| Which of the following is not a variable in the effectiveness of hemostasis? | View Page |
| Which of the following mechanisms involve a series of interrelated chemical processes that lead to the formation of durable fibrin strands? | View Page |
| An Introduction to the Fundamentals of Coagulation The ability of the body to maintain a state of homeostasis, or physiological equilibrium, is absolutely essential for effective, efficient functionality of all body systems.
The mechanisms involved in blood coagulation, also known as hemostasis or blood clotting, serve to illustrate this concept.
Hemostasis is the cessation of free blood flow, external to the vascular system, when a vessel wall has been breached.
With the maintenance of homeostasis in mind, it is vital that the body be able to rapidly repair vascular damage, arresting blood flow in the process, while simultaneously maintaining blood in a fluid state within the vascular compartment.
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| An Introduction to the Fundamentals of Coagulation As we will discover later in the course, there are other variables which impact the effectiveness of hemostatic mechanisms as well, such as acquired disease states, and inborn metabolic pathway defects.
For now, however, our focus will be on the mechanisms, processes, and components which work together to achieve coagulation, or the cessation of blood flow from a damaged vessel.
Note: The terms coagulation and hemostasis are used interchangeably throughout this course.
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| Introduction to Primary Hemostasis Primary hemostasis is considered the starting point for the hemostatic response mounted by the body, subsequent to vascular damage. Its activation serves as a trigger for ensuing hemostatic processes, as the mechanisms are all interrelated. Primary hemostasis consists of two key parts: The vascular system Platelets (thrombocytes) | View Page |
| Primary Hemostasis – The Vascular System Our blood circulates freely through undamaged, intact vessels. The design of the vasculature, or blood vessels, is such that the walls of the vessels are chemically inert to both coagulation factors and platelets under normal conditions. Damage to a vessel breaks that inert epithelial lining, exposing the subendothelium and collagen, and releasing chemical signals that trigger subsequent hemostatic mechanisms. | View Page |
| Primary Hemostasis – The Vascular System Overview of Vascular System Involvement in Primary Hemostasis:
Vasoconstriction
Reroute blood flow
Platelet aggregation
Contact activation of coagulation system (start of secondary hemostasis at this point) | View Page |
| Primary Hemostasis – The Vascular System The first specific, recognizable hemostatic mechanism is a process known as vasoconstriction, which is initiated by chemical signals stemming from a breach of the vasculature. Vasoconstriction, or vascular constriction, immediately reduces the quantity of blood flowing through the damaged area. Its action is the physical decrease in the size of the vessel, and the redirection of blood flow around, and away from, the damaged area. Vasoconstriction is akin to putting a clamp on a pliable piece of plastic tubing. A short process in terms of the overall time elapsed, the entire vascular response typically lasts less than one minute! Though fleeting, vasoconstriction is an exceedingly important hemostatic mechanism as it prepares the damaged vessel for subsequent repair activities. | View Page |
| Primary Hemostasis – The Vascular System & Platelet Involvement As discussed earlier, a break in the vessel endothelium leads to exposure of collagen and the vessel's subendothelial surface. Ruptured endothelial cells leak ADP and Serotonin, which are the chemical triggers that induce platelet adhesion, the next step in the sequence of hemostatic events. Circulating platelets are drawn to the area by those liberated chemical signals, and begin to physically attach themselves to the rough, damaged surfaces of the breach. As platelets continue to arrive and bind to the exposed collagen and basement membrane, a rudimentary barrier begins to form, as the platelets themselves serve to fill in the breached vessel wall. Platelets possess an inherent “sticky” property which enables them to adhere to one another, and not just to the damaged vessel endothelium. The process by which platelets bind to one another is referred to as platelet aggregation, and is vital because it allows for a platelet plug to be formed. The platelet plug is the structure responsible for plugging the hole in the vessel wall. | View Page |
| Primary Hemostasis – The Vascular System & Platelet Involvement Platelet aggregation is essential to platelet plug formation because the platelets are now able to span the center of the breach, where there was obviously no exposed collagen to bind to before, as well as clumping behind one another to buttress or strengthen the barrier.
The formation of the platelet plug signals the end of primary hemostasis, and serves to initiate upcoming processes associated with secondary hemostasis.
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| All of the following processes occur during primary hemostasis except: | View Page |
| Primary Hemostasis – Characteristics of the Platelet Platelets are produced in the bone marrow by highly specialized cells called megakaryocytes. About 70-80% of a person's total platelet count is circulating in the vasculature at any given time. Approximately 20-30% of a person's total platelet count is pooled in the spleen. The average lifespan of a platelet is 9-12 days. | View Page |
| Primary Hemostasis – Characteristics of the Platelet Platelets play a significant role in primary hemostasis, as they are the “bricks”, or building blocks of the developing platelet plug, the forerunner to the end stage fibrin clot. Platelets have inherent adhesive properties which are essential for adherence to the site of vascular damage, and for binding to one another in aggregation activities. Platelets must be present in sufficient number, and be functionally active for optimal clotting to occur. Platelet functionality tends to be more crucial than the number of platelets available, however, as patients with lower platelet counts can still clot relatively effectively as compared to those patients with intrinsic platelet defects. | View Page |
| Primary Hemostasis – Platelet Function Platelets have three primary functions: Maintenance of Vascular Integrity – Platelets contain chemicals within their granules that are vital to the normal growth and maintenance of the vascular system. Platelet Plug Formation – Platelets are the fundamental components of the physical barrier that initially fills the breach in the compromised vessel. Stabilization of the Platelet Plug – Inherent platelet stickiness acts as a strong bond between the platelet and the exposed subendothelium, as well as between platelets themselves. Fibrin strands will weave in amongst the bound platelets that make up the platelet plug, further compressing and solidifying the structure and creating a fibrin clot. | View Page |
| Primary Hemostasis – Platelet Kinetics Kinetic Processes Specific to Platelets. Adhesion – When platelets adhere to exposed collagen, they take on a characteristic “spiny” shape. Their inherent stickiness, and the aforementioned spiny shape serve to compliment each other during this process. Von Willebrands Factor (vWF) is absorbed by surface receptors on both the platelet and exposed subendothelial tissue, thereby linking the platelets to the tissue. Release – This process occurs prior to aggregation. Platelets dump the contents of their granules (ADP, Serotonin, & Calcium), which aids the upcoming aggregation process by acting as a chemical signal. Aggregation – Platelets physically bind to each other, not just to the exposed subendothelial walls and collagen of the breached vessel. Platelet aggregation requires sufficient chemical signal stimulation. Stabilization (technically part of secondary hemostasis as fibrin is a product of secondary hemostasis)– This process strengthens the platelet plug with the addition of interwoven fibrin strands, ultimately producing a fibrin clot. The durable fibrin clot is the ultimate goal of hemostatic processes. | View Page |
| Summary of Primary Hemostasis In summation, we have covered the following sequence of events which comprise primary hemostasis. The process begins with damage to a vessel wall, as blood flows outside the vasculature. The body responds with vasoconstriction, decreasing blood flow to the affected area. Platelets begin sticking to the damaged vessel walls. As the platelets stick, they release chemicals which signal other platelets to respond. As other platelets arrive, they begin sticking to one another, clumping together, forming a plug to fill in the breach. This plug, while strong, is a temporary fix, and must be reinforced with fibrin strands to effectively fill the breach during the vessel repair process. Construction of the fibrin strands occurs during secondary hemostasis, our next topic to be covered. | View Page |
| Overview of Secondary Hemostasis Secondary hemostasis is the series of interrelated chemical processes which lead to the formation of durable fibrin strands, as well as being involved in their incorporation into the existing platelet plug, creating a fibrin clot. The fibrin strands themselves are manufactured through the interaction of various coagulation factors, via a process known as the coagulation cascade. After strand construction, these fibrin monomers are woven into the framework of the platelet plug, adding greater strength and stability. Once woven into the platelet plug, and further stabilized with covalent cross-linking, a fibrin clot (the end goal of secondary hemostasis) is achieved. The fibrin clot is more durable than the platelet plug, and is more of a long term fix, allowing time for continued vascular repair. | View Page |
| Secondary Hemostasis – Fibrin Formation via the Coagulation Cascade The formation of fibrin involves three interconnected biochemical pathways; the intrinsic, extrinsic, and common pathways. These pathways allow for the interaction of coagulation factors via a finely tuned sequence of chemical processes, where the factors themselves control the activity of the pathway. Most coagulation factors are stimulated and activated by the preceding factor , hence the term, "coagulation cascade." Since factor activation requires the activation of a preceding factor, a deficiency in the functionality or availability of any factor would seriously impact the effectiveness of the coagulation process. Factor deficiencies do occur, however, and often lead to impaired vascular repair and depressed hemostatic activity. | View Page |
| Secondary Hemostasis – The Coagulation Cascade Consequences linked to deficiencies in coagulation factors;
Coagulation does not proceed at its usual pace, it is much slower than normal.
Activation of subsequent factors may be delayed or inhibited all together.
The time required for a clot to form is prolonged.
The breach fails to seal, and free bleeding continues.
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| The ultimate goal of secondary hemostasis is: | View Page |
| Secondary Hemostasis – The Extrinsic Pathway The shortest, and least complex of the three pathways, the extrinsic pathway primarily focuses on the interaction of tissue factor with factor VII, leading to the activation of factor VII. Tissue factor, a substance expressed on the surface of cells such as fibroblasts and macrophages found outside the vasculature, initiates coagulation when plasma contained within the vessel walls leaks outside the broken vessel, and comes into contact with these cells. The nomenclature, extrinsic pathway, comes from the fact that tissue factor is external to the vasculature. | View Page |
| Secondary Hemostasis – The Extrinsic Pathway It should be noted that this pathway is sometimes referred to as the Tissue Factor Pathway.
Once a vessel has been breached, tissue factor is exposed to circulating factor VII, and the two substances bind to form a complex.
The newly formed tissue factor/factor VII complex is thought to be the primary physiological stimulus for blood coagulation.
In other words, more hemostatic activities are initiated by the extrinsic pathway than the intrinsic.
This complex leads to the activation of factor VII (factor VIIa) which is now ready to catalyze the conversion of factor X to factor Xa as part of the common pathway.
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| Secondary Hemostasis – The Extrinsic Pathway Functional control of the extrinsic pathway is mediated by Tissue Factor Pathway Inhibitor (TFPI) which binds to and inhibits factor X. Remember, for hemostatic processes to continue, factor VIIa must be able to promote the chemical conversion of factor X into factor Xa. TFPI effectively blocks this action, thereby controlling the initiation of the common pathway. The Prothrombin time (PT) is used to monitor the extrinsic pathway, and the activity of oral anticoagulants such as Coumarin. | View Page |
| Secondary Hemostasis – The Intrinsic Pathway Exposure to contact substances, such as collagen, can activate the intrinsic pathway.
The exposed collagen is the location where a complex between High Molecular Weight Kininogen (HMWK), Prekallikrein (also known as Fletcher Factor, which activates to Kallikrein), and factor XII (Hageman Factor) forms.
Together, this three biochemical complex, adhered to the collagen binding site, catalyzes the conversion of factor XII to its activated form, XIIa, thereby triggering the intrinsic pathway.
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| Secondary Hemostasis – The Intrinsic Pathway Here is where the “cascade” or “waterfall” nomenclature becomes evident as each activated factor triggers the conversion and activation of subsequent factors.
Factor XIIa, activated previously, catalyzes the conversion of factor XI into XIa, while in the presence of HMWK.
Note: This reaction can occur without HMWK, but will be much slower.
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| Secondary Hemostasis – The Intrinsic Pathway Next, activated factor XI (XIa) catalyzes the conversion of factor IX into IXa, in the presence of ionized Calcium. | View Page |
| Secondary Hemostasis – The Intrinsic Pathway Then, activated factor IX (IXa), along with ionized calcium, platelet factor, and factor VIII, activate factor X, which is a direct precursor of thrombin in the common pathway. The activation of factor X signals the beginning of the common pathway. The activated partial thromboplastin time (APTT) is used to monitor the intrinsic pathway and the effectiveness of heparin therapy. | View Page |
| Secondary Hemostasis – The Intrinsic Pathway | View Page |
| Secondary Hemostasis – The Common Pathway The common pathway is sometimes referred to as the final common pathway or thrombin pathway. The common pathway starts with the activation of factor X by way of the intrinsic pathway, the extrinsic pathway, or both. Factor X is also referred to as either Stuart-Prower Factor or Thrombokinase. Factor X is manufactured in the liver, and is vitamin K dependant. | View Page |
| Secondary Hemostasis – The Common Pathway Activated factor X (Xa), in the presence of factor V (necessary cofactor), ionized calcium, and platelet factor act to convert prothrombin (precursor) to its active form, thrombin, by cleaving the prothrombin molecule. | View Page |
| Secondary Hemostasis – The Common Pathway Thrombin, after its conversion from prothrombin, catalyzes the conversion of fibrinogen into a fibrin monomer. Additionally, thrombin triggers the conversion of factor XIII into factor XIIIa which forms covalent bonds that crosslink and stabilize the fibrin monomers. Finally, thrombin feeds back into the intrinsic and common pathways, accelerating the action of factors XI, V, and VIII. | View Page |
| Secondary Hemostasis - Factor Characteristics | View Page |
| Tests of Hemostatic Function - Platelet Function Assay A platelet function assay (PFA) is a screening test for the evaluation of platelets/primary hemostasis. Common clinical applications include the following: Preoperative evaluation of platelet function Determining the presence of drug-induced platelet dysfunction Determining platelet functionality in high-risk pregnancy Evaluation of patients with suspected inherited or acquired platelet disorders such as von Willebrand disease Evaluation of a bleeding patientA PFA instrument is able to differentiate between drug-induced platelet defects and other platelet defects. PFA tests are superior to the bleeding time test. The bleeding time is often not reproducible and, in spite of attempts at standardization, remains prone to variations in test results between persons performing the test. It is also relatively insensitive to platelet function. The bleeding time cannot be used to identify patients who may have recently ingested aspirin or non-steroidal anti-inflammatory drugs or patients who may have a platelet defect attributable to these drugs. The bleeding time is used to assess platelet function, but may be affected by platelet quantity. NOTE: Aspirin, and some other drugs, may falsely prolong bleeding times. Patients must be asked about aspirin use, and be aspirin free for 7-10 days prior to testing, for valid results. | View Page |