red cells include dietary deficiency of vitamin B₁₂ or folic acid.

Iron deficiency is the most common cause of a microcytic anemia, whereas the major causes of a macrocytic (or megaloblastic) anemia are vitamin B₁₂ and/or folate deficiency.

Polycythemia is an abnormally high level of circulating red blood cells, and is less commonly encountered than anemia. A high red cell mass increases the viscosity of blood and increases the risk of thrombosis (formation of an intravascular blood clot). The most common causes of polycythemia are associated with increased levels of EPO. For example, conditions that result in reduced arterial O₂ content, such as lung disease, carbon monoxide poisoning, or living at a high altitude, all stimulate EPO secretion.

An essential property of the blood is the ability to clot, which aids in preventing blood loss. Precisely regulated interactions between blood vessel walls, circulating platelets, and clotting proteins in the plasma account for the ability to quickly stop blood loss (i.e., hemostasis). The formation of a stable blood clot is followed by the orderly breakdown of the clot (fibrinolysis) as wound healing proceeds. A fine balance must be achieved between the activation of hemostatic mechanisms to prevent bleeding and excessive activation, which can cause intravascular thrombosis and embolism (blood vessel occlusion).

Overview of Hemostasis

Three physiologic mechanisms interact to prevent hemorrhage:

1. Vasoconstriction of small vessels reduces blood flow and increases the likelihood of vessel closure. At a site of injury, platelet activation and clot formation result in the release of the vasoconstrictors. Platelets release the vasoconstrictors serotonin and thromboxane A₂; the clotting protein thrombin stimulates endothelial cells to secrete the highly potent vasoconstrictor endothelin-1.

2. Platelet plug formation occurs at the site of damage in capillaries, arterioles, and venules. Plug formation is called primary hemostasis.

3. Clot formation occurs in which a fibrin mesh forms together with platelets and other trapped blood cells. Clot formation is called secondary hemostasis, and is closely coordinated with primary hemostasis.

Increased tissue pressure is an effective means to prevent bleeding; for example, this can be achieved by applying a surgical clamp, a tourniquet, or by applying direct pressure to a wound. Increasing the tissue pressure reduces blood vessel radius and decreases local blood flow; hemostatic mechanisms are facilitated by preventing the washout of procoagulant factors.

Key elements in a patient's history can help to differentiate bleeding due to a platelet disorder versus bleeding due to a plasma coagulation defect (coagulopathy). Bleeding that results from defective platelet function typically occurs superficially in sites such as the skin (e.g., petechiae and ecchymosis) and mucous membranes. In contrast, patients who bleed secondary to clotting factor dysfunction will suffer “deep” bleeds, such as in the deep subcutaneous tissues or muscles causing a hematoma, or in the joints causing hemarthrosis.

Platelet Function

There are three phases of primary hemostasis:

1. Platelet adhesion to the exposed subendothelial extracellular matrix occurs at a site of injury (Figure 3-3). Adhesion is mediated by a variety of different platelet receptors, including:

   • Receptors of the integrin family are important for binding to extracellular matrix proteins (e.g., collagen).

   • The von Willebrand factor (vWF) is a ligand present in plasma and is released by endothelial cells and by activated platelets. The binding of vWF to a type of platelet receptor promotes platelet adhesion; vWF forms cross-links between platelets and with collagen.

   • Other receptor types bind ligands associated with platelet activation (e.g., thromboxane A₂) and blood clotting (e.g., thrombin), which further promote platelet adhesion and activation.

2. Platelet activation. Several intracellular signaling pathways are activated when ligands bind to platelet receptors. The result is exocytosis of the contents of platelet granules and a morphologic change from a smooth membrane surface to one with finger-like cytoplasmic projections. Adherent platelets therefore release many locally acting agents and undergo physical changes that increase adhesion and aggregation with other platelets. Activated platelets release many factors that promote hemostasis, including:

   • Adenosine diphosphate (ADP) is a potent activator of other platelets, amplifying the platelet activation response.

   • Serotonin and thromboxane A₂ assist hemostasis as vasoconstrictors.

   • vWF augments platelet adhesion and aggregation.

   • Ca²⁺ and the clotting factors fibrinogen and factor V facilitate coagulation.

   • Platelet-derived growth factor promotes wound healing by stimulating growth and migration of fibroblasts and smooth muscle cells at the site of injury.

3. Platelet aggregation completes the formation of a platelet plug (Figure 3-3). The signaling molecules released during platelet activation amplify the platelet adhesion and activation responses and recruit more platelets to the site of injury. The platelet plug is prevented from extending beyond the site of injury by prostacyclin and nitric oxide, which are secreted from intact endothelial cells and inhibit platelet activation.

von Willebrand's disease (vWD) is the most common inherited bleeding disorder and is characterized by either a deficiency or a functional defect in vWF. Because vWF is also a cofactor for the procoagulant factor VIII, a defect in vWF will result in both platelet dysfunction and coagulopathy, a characteristic feature of this disease that is shared with very few other conditions (e.g., disseminated intravascular coagulopathy [DIC]).

Antiplatelet drugs are used to prevent thrombosis (e.g., cardiovascular disease) and target many of the steps of platelet activation and adhesion. A few examples include:

• Aspirin and NSAIDs, which inhibit the production of thromboxane A₂ by blocking the key enzyme cyclooxygenase (COX). These agents therefore inhibit platelet activation and secretion.

• Clopidogrel (Plavix) and ticlopidine (Ticlid), which antagonize the actions of adenosine diphosphate and therefore inhibit platelet activation.

• Eptifibatide (Integrilin) is a competitive inhibitor of glycoprotein IIb/IIIa, a platelet specific adhesion molecule in the integrin family. Blockade of this important adhesion receptor prevents the binding of fibrinogen, vWF, and other adhesion molecules to activated platelets.

Thrombocytopenia (low platelet count) has many causes, which can be broadly classified into three categories:

1. Decreased platelet production by the bone marrow (e.g., leukemia or tumor infiltration of the marrow).

2. Splenic sequestration of platelets (e.g., splenomegaly secondary to portal hypertension).

3. Increased destruction of platelets (e.g., antibody-mediated platelet destruction in idiopathic thrombocytopenic purpura [ITP]).

Regardless of the cause, thrombocytopenia results in an increased bleeding tendency.

The Blood Coagulation Mechanism

A blood clot is formed when the plasma protein fibrinogen is proteolytically cleaved to produce fibrin, which subsequently becomes cross-linked into a stable mesh. Clotting can occur in the absence of platelet activation, but there is usually parallel activation of primary and secondary hemostasis and cross talk between the pathways to ensure coordinated activation. In the classic description of coagulation, clotting may be initiated by one of two pathways: the intrinsic pathway or the extrinsic pathway (Figure 3-4). Each pathway consists of a cascade of reactions, in which inactive circulating precursor proteins (“clotting factors”) become activated, in most cases by proteolytic cleavage. These chain reactions normally are not activated in the circulation because clotting factors are present at low concentration in plasma. The intrinsic pathway is triggered when blood contacts a negatively charged surface (e.g., exposed subendothelial collagen; aggregations of platelets). The extrinsic pathway is activated when blood contacts cells outside the vascular endothelium; nonvascular cells express a ubiquitous membrane protein called tissue factor, which initiates the extrinsic pathway.

The intrinsic and extrinsic pathways converge at the common pathway for coagulation, which begins with the activation of factor X. The plasma protein prothrombin is cleaved by activated factor X to produce the protease thrombin (Figure 3-4). Thrombin is responsible for the cleavage of fibrinogen and for the activation of factor XIII, which cross-links fibrin into a stable mesh. The extrinsic pathway is thought to be the most important pathway for initiating thrombin activation, whereas the intrinsic pathway is thought to be more important for maintaining thrombin generation.

The current understanding of the clotting pathways shows that rather than the intrinsic, extrinsic, and common pathways being independent, they form a network with reciprocal connections. For example, activated factor IX of the intrinsic pathway activates factor VII of the extrinsic pathway. In the reverse direction, a complex formed from tissue factor, activated factor VII, and Ca²⁺ in the extrinsic pathway can activate factors IX and XI of the intrinsic pathway. Thrombin plays a central role in the coordination of the clotting cascades because it stimulates the downstream events in the common pathway, resulting in the formation of a fibrin clot, and it mediates positive feedback stimulation at upstream points in the intrinsic and extrinsic pathways (Figure 3-4).

Clotting Indices

Several tests are used to assess the function of primary and secondary hemostatic mechanisms, including:

• The bleeding time is a sensitive test of platelet function. A small standardized incision is made in the underside of the forearm, and the amount of time it takes for bleeding to stop is recorded. Drugs such as aspirin, which inhibits platelet function, increase the bleeding time.

• The prothrombin time (PT) evaluates the extrinsic coagulation pathway. A sample of blood plasma is incubated with tissue factor in the presence of an excess of Ca²⁺. Because there are variations between assays, corrections may be applied that normalize the prothrombin time of a sample to that of a normal sample (e.g., the prothrombin ratio and international normalized ratio [INR]). The anticoagulant drug warfarin increases the prothrombin time.

• The partial thromboplastin time (PTT) indicates the performance of the intrinsic pathway. The intrinsic pathway is triggered by adding an activator surface (e.g., silica) plus phospholipid and Ca²⁺ to a plasma sample. The anticoagulant drug heparin increases the partial thromboplastin time.

Clotting factors II, VII, IX, and X (and the anticoagulant proteins C and S) are produced in the liver and rely upon a vitamin K-dependent enzymatic reaction, γ-carboxylation of a glutamyl residue, which has significant clinical implications. In select patients (e.g., those at risk of developing a blood clot secondary to atrial fibrillation), it can be advantageous to block the function of γ-glutamyl carboxylase to induce a coagulopathy. The anticoagulant medication warfarin (Coumadin) inhibits the enzyme vitamin K epoxide reductase, making vitamin K unavailable for the γ-carboxylation reaction, which will halt the production of the vitamin K-dependent coagulation factors. Careful monitoring of the prothrombin time or the INR is required to ensure that excessive anticoagulation does not occur.

The bleeding disorders hemophilia A (classic hemophilia) and hemophilia B (Christmas disease) are X-linked recessive disorders that result in the deficiency of clotting factors VIII and IX, respectively. An increase in PTT is expected in both conditions because both factor VIII and factor IX are part of the intrinsic pathway.

Anticoagulants

The blood clotting cascades have the effect of amplifying a stimulus for coagulation, so it is essential that blood clotting is restricted to the site where it is needed. Random blood clotting in the circulation is normally prevented by the presence of anticoagulant factors. The capillary endothelium is the main source of anticoagulant factors.

• Tissue factor pathway inhibitor (TFPI) is anchored to the endothelial cell membrane and blocks the action of activated factor VII in the extrinsic pathway.

• Antithrombin III inhibits coagulation by binding to activated factor X and thrombin; the binding of antithrombin III is augmented by heparan sulfate molecules on the surface of endothelial cells.

• Thrombomodulin inhibits coagulation by binding to thrombin.

• Proteins C and S act together to inactivate activated factors V and VIII, which are cofactors in the clotting cascades.

Heparin is a glycosaminoglycan molecule that is released endogenously from mast cells and basophils, and is widely used as an anticoagulant drug. Heparin functions by augmenting the anticoagulant effects of antithrombin III.

Hypercoagulable states occur when there is a defect in the anticoagulant pathway. For example, a deficiency of antithrombin III, protein C, or protein S will increase a patient's risk for thrombosis. The most common inherited cause of hypercoagulability is the factor V Leiden mutation. In this condition, factor V undergoes a mutation that renders it resistant to the anticoagulant actions of protein C, which allows factor V to remain activated, prolonging its thrombogenic effect.

Fibrinolysis

When a blood clot is initially formed, it is a semisolid mass consisting of platelets and a fibrin mesh, which also traps red and white blood cells and contains plasma. The clot solidifies as platelets contract and squeeze out plasma water. The protein plasminogen is among the serum proteins that are adsorbed into the clot at the time of its formation. The cleavage of plasminogen produces the protease plasmin, which breaks down fibrin and fibrinogen. The breakdown products of the fibrin mesh are released from the blood clot and are scavenged by macrophages.

Fibrinolysis begins soon after a clot is formed. The activation of plasminogen is mainly regulated by two factors, which are released from capillary endothelial cells: tissue plasminogen activator and urokinase-type plasminogen activator (Figure 3-5). It is important that plasmin is not active in the circulation because it can also break down fibrinogen. If plasmin escapes from the clot, it is rapidly bound and inactivated by the circulating plasma protein α₂-antiplasmin.