Guyton 37: Hemostasis and Blood Coagulation PDF

CHAPTER 37 UNIT VI Hemostasis and Blood Coagulation HEMOSTASIS EVENTS fragme...

CHAPTER 37 UNIT VI Hemostasis and Blood Coagulation HEMOSTASIS EVENTS fragment into the minute platelets in the bone marrow or soon after entering the blood, especially as they squeeze The term hemostasis means prevention of blood loss. through capillaries. The normal concentration of platelets Whenever a vessel is severed or ruptured, hemostasis is in the blood is between 150,000 and 450,000/µl. achieved by several mechanisms: (1) vascular constric- Platelets have many functional characteristics of whole tion; (2) formation of a platelet plug; (3) formation of a cells, even though they do not have nuclei and cannot blood clot as a result of blood coagulation; and (4) even- reproduce. In their cytoplasm are the following: (1) actin tual growth of fibrous tissue into the blood clot to close and myosin molecules, which are contractile proteins the hole in the vessel permanently. similar to those found in muscle cells, and still another VASCULAR CONSTRICTION contractile protein, thrombosthenin, that can cause the platelets to contract; (2) residuals of both the endo- Immediately after a blood vessel has been cut or rup- plasmic reticulum and Golgi apparatus that synthesize tured, the trauma to the vessel wall causes smooth muscle various enzymes and especially store large quantities of in the wall to contract; this instantaneously reduces the calcium ions; (3) mitochondria and enzyme systems that flow of blood from the ruptured vessel. The contraction are capable of forming adenosine triphosphate (ATP) and results from the following: (1) local myogenic spasm; (2) adenosine diphosphate (ADP); (4) enzyme systems that local autacoid factors from the traumatized tissues, vas- synthesize prostaglandins, which are local hormones that cular endothelium, and blood platelets; and (3) nervous cause many vascular and other local tissue reactions; (5) reflexes. The nervous reflexes are initiated by pain nerve an important protein called fibrin-stabilizing factor, which impulses or other sensory impulses that originate from the we discuss later in relation to blood coagulation; and (6) traumatized vessel or nearby tissues. However, even more a growth factor that causes vascular endothelial cells, vas- vasoconstriction probably results from local myogenic cular smooth muscle cells, and fibroblasts to multiply and contraction of the blood vessels initiated by direct dam- grow, thus causing cellular growth that eventually helps age to the vascular wall. And, for the smaller vessels, the repair damaged vascular walls. platelets are responsible for much of the vasoconstriction On the platelet cell membrane surface is a coat of glyco- by releasing a vasoconstrictor substance, thromboxane A2. proteins that repulses adherence to normal endothelium The more severely a vessel is traumatized, the greater and yet causes adherence to injured areas of the vessel the degree of vascular spasm. The spasm can last for many wall, especially to injured endothelial cells and even more minutes or even hours, during which time the processes so to any exposed collagen from deep within the vessel of platelet plugging and blood coagulation can take place.! wall. In addition, the platelet membrane contains large amounts of phospholipids that activate multiple stages in FORMATION OF THE PLATELET PLUG the blood-clotting process, as discussed later. If the cut in the blood vessel is very small—many very Thus, the platelet is an active structure. It has a half-life small vascular holes develop throughout the body each in the blood of only 8 to 12 days, so over several weeks its day—the cut is often sealed by a platelet plug rather than functional processes run out; it is then eliminated from by a blood clot. To understand this process, it is important the circulation mainly by the tissue macrophage system. that we first discuss the nature of platelets themselves. More than half of the platelets are removed by macro- phages in the spleen, where the blood passes through a Physical and Chemical Characteristics latticework of tight trabeculae.! Platelets (also called thrombocytes) are minute discs 1 to 4 micrometers in diameter. They are formed in the bone Mechanism of Platelet Plug Formation marrow from megakaryocytes, which are extremely large Platelet repair of vascular openings is based on several hematopoietic cells in the marrow; the megakaryocytes important functions of the platelet. When platelets 477 UNIT VI Blood Cells, Immunity, and Blood Coagulation Figure 37-1. Formation of a platelet plug in a severed blood vessel. Endothelial in- jury and exposure of the vascular extracel- Shape Granule Recruitment lular matrix facilitates platelet adhesions Adhesion change release and activation, which changes their shape (ADP, PAF, TXA2) and causes release of adenosine diphos- phate (ADP), thromboxane A2 (TXA2), Gplb Aggregation and platelet-activating factor (PAF). These platelet-secreted factors recruit additional platelets (aggregation) to form a hemo- static plug. Von Willebrand factor (vWF) serves as an adhesion bridge between sub- Endothelium vWF endothelial collagen and the glycoprotein Basement Damaged blood Ib (GpIb) platelet receptor. membrane vessel wall come in contact with a damaged vascular surface, especially with collagen fibers in the vascular wall, the platelets rapidly change their own characteristics dras- tically (Figure 37-1). They begin to swell, they assume irregular forms with numerous irradiating pseudo- 1. Severed vessel 2. Platelets agglutinate pods protruding from their surfaces, their contractile proteins contract forcefully and cause the release of granules that contain multiple active factors, and they become sticky so that they adhere to collagen in the tissues and to a protein called von Willebrand factor 3. Fibrin appears 4. Fibrin clot forms (vWF), which leaks into the traumatized tissue from the plasma. The platelet surface glycoproteins bind to vWF in the exposed matrix below the damaged endo- thelium. The platelets then secrete increased quanti- ties of ADP and platelet- activating factor (PAF), and their enzymes form thromboxane A2. Thromboxane is 5. Clot retraction occurs a vasoconstrictor and, along with ADP and PAF, acts on Figure 37-2. Clotting process in a traumatized blood vessel. (Modi- nearby platelets to activate them as well; the stickiness fied from Seegers WH: Hemostatic Agents. Springfield, IL: Charles C of these additional activated platelets causes them to Thomas, 1948.) adhere to the original activated platelets. Therefore, at the site of a puncture in a blood vessel wall, the damaged vascular wall activates successively BLOOD COAGULATION IN THE RUPTURED increasing numbers of platelets that attract more and VESSEL more additional platelets, thus forming a platelet plug. The third mechanism for hemostasis is formation of the This plug is loose at first but is usually successful in block- blood clot. The clot begins to develop in 15 to 20 seconds ing blood loss if the vascular opening is small. Then, dur- if the trauma to the vascular wall is severe and in 1 to 2 ing the subsequent process of blood coagulation, fibrin minutes if the trauma is minor. Activator substances from threads form. These threads attach tightly to the platelets, the traumatized vascular wall, from platelets, and from thus constructing an unyielding plug. blood proteins adhering to the traumatized vascular wall Importance of Platelet Mechanism for Closing initiate the clotting process. The physical events of this Vascular Holes. The platelet- plugging mechanism is process are shown in Figure 37-2; Table 37-1 lists the extremely important for closing minute ruptures in most important clotting factors. very small blood vessels that occur many thousands of Within 3 to 6 minutes after rupture of a vessel, the times daily. Indeed, multiple small holes through the entire opening or broken end of the vessel is filled with endothelial cells themselves are often closed by plate- clot if the vessel opening is not too large. After 20 to 60 lets actually fusing with the endothelial cells to form minutes, the clot retracts, which closes the vessel still additional endothelial cell membranes. Literally thou- further. Platelets also play an important role in this clot sands of small hemorrhagic areas develop each day un- retraction, as discussed later.! der the skin (petechiae, which appear as purple or red FIBROUS ORGANIZATION OR dots on the skin) and throughout the internal tissues DISSOLUTION OF BLOOD CLOTS of a person who has few blood platelets. This phenom- enon does not occur in persons with normal numbers Once a blood clot has formed, it can follow one of two of platelets.! courses: (1) it can become invaded by fibroblasts, which 478  Chapter 37 Hemostasis and Blood Coagulation Table 37-1 Clotting Factors in Blood and Their Prothrombin Synonymsa Prothrombin Ca2+ Clotting Factor Synonym(s) activator Fibrinogen Factor I Thrombin Prothrombin Factor II UNIT VI Tissue factor Factor III; tissue thromboplastin Fibrinogen Fibrin monomer Calcium Factor IV Ca2+ Factor V Proaccelerin; labile factor; Ac- globulin (Ac-G) Fibrin fibers Factor VII Serum prothrombin conversion Thrombin Activated accelerator (SPCA); proconvertin; fibrin-stabilizing factor stable factor Factor VIII Antihemophilic factor (AHF); Cross-linked fibrin fibers antihemophilic globulin (AHG); Figure 37-3 Schema for conversion of prothrombin to thrombin and antihemophilic factor A polymerization of fibrinogen to form fibrin fibers. Factor IX Plasma thromboplastin component (PTC); Christmas factor; antihemophilic factor B in the blood vessels. However, when a vessel is ruptured, Factor X Stuart factor; Stuart-Prower factor procoagulants from the area of tissue damage become Factor XI Plasma thromboplastin antecedent activated and override the anticoagulants, and then a clot (PTA); antihemophilic factor C does develop. Factor XII Hageman factor Clotting takes place in three essential steps: Factor XIII Fibrin-stabilizing factor 1. In response to rupture of the vessel or damage to the blood itself, a complex cascade of chemical re- Prekallikrein Fletcher factor actions occurs in the blood involving more than 12 High-molecular- Fitzgerald factor; high-molecular- blood coagulation factors. The net result is the for- weight kininogen weight kininogen (HMWK) mation of a complex of activated substances collec- Platelets tively called prothrombin activator. aThese are listed here mainly for historical interest. 2. The prothrombin activator catalyzes the conversion of prothrombin into thrombin. subsequently form connective tissue all through the clot; 3. The thrombin acts as an enzyme to convert fibrino- or (2) it can dissolve. The usual course for a clot that forms gen into fibrin fibers that enmesh platelets, blood in a small hole of a vessel wall is invasion by fibroblasts, cells, and plasma to form the clot. beginning within a few hours after the clot is formed, We will first discuss the mechanism whereby the blood which is promoted at least partially by growth factor clot is formed, beginning with conversion of prothrombin secreted by platelets. This process continues to complete to thrombin, and then come back to the initiating stages organization of the clot into fibrous tissue within about 1 in the clotting process whereby prothrombin activator is to 2 weeks. formed.! Conversely, when excess blood has leaked into the CONVERSION OF PROTHROMBIN TO tissues, and tissue clots have formed where they are not THROMBIN needed, special substances in the clot usually become activated. These substances function as enzymes to dis- 1. Prothrombin activator is formed as a result of rup- solve the clot, as discussed later in the chapter.! ture of a blood vessel or as a result of damage to special substances in the blood. 2. Prothrombin activator, in the presence of sufficient MECHANISM OF BLOOD COAGULATION amounts of ionic calcium (Ca2+), causes conver- sion of prothrombin to thrombin (Figure 37-3 and GENERAL MECHANISM 37-4). More than 50 important substances that cause or affect 3. Thrombin causes polymerization of fibrinogen blood coagulation have been found in the blood and in molecules into fibrin fibers within another 10 to 15 the tissues—some that promote coagulation, called pro- seconds. coagulants, and others that inhibit coagulation, called Thus, the rate-limiting factor in causing blood coagula- anticoagulants. Whether blood will coagulate depends on tion is usually the formation of prothrombin activator and the balance between these two groups of substances. In not the subsequent reactions beyond that point because the blood stream, the anticoagulants normally predomi- these terminal steps normally occur rapidly to form the nate, so the blood does not coagulate while it is circulating clot. 479 UNIT VI Blood Cells, Immunity, and Blood Coagulation Prothrombin Thrombin Cross-linked Figure 37-4. Coagulation cascade after Fibrinogen Fibrin Platelet fibrin vascular injury. Exposure of blood to the Release of phospholipid vascular wall causes release of tissue factor tissue factor complex (also called factor III or thromboplastin) from endothelial cells, phospholipid expression, activation of thrombin, which then acts on fibrinogen to form fibrin, and fibrin polym- erization to form a meshwork that stabilizes the platelet plug. Endothelium Fibrin clot Platelets also play an important role in the conversion Action of Thrombin on Fibrinogen to Form Fibrin. of prothrombin to thrombin because much of the pro- Thrombin is a protein enzyme with weak proteolytic thrombin first attaches to prothrombin receptors on the capabilities. It acts on fibrinogen to remove four low- platelets that are already bound to the damaged tissue. molecular-weight peptides from each molecule of fi- brinogen, forming one molecule of fibrin monomer that Prothrombin and Thrombin. Prothrombin is a plasma has the automatic capability to polymerize with other fi- protein, an α2-globulin, having a molecular weight of brin monomer molecules to form fibrin fibers. Therefore, 68,700. It is present in normal plasma in a concentration many fibrin monomer molecules polymerize within sec- of about 15 mg/dl. It is an unstable protein that can split onds into long fibrin fibers that constitute the reticulum of easily into smaller compounds, one of which is thrombin, the blood clot. which has a molecular weight of 33,700, almost half that In the early stages of polymerization, the fibrin mono- of prothrombin. mer molecules are held together by weak noncovalent Prothrombin is formed continually by the liver, and it hydrogen bonding, and the newly forming fibers are not is continually being used throughout the body for blood cross-linked with one another. Therefore, the resultant clotting. If the liver fails to produce prothrombin, in a day clot is weak and can be broken apart with ease. However, or so prothrombin concentration in the plasma falls too another process occurs during the next few minutes that low to provide normal blood coagulation. greatly strengthens the fibrin reticulum. This process Vitamin K is required by the liver for normal activa- involves a substance called fibrin stabilizing factor that tion of prothrombin, as well as a few other clotting fac- is present in small amounts in normal plasma globulins tors. Therefore, lack of vitamin K or the presence of liver but is also released from platelets entrapped in the clot. disease that prevents normal prothrombin formation can Before fibrin stabilizing factor can have an effect on the decrease the prothrombin to such a low level that a bleed- fibrin fibers, it must be activated. The same thrombin that ing tendency results.! causes fibrin formation also activates the fibrin stabiliz- ing factor. This activated substance then operates as an enzyme to form covalent bonds between more and more CONVERSION OF FIBRINOGEN TO of the fibrin monomer molecules, as well as multiple FIBRIN—FORMATION OF THE CLOT cross-linkages between adjacent fibrin fibers, thus adding Fibrinogen Formed in the Liver Essential for Clot For- tremendously to the three-dimensional strength of the mation. Fibrinogen is a high-molecular-weight protein fibrin meshwork.! (molecular weight ≈340,000) that occurs in the plasma in Blood Clot. The clot is composed of a meshwork of fi- quantities of 100 to 700 mg/dl. Fibrinogen is formed in brin fibers running in all directions and entrapping blood the liver, and liver disease can decrease the concentration of circulating fibrinogen, as it does the concentration of cells, platelets, and plasma (see Figure 37-4). The fibrin prothrombin, noted earlier. fibers also adhere to damaged surfaces of blood vessels; therefore, the blood clot becomes adherent to any vascu- Because of its large molecular size, little fibrinogen lar opening and thereby prevents further blood loss.! normally leaks from the blood vessels into the intersti- tial fluids, and because fibrinogen is one of the essential Clot Retraction and Expression of Serum. Within a few factors in the coagulation process, interstitial fluids ordi- minutes after a clot is formed, it begins to contract and narily do not coagulate. Yet, when the permeability of the usually expresses most of the fluid from the clot within capillaries becomes pathologically increased, fibrinogen 20 to 60 minutes. The fluid expressed is called serum be- does leak into the tissue fluids in sufficient quantities to cause all its fibrinogen and most of the other clotting fac- allow clotting of these fluids in much the same way that tors have been removed; in this way, serum differs from plasma and whole blood can clot.! plasma and cannot clot because it lacks these factors. 480  Chapter 37 Hemostasis and Blood Coagulation Platelets are necessary for clot retraction to occur. (1) Tissue trauma Therefore, failure of clot retraction is an indication that the number of platelets in the circulating blood might be low. Electron micrographs of platelets in blood clots show that they become attached to the fibrin fibers in such a way that Tissue factor UNIT VI they actually bond different fibers together. Furthermore, platelets entrapped in the clot continue to release proco- agulant substances, one of the most important of which (2) Vll VIIa is fibrin stabilizing factor, which causes more and more cross-linking bonds between adjacent fibrin fibers. In addi- X Activated X (Xa) tion, the platelets contribute directly to clot contraction by activating platelet thrombosthenin, actin, and myosin Ca2+ molecules, which are all contractile proteins in the plate- V Ca2+ lets; they cause strong contraction of the platelet spicules (3) Prothrombin attached to the fibrin. This action also helps compress the activator Platelet fibrin meshwork into a smaller mass. The contraction is phospholipids activated and accelerated by thrombin and by calcium ions Prothrombin Thrombin released from calcium stores in the mitochondria, endo- plasmic reticulum, and Golgi apparatus of the platelets. As the clot retracts, the edges of the broken blood ves- Ca2+ sel are pulled together, thus contributing still further to Figure 37-5. Extrinsic pathway for initiating blood clotting. hemostasis.! POSITIVE FEEDBACK OF CLOT Prothrombin activator is generally considered to be FORMATION formed in two ways, although, in reality, the two ways interact constantly with each other: (1) by the extrinsic Once a blood clot starts to develop, it normally extends pathway that begins with trauma to the vascular wall and within minutes into the surrounding blood—that is, the surrounding tissues; and (2) by the intrinsic pathway that clot initiates a positive feedback to promote more clot- begins in the blood. ting. One of the most important causes of this clot pro- In both the extrinsic and the intrinsic pathways, a motion is that the proteolytic action of thrombin allows series of different plasma proteins called blood-clotting it to act on many of the other blood-clotting factors in factors plays a major role. Most of these proteins are inac- addition to fibrinogen. For example, thrombin has a direct tive forms of proteolytic enzymes. When converted to the proteolytic effect on prothrombin, tending to convert it active forms, their enzymatic actions cause the successive, into still more thrombin, and it acts on some of the blood- cascading reactions of the clotting process. clotting factors responsible for formation of prothrombin Most of the clotting factors listed in Table 37-1 are activator. (These effects, discussed in subsequent para- designated by Roman numerals. To indicate the activated graphs, include acceleration of the actions of factors VIII, form of the factor, a small letter “a” is added after the IX, X, XI, and XII and aggregation of platelets.) Once a Roman numeral, such as factor VIIIa to indicate the acti- critical amount of thrombin is formed, a positive feedback vated state of factor VIII. develops that causes still more blood clotting and more and more thrombin to be formed; thus, the blood clot Extrinsic Pathway for Initiating Clotting continues to grow until blood leakage ceases.! The extrinsic pathway for initiating the formation of prothrombin activator begins with a traumatized vas- INITIATION OF COAGULATION: cular wall or traumatized extravascular tissues that FORMATION OF PROTHROMBIN come in contact with the blood. This condition leads ACTIVATOR to the following steps, as shown in Figure 37-4 and Now that we have discussed the clotting process, the Figure 37-5: more complex mechanisms that initiate clotting in the 1. Release of tissue factor. Traumatized tissue releases first place will be described. These mechanisms are set a complex of several factors called tissue factor or into play by the following: (1) trauma to the vascular wall tissue thromboplastin. This factor is composed es- and adjacent tissues; (2) trauma to the blood; or (3) con- pecially of phospholipids from the membranes of tact of the blood with damaged endothelial cells or with the tissue plus a lipoprotein complex that functions collagen and other tissue elements outside the blood ves- mainly as a proteolytic enzyme. sel. In each case, this leads to the formation of prothrom- 2. Activation of factor X—role of factor VII and tissue bin activator, which then causes prothrombin conversion factor. The lipoprotein complex of tissue factor fur- to thrombin and all the subsequent clotting steps. ther complexes with blood coagulation factor VII 481 UNIT VI Blood Cells, Immunity, and Blood Coagulation and, in the presence of calcium ions, acts enzymati- Intrinsic Pathway for Initiating Clotting cally on factor X to form activated factor X (Xa). The second mechanism for initiating formation of pro- 3. Effect of Xa to form prothrombin activator—role thrombin activator, and therefore for initiating clotting, of factor V. The activated factor X combines im- begins with trauma to the blood or exposure of the blood mediately with tissue phospholipids that are part to collagen from a traumatized blood vessel wall. Then the of tissue factors or with additional phospholipids process continues through the series of cascading reac- released from platelets, as well as with factor V, tions shown in Figure 37-6. to form the complex called prothrombin activator. 1. Blood trauma causes (1) activation of factor XII and Within a few seconds, in the presence of Ca2+, pro- (2) release of platelet phospholipids. Trauma to the thrombin is split to form thrombin, and the clot- blood or exposure of the blood to vascular wall col- ting process proceeds as already explained. At first, lagen alters two important clotting factors in the the factor V in the prothrombin activator complex blood: factor XII and the platelets. When factor XII is inactive, but once clotting begins and thrombin is disturbed, such as by coming into contact with begins to form, the proteolytic action of thrombin collagen or with a wettable surface such as glass, it activates factor V. This activation then becomes an takes on a new molecular configuration that con- additional strong accelerator of prothrombin ac- verts it into a proteolytic enzyme called activated tivation. Thus, in the final prothrombin activator factor XII. Simultaneously, the blood trauma also complex, activated factor X is the actual protease damages the platelets because of adherence to col- that causes splitting of prothrombin to form throm- lagen or to a wettable surface (or by damage in other bin. Activated factor V greatly accelerates this pro- ways); this releases platelet phospholipids that con- tease activity, and platelet phospholipids act as a tain the lipoprotein called platelet factor 3, which vehicle that further accelerates the process. Note also plays a role in subsequent clotting reactions. especially the positive feedback effect of thrombin, 2. Activation of factor XI. The activated factor XII also acting through factor V, to accelerate the entire pro- acts enzymatically on factor XI to activate this fac- cess once it begins.! tor, which is the second step in the intrinsic path- Blood trauma or contact with collagen (1) XII Activated XII (XIIa) (HMW kininogen, prekallikrein) (2) XI Activated XI (XIa) Ca2+ (3) IX Activated IX (IXa) VIII Thrombin VIIIa Ca2+ (4) X Activated X (Xa) (5) Platelet phospholipids Thrombin Ca2+ V Prothrombin activator Platelet phospholipids Prothrombin Thrombin Figure 37-6. Intrinsic pathway for initiating blood clotting. HMW, High-molecular weight. Ca2+ 482  Chapter 37 Hemostasis and Blood Coagulation way. This reaction also requires high-molecular- the blood. With severe tissue trauma, clotting can occur weight kininogen and is accelerated by prekallikrein. in as little as 15 seconds. The intrinsic pathway is much 3. Activation of factor IX by activated factor XI. The slower to proceed, usually requiring 1 to 6 minutes to activated factor XI then acts enzymatically on fac- cause clotting.! tor IX to activate this factor as well. Intravascular Anticoagulants Prevent Blood UNIT VI 4. Activation of factor X—role of factor VIII. The acti- vated factor IX, acting in concert with activated fac- Clotting in the Normal Vascular System tor VIII and the platelet phospholipids and factor III Endothelial Surface Factors. Probably the most impor- from the traumatized platelets, activates factor X. It tant factors for preventing clotting in the normal vas- is clear that when either factor VIII or platelets are cular system are the following: (1) the smoothness of the in short supply, this step is deficient. Factor VIII is endothelial cell surface, which prevents contact activation the factor that is missing in a person who has clas- of the intrinsic clotting system; (2) a layer of glycocalyx on sic hemophilia, so it is called antihemophilic factor. the endothelium (glycocalyx is a mucopolysaccharide ad- Platelets are the clotting factor that is lacking in the sorbed to the surfaces of the endothelial cells), which repels bleeding disease called thrombocytopenia. clotting factors and platelets, thereby preventing activation 5. Action of activated factor X to form prothrombin of clotting; and (3) a protein bound with the endothelial activator—role of factor V. This step in the intrinsic membrane, thrombomodulin, which binds thrombin. Not pathway is the same as the last step in the extrinsic only does the binding of thrombin with thrombomodulin pathway. That is, activated factor X combines with slow the clotting process by removing thrombin, but the factor V and platelet or tissue phospholipids to form thrombomodulin-thrombin complex also activates a plas- the complex called prothrombin activator. The pro- ma protein, protein C, that acts as an anticoagulant by inac- thrombin activator, in turn, initiates the cleavage tivating activated factors V and VIII. of prothrombin to form thrombin within seconds, When the endothelial wall is damaged, its smoothness thereby setting into motion the final clotting pro- and glycocalyx-thrombomodulin layer are lost, which cess, as described earlier.! activates both factor XII and the platelets, thus setting off Role of Calcium Ions in the Intrinsic and the intrinsic pathway of clotting. If factor XII and platelets come into contact with the subendothelial collagen, the Extrinsic Pathways activation is even more powerful. Except for the first two steps in the intrinsic pathway, cal- Intact endothelial cells also produce other substances cium ions are required for promotion or acceleration of such a prostacyclin and nitric oxide (NO) that inhibit all the blood-clotting reactions. Therefore, in the absence platelet aggregation and initiation of blood clotting. Pros- of calcium ions, blood clotting by either pathway does not tacyclin, also called prostaglandin I2 (PGI2), is a member occur. of the eicosanoid family of lipids and is a vasodilator, as In the living body, the calcium ion concentration well as an inhibitor of platelet aggregation. As discussed seldom falls low enough to affect blood-clotting kinet- in Chapter 17, NO is a powerful vasodilator released from ics significantly. However, when blood is removed from healthy vascular endothelial cells throughout the body, someone, it can be prevented from clotting by reducing and it is an important inhibitor of platelet aggregation. the calcium ion concentration below the threshold level When endothelial cells are damaged, their production of for clotting by deionizing the calcium by causing it to prostacyclin and NO is greatly diminished.! react with substances such as citrate ion or by precipitat- ing the calcium with substances such as oxalate ion.! Antithrombin Action of Fibrin and Antithrombin III. Among the most important anticoagulants in the blood Interaction Between Extrinsic and are those that remove thrombin from the blood. The most Intrinsic Pathways—Summary of Blood- powerful of these are the following: (1) the fibrin fibers Clotting Initiation that are formed during the process of clotting; and (2) It is clear from the schemas of the intrinsic and extrinsic an α globulin called antithrombin III or antithrombin- systems that after blood vessels rupture, clotting occurs heparin cofactor. by both pathways simultaneously. Tissue factor initiates While a clot is forming, about 85% to 90% of the throm- the extrinsic pathway, whereas contact of factor XII and bin formed from the prothrombin becomes adsorbed to platelets with collagen in the vascular wall initiates the the fibrin fibers as they develop. This adsorption helps intrinsic pathway. prevent the spread of thrombin into the remaining blood An especially important difference between the and, therefore, prevents excessive spread of the clot. extrinsic and intrinsic pathways is that the extrin- The thrombin that does not adsorb to the fibrin sic pathway can be explosive; once initiated, its speed fibers soon combines with antithrombin III. This further of completion to the final clot is limited only by the blocks the effect of thrombin on the fibrinogen and then amount of tissue factor released from the traumatized also inactivates thrombin itself during the next 12 to 20 tissues and by the quantities of factors X, VII, and V in minutes.! 483 UNIT VI Blood Cells, Immunity, and Blood Coagulation Heparin. Heparin is another powerful anticoagulant but, converts plasminogen to plasmin, which in turn removes because its concentration in the blood is normally low, the remaining unnecessary blood clot. In fact, many small it has significant anticoagulant effects only under special blood vessels in which blood flow has been blocked by physiological conditions. However, heparin is used widely clots are reopened by this mechanism. Thus, an especially as a pharmacological agent in medical practice in much important function of the plasmin system is to remove higher concentrations to prevent intravascular clotting. minute clots from millions of tiny peripheral vessels that The heparin molecule is a highly negatively charged eventually would become occluded were there no way to conjugated polysaccharide. By itself, it has little or no clear them.! anticoagulant properties, but when it combines with antithrombin III, the effectiveness of antithrombin III for removing thrombin increases by a hundredfold to a CONDITIONS THAT CAUSE EXCESSIVE thousandfold and thus acts as an anticoagulant. There- BLEEDING IN HUMANS fore, in the presence of excess heparin, the removal of free Excessive bleeding can result from a deficiency of any of thrombin from the circulating blood by antithrombin III the many blood-clotting factors. Three particular types of is almost instantaneous. bleeding tendencies that have been studied to the greatest The complex of heparin and antithrombin III removes extent are discussed here—bleeding caused by (1) vitamin several other activated coagulation factors in addition to K deficiency, (2) hemophilia, and (3) thrombocytopenia thrombin, further enhancing the effectiveness of antico- (platelet deficiency). agulation. The others include activated factors IX through XII. DECREASED PROTHROMBIN, FACTOR VII, Heparin is produced by many different cells of the FACTOR IX, AND FACTOR X CAUSED BY body, but the largest quantities are formed by the baso- VITAMIN K DEFICIENCY philic mast cells located in the pericapillary connec- tive tissue throughout the body. These cells continually With few exceptions, almost all the blood-clotting fac- secrete small quantities of heparin that diffuse into the tors are formed by the liver. Therefore, diseases of the circulatory system. The basophil cells of the blood, which liver such as hepatitis, cirrhosis, and acute yellow atrophy are functionally almost identical to the mast cells, release (degeneration of the liver caused by toxins, infections, or small quantities of heparin into the plasma. other agents) can sometimes depress the clotting system Mast cells are abundant in tissue surrounding the cap- so much that the patient develops a severe tendency to illaries of the lungs and, to a lesser extent, capillaries of the bleed. liver. It is easy to understand why large quantities of hepa- Another cause of depressed formation of clotting rin might be needed in these areas because the capillaries factors by the liver is vitamin K deficiency. Vitamin K of the lungs and liver receive many embolic clots that have is an essential factor to a liver carboxylase that adds a formed in slowly flowing venous blood; sufficient produc- carboxyl group to glutamic acid residues on five of the tion of heparin prevents further growth of the clots.! important clotting factors—prothrombin, factor VII, fac- tor IX, factor X, and protein C. On adding the carboxyl PLASMIN CAUSES LYSIS OF BLOOD CLOTS group to glutamic acid residues on the immature clot- ting factors, vitamin K is oxidized and becomes inactive. The plasma proteins contain a euglobulin called plas- Another enzyme, vitamin K epoxide reductase complex 1 minogen (profibrinolysin) that when activated, becomes a (VKORC1), reduces vitamin K back to its active form. In substance called plasmin (fibrinolysin). Plasmin is a pro- the absence of active vitamin K, subsequent insufficiency teolytic enzyme that resembles trypsin, the most impor- of these coagulation factors in the blood can lead to seri- tant proteolytic digestive enzyme of pancreatic secretion. ous bleeding tendencies. Plasmin digests fibrin fibers and some other protein Vitamin K is continually synthesized in the intestinal coagulants, such as fibrinogen, factor V, factor VIII, pro- tract by bacteria, so vitamin K deficiency seldom occurs thrombin, and factor XII. Therefore, whenever plasmin in healthy persons as a result of the absence of vitamin K is formed, it can cause lysis of a clot by destroying many from the diet (except in neonates, before they establish of the clotting factors, thereby sometimes even causing their intestinal bacterial flora). However, in persons with hypocoagulability of the blood. gastrointestinal disease, vitamin K deficiency often occurs Activation of Plasminogen to Form Plasmin, Then as a result of poor absorption of fats from the gastrointes- Clot Lysis. When a clot is formed, a large amount of plas- tinal tract because vitamin K is fat-soluble and is ordinar- minogen is trapped in the clot, along with other plasma ily absorbed into the blood along with the fats. proteins. This will not become plasmin or cause lysis of One of the most prevalent causes of vitamin K defi- the clot until it is activated. The injured tissues and vascu- ciency is failure of the liver to secrete bile into the gas- lar endothelium very slowly release a powerful activator trointestinal tract, which occurs as a result of obstruction called tissue plasminogen activator (t-PA); a few days later, of the bile ducts or of liver disease. Lack of bile prevents after the clot has stopped the bleeding, t-PA eventually adequate fat digestion and absorption and, therefore, 484  Chapter 37 Hemostasis and Blood Coagulation depresses vitamin K absorption as well. Thus, liver dis- disease with somewhat different characteristics, called ease often causes decreased production of prothrombin von Willebrand disease, results from loss of the large and some other clotting factors because of poor vitamin component. K absorption and because of the diseased liver cells. As When a person with classic hemophilia experiences a result, vitamin K is injected into surgical patients with severe prolonged bleeding, almost the only therapy that is UNIT VI liver disease or with obstructed bile ducts before the truly effective is injection of purified factor VIII or factor surgical procedure is performed. Ordinarily, if vitamin IX. Both these clotting factors are now available as recom- K is given to a deficient patient 4 to 8 hours before the binant proteins, although they are expensive and their operation and the liver parenchymal cells are at least half- half-lives are relatively short; therefore, these products are normal in function, sufficient clotting factors will be pro- not readily available for many patients with hemophilia, duced to prevent excessive bleeding during the operation.! especially in economically disadvantaged countries.! HEMOPHILIA THROMBOCYTOPENIA Hemophilia is a bleeding disease that occurs almost Thrombocytopenia means the presence of very low num- exclusively in males. In 85% of cases, it is caused by an bers of platelets in the circulating blood. People with abnormality or deficiency of factor VIII; this type of hemo- thrombocytopenia have a tendency to bleed, as do hemo- philia is called hemophilia A or classic hemophilia. About philiacs, except that the bleeding is usually from many 1 of every 10,000 males in the United States has classic small venules or capillaries, rather than from larger ves- hemophilia. In the other 15% of patients with hemophilia sels, as in hemophilia. As a result, small punctate hem- B, the bleeding tendency is caused by deficiency of factor orrhages occur throughout all the body tissues. The skin IX. Both these factors are transmitted genetically by way of such a person displays many small petechiae, red or of the female (X) chromosome and are recessive in their purplish blotches, giving the disease the name throm- inheritance. Therefore, a woman will rarely have hemo- bocytopenic purpura. As noted, platelets are especially philia because at least one of her two X chromosomes will important for the repair of minute breaks in capillaries have the appropriate genes. If one of her X chromosomes and other small vessels. is deficient, she will be a hemophilia carrier; her male off- Platelet counts below 30,000/µl, compared with the spring will have a 50% chance of inheriting the illness, and normal value of 150,000 to 450,000/µl, increase the risk her female offspring will have a 50% chance of inheriting for excessive bleeding after surgery or injury. Spontane- the carrier status. ous bleeding, however, will not ordinarily occur until the Although female carriers have one normal allele and number of platelets in the blood falls below 30,000/µl. usually do not develop symptomatic hemophilia, some Levels as low as 10,000/µl are frequently lethal. may experience a mild bleeding trait. It is also possible Even without determining specific platelet counts in for female carriers to develop mild hemophilia due to the blood, sometimes one can suspect the existence of loss of part or all of the normal X chromosome (as in thrombocytopenia if the person’s blood clot fails to retract. Turner syndrome) or inactivation (lyonization) of the As noted earlier, clot retraction is normally dependent X-chromosomes. For a female to inherit full-blown on release of multiple coagulation factors from the large symptomatic hemophilia A or B, she must receive two numbers of platelets entrapped in the fibrin mesh of the deficient X-chromosomes, one from her carrier mother clot. and the other from her father, who must have hemophilia. The major causes of thrombocytopenia include the Most cases of hemophilia are inherited, but approxi- following: (1) decreased platelet production in the bone mately one-third of hemophilia patients do not have a marrow due to infections or sepsis, nutrient deficiencies, family history of the disease, which appears to be caused or myelodysplastic disorders, which usually also reduce by novel mutation events. production of other cells (red blood cells [RBCs] and The bleeding trait in hemophilia can have various white blood cells); (2) peripheral platelet destruction by degrees of severity, depending on the genetic deficiency. antibodies; (3) sequestration (pooling) of platelets in the Bleeding usually does not occur except after trauma, but spleen, especially in individuals with portal hypertension in some patients, the degree of trauma required to cause and excessively large spleens (splenomegaly); (4) con- severe and prolonged bleeding may be so mild that it is sumption of platelets in thrombi; and (4) dilution of the hardly noticeable. For example, bleeding can often last for blood from fluid resuscitation or massive transfusion. days after extraction of a tooth. Most people with thrombocytopenia have the disease Factor VIII has two active components, a large compo- known as idiopathic thrombocytopenia, which means nent with a molecular weight in the millions and a smaller thrombocytopenia of unknown cause. In most of these component with a molecular weight of about 230,000. people, it has been discovered that, for unknown reasons, The smaller component is most important in the intrinsic specific antibodies have formed and react against the plate- pathway for clotting, and it is deficiency of this part of fac- lets to destroy them. Relief from bleeding for 1 to 4 days tor VIII that causes classic hemophilia. Another bleeding can often be effected in a patient with thrombocytopenia 485 UNIT VI Blood Cells, Immunity, and Blood Coagulation by giving fresh whole blood transfusions that contain large artery is blocked, death may not occur, or the embolism numbers of platelets. Also, splenectomy may be helpful, may lead to death a few hours to several days later because sometimes resulting in an almost complete cure because of further growth of the clot in the pulmonary vessels. the spleen normally removes large numbers of platelets However, again, t-PA therapy can be a lifesaver.! from the blood.! DISSEMINATED INTRAVASCULAR COAGULATION THROMBOEMBOLIC CONDITIONS Occasionally, the clotting mechanism becomes activated Thrombi and Emboli. An abnormal clot that develops in widespread areas of the circulation, giving rise to the in a blood vessel is called a thrombus. Once a clot has de- condition called disseminated intravascular coagulation veloped, continued flow of blood past the clot is likely to (DIC). This condition often results from the presence break it away from its attachment and cause the clot to of large amounts of traumatized or dying tissue in the flow with the blood; such freely flowing clots are known body that releases great quantities of tissue factor into as emboli. Also, emboli that originate in large arteries or the blood. Frequently, the clots are small but numerous, in the left side of the heart can flow peripherally and plug and they plug a large share of the small peripheral blood arteries or arterioles in the brain, kidneys, or elsewhere. vessels. This process occurs especially in patients with Emboli that originate in the venous system or in the right widespread septicemia, in which circulating bacteria or side of the heart generally flow into the lungs to cause pul- bacterial toxins—especially endotoxins—activate the clot- monary arterial embolism.! ting mechanisms. The plugging of small peripheral vessels greatly diminishes delivery of oxygen and other nutrients Causes of Thromboembolic Conditions. The causes of to the tissues, a situation that leads to or exacerbates cir- thromboembolic conditions in people are usually twofold: culatory shock. It is partly for this reason that septicemic (1) a roughened endothelial surface of a vessel—as may be shock is lethal in 35% to 50% of patients. caused by arteriosclerosis, infection, or trauma—is likely A peculiar effect of disseminated intravascular coagu- to initiate the clotting process; and (2) blood often clots lation is that the patient, on occasion, begins to bleed. The when it flows very slowly through blood vessels, where reason for this bleeding is that so many of the clotting fac- small quantities of thrombin and other procoagulants are tors are removed by the widespread clotting that too few always being formed.! procoagulants remain to allow normal hemostasis of the Use of Tissue Plasminogen Activator in Treating In- remaining blood.! travascular Clots. Genetically engineered tissue plas- minogen activator (t-PA) is available. When delivered ANTICOAGULANTS FOR CLINICAL USE through a catheter to an area with a thrombus, it is effec- tive in activating plasminogen to plasmin, which in turn In some thromboembolic conditions, it is desirable to can dissolve some intravascular clots. For example, if used delay the coagulation process. Various anticoagulants within the 1 or 2 hours after thrombotic occlusion of a have been developed for this purpose. The ones most coronary artery, the heart is often spared serious damage.! clinically useful are heparin and the coumarins. FEMORAL VENOUS THROMBOSIS AND HEPARIN—INTRAVENOUS ANTICOAGULANT MASSIVE PULMONARY EMBOLISM Commercial heparin is extracted from several differ- Because clotting almost always occurs when blood flow ent animal tissues and prepared in almost pure form. is blocked for many hours in any vessel of the body, the Injection of relatively small quantities, about 0.5 to 1 immobility of patients confined to bed, plus the practice mg/kg of body weight, causes the blood-clotting time of propping the knees up with pillows, often causes intra- to increase from a normal of about 6 minutes to 30 or vascular clotting because of blood stasis in one or more more minutes. Furthermore, this change in clotting time of the leg veins for hours at a time. Then the clot grows, occurs instantaneously, thereby immediately preventing mainly in the direction of the slowly moving venous or slowing further development of a thromboembolic blood, sometimes growing the entire length of the leg condition. veins and occasionally even up into the common iliac vein The action of heparin lasts about 1.5 to 4 hours. The and inferior vena cava. About 10% of the time, a large part injected heparin is destroyed by an enzyme in the blood of the clot disengages from its attachments to the vessel known as heparinase.! wall and flows freely with the venous blood through the COUMARINS AS ANTICOAGULANTS right side of the heart and into the pulmonary arteries to cause massive blockage of the pulmonary arteries; this is When a coumarin, such as warfarin, is given to a patient, called a massive pulmonary embolism. If the clot is large the amounts of active prothrombin and factors VII, IX, enough to occlude both pulmonary arteries at the same and X, all formed by the liver, begin to fall. Warfarin time, immediate death ensues. If only one pulmonary causes this effect by inhibiting the enzyme VKORC1. As 486  Chapter 37 Hemostasis and Blood Coagulation discussed previously, this enzyme converts the inactive, polymerized into glucose or metabolized directly for oxidized form of vitamin K to its active, reduced form. energy. Consequently, 500 milliliters of blood that has By inhibiting VKORC1, warfarin decreases the avail- been rendered noncoagulable by citrate can ordinarily be able active form of vitamin K in the tissues. When this transfused into a recipient within a few minutes, without decrease occurs, the coagulation factors are no longer car- dire consequences. However, if the liver is damaged, or UNIT VI boxylated and are biologically inactive. Over several days, if large quantities of citrated blood or plasma are given the body stores of the active coagulation factors degrade too rapidly (within fractions of a minute), the citrate ion and are replaced by inactive factors. Although the coagu- may not be removed quickly enough, and the citrate can, lation factors continue to be produced, they have greatly under these conditions, greatly depress the level of cal- decreased coagulant activity. cium ion in the blood, which can result in tetany and con- After administration of an effective dose of warfarin, vulsive death.! the coagulant activity of the blood decreases to about 50% of normal by the end of 12 hours and to about 20% of BLOOD COAGULATION TESTS normal by the end of 24 hours. In other words, the coagu- lation process is not blocked immediately but must await BLEEDING TIME the degradation of the active prothrombin and the other affected coagulation factors already present in the plasma. When a sharp-pointed knife is used to pierce the tip of the Normal coagulation usually returns 1 to 3 days after dis- finger or earlobe, bleeding ordinarily lasts for 1 to 6 min- continuing coumarin therapy.! utes. This time depends largely on the depth of the wound and degree of hyperemia in the finger or earlobe at the PREVENTION OF BLOOD COAGULATION time of the test. Lack of any one of several of the clotting OUTSIDE THE BODY factors can prolong the bleeding time, but it is especially prolonged by lack of platelets.! Although blood removed from the body and held in a glass test tube normally clots in about 6 minutes, blood CLOTTING TIME collected in siliconized containers often does not clot for Many methods have been devised for determining blood- 1 hour or more. The reason for this delay is that preparing the surfaces of the containers with silicone prevents con- clotting time. The one most widely used is to collect blood in a chemically clean glass test tube and then to tact activation of platelets and factor XII, the two princi- pal factors that initiate the intrinsic clotting mechanism. tip the tube back and forth about every 30 seconds until Conversely, untreated glass containers allow contact acti- the blood has clotted. By this method, the normal clot- ting time is 6 to 10 minutes. Procedures using multiple vation of the platelets and factor XII, with the rapid devel- opment of clots. test tubes have also been devised for determining clotting time more accurately. Heparin can be used for preventing coagulation of blood outside the body, as well as in the body. Heparin is Unfortunately, the clotting time varies widely, depend- ing on the method used for measuring it, so it is no lon- especially used in surgical procedures in which the blood ger used in many clinics. Instead, measurements of the must be passed through a heart-lung machine or artificial kidney machine and then back into the patient. clotting factors themselves are made, using sophisticated Various substances that decrease the concentration of chemical procedures.! calcium ions in the blood can also be used for preventing PROTHROMBIN TIME AND blood coagulation outside the body. For example, a solu- INTERNATIONAL NORMALIZED RATIO ble oxalate compound mixed in a very small quantity with a sample of blood causes precipitation of calcium oxalate The prothrombin time indicates the concentration of from the plasma and thereby decreases the ionic calcium prothrombin in the blood. Figure 37-7 shows the rela- level so much that blood coagulation is blocked. tionship of prothrombin concentration to prothrombin Any substance that deionizes the blood calcium will time. The method for determining prothrombin time is prevent coagulation. The negatively charged citrate ion is the following. especially valuable for this purpose; it is mixed with blood Blood removed from the patient is immediately oxa- usually in the form of sodium, ammonium, or potassium lated so that none of the prothrombin can change into citrate. The citrate ion combines with calcium in the thrombin. Then, a large excess of calcium ion and tis- blood to produce a nonionized calcium compound, and sue factor is quickly mixed with the oxalated blood. The the lack of ionic calcium prevents coagulation. Citrate excess calcium nullifies the effect of the oxalate, and the anticoagulants have an important advantage over the oxa- tissue factor activates the prothrombin to thrombin reac- late anticoagulants because oxalate is toxic to the body, tion by means of the extrinsic clotting pathway. The time whereas moderate quantities of citrate can be injected required for coagulation to take place is known as the intravenously. After injection, the citrate ion is removed prothrombin time. The shortness of the time is determined from the blood within a few minutes by the liver and is mainly by the prothrombin concentration. The normal 487 UNIT VI Blood Cells, Immunity, and Blood Coagulation 100 Tests similar to that for prothrombin time and INR have been devised to determine the quantities of other blood- Concentration (percent of normal) clotting factors. In each of these tests, excesses of calcium ions and all the other factors in addition to the one being tested are added to oxalated blood all at once. Then, the time required for coagulation is determined in the same manner as for prothrombin time. If the factor being tested is defi- 50.0 cient, the coagulation time is prolonged. The time itself can then be used to quantitate the concentration of the factor. 25.0 Bibliography 12.5 Becker RC, Sexton T, Smyth SS: Translational implications of platelets 6.25 as vascular first responders. Circ Res 122:506, 2018. 0 Furie B, Furie BC: Mechanisms of thrombus formation. N Engl J Med 0 10 20 30 40 50 60 359:938, 2008. Gupta S, Shapiro AD: Optimizing bleed prevention throughout the Prothrombin time (seconds) lifespan: womb to tomb. Haemophilia 24 Suppl 6:76, 2018. Hess CN, Hiatt WR: Antithrombotic therapy for peripheral artery dis- Figure 37-7. Relationship of prothrombin concentration in the blood ease in 2018. JAMA 319:2329, 2018. to prothrombin time. Hunt BJ: Bleeding and coagulopathies in critical care. N Engl J Med 370:847, 2014. Koupenova M, Clancy L, Corkrey HA, Freedman JE: Circulating plate- prothrombin time is about 12 seconds. In each laboratory, lets as mediators of immunity, inflammation, and thrombosis. Circ a curve relating prothrombin concentration to prothrom- Res 122:337, 2018. bin time, such as that shown in Figure 37-7, is drawn for Kucher N: Clinical practice. Deep-vein thrombosis of the upper ex- the method used so that the prothrombin in the blood can tremities. N Engl J Med 364:861, 2011. Leebeek FW, Eikenboom JC: Von Willebrand’s disease. N Engl J Med be quantified. 375:2067, 2016. The results obtained for prothrombin time may vary Luyendyk JP, Schoenecker JG, Flick MJ: The multifaceted role of fi- considerably, even in the same individual if there are brinogen in tissue injury and inflammation. Blood 133:511, 2019. differences in activity of the tissue factor and the ana- Maas C, Renné T: Coagulation factor XII in thrombosis and inflamma- lytical system used to perform the test. Tissue factor tion. Blood 131:1903, 2018. McFadyen JD, Schaff M, Peter K: Current and future antiplatelet is isolated from human tissues, such as placental tis- therapies: emphasis on preserving haemostasis. Nat Rev Cardiol sue, and different batches may have different activity. 15:181, 2018. The international normalized ratio (INR) was devised Mohammed BM, Matafonov A, Ivanov I, et al: An update on factor XI as a way to standardize measurements of prothrombin structure and function. Thromb Res 161:94, 2018. time. For each batch of tissue factor, the manufacturer Nachman RL, Rafii S: Platelets, petechiae, and preservation of the vas- cular wall. N Engl J Med 359:1261, 2008. assigns an international sensitivity index (ISI), which Negrier C, Shima M, Hoffman M: The central role of thrombin in bleed- indicates the activity of the tissue factor with a stan- ing disorders. Blood Rev 2019 May 22. pii: S0268-960X(18)30097-3. dardized sample. The ISI usually varies between 1.0 https://www.doi.org/10.1016/j.blre.2019.05.006 and 2.0. The INR is the ratio of the person’s prothrom- Peters R, Harris T: Advances and innovations in haemophilia treat- bin time (PT) to a normal control sample raised to the ment. Nat Rev Drug Discov 17:493, 2018. Samuelson Bannow B, Recht M, Négrier C, et al: Factor VIII: long- power of the ISI: established role in haemophilia A and emerging evidence beyond ISI haemostasis. Blood Rev 35:43, 2019. INR = ⎛ PTtest ⎝ Tillman BF, Gruber A, McCarty OJT, Gailani D: Plasma contact factors ⎝PT normal ⎛ as therapeutic targets. Blood Rev 32:433, 2018. The normal range for INR in a healthy person is 0.9 to van der Meijden PEJ, Heemskerk JWM: Platelet biology and functions: 1.3. A high INR level (e.g., 4 or 5) indicates a high risk of new concepts and clinical perspectives. Nat Rev Cardiol 16:166, 2019. Wells PS, Forgie MA, Rodger MA: Treatment of venous thromboem- bleeding, whereas a low INR (e.g., 0.5) suggests that there bolism. JAMA 311:717, 2014. is a chance of having a clot. Patients undergoing warfarin Weyand AC, Pipe SW: New therapies for hemophilia. Blood 133:389, therapy usually have an INR of 2.0 to 3.0. 2019. 488