Blood

The second wave agonists

(TXA2 and ADP) generate an additional increase in intracellular calcium, likely through release from intracellular stores.

ADDITIONAL NOTES:

- Thrombin can also bind to thrombomodulin that is expressed on EC surfaces. This prevents it from cleaving fibrinogen. Instead, it cleaves and activates protein C (in the presence of a protein cofactor Va) which inhibits clotting by cleaving and inactivating two pro-coagulant factors, Va and VIIIa. - Protein C is a natural plasma protein. Together with its co-factor protein S (which results in activated protein C), it degrades factors Va and VIIIa, and thus provides natural anticoagulation by inhibiting activation of Factor X and Prothrombin. - Antithrombin is an endogenous anticoagulant.

LEARNING OBJECTIVES

- Understand hemostasis-cessation of blood loss - Define arterial and venous thrombosis - Outline platelet adhesion, activation, and aggregation - Summarize the process fibrin formation

THroumbus vs embolus

A clot that adheres to the interior wall of an artery or vein is called a thrombus, whereas a blood clot that has been dislodged and is travelling throughout the blood stream is termed an embolus.

generation of thrombin

Both the Intrinsic and Extrinsic pathways converge at a final common pathway which leads to the generation of thrombin, fibrinogen, and finally the formation of insoluble fibrin. Fibrin undergoes covalent cross bridging between adjacent fibrin monomers. The fibrin "mesh" of cross-linked fibrin monomers can be seen as a white stringlike substance trapping red blood cells in a fresh clot. The red cells do not stick together; they are held together by fibrin.

why thrombi and emboli are dangerous?

Both thrombi and emboli are dangerous because they may occlude blood vessels, obstruct the flow of blood through the circulatory system, and deprive tissues of oxygen and nutrients.

what are the causes of deep vein thrombosis?

Causes of deep vein thrombosis include surgery, injury like a broken leg, sitting or inactivity, and birth control. Largely a consequence of activation of the clotting (coagulation) system and hence are made up of RBC and fibrin (venous thrombi are soft, deep red and have a higher cell content).

how does the bridging action happen?

GPIb can indirectly interact with collagen via its binding to the von Willebrand factor (vWF), a unique adhesive glycoprotein (that is released from both the injured endothelial cell and platelets), which allows platelets to remain attached/anchored to the vessel wall despite the high shear forces generated within the vascular lumen. vWF binds to both GPIb on the platelet membrane and to the exposed collagen ("bridging action"). In this way, platelets have an additional mechanism by which they get anchored to the site of the injured endothelium.

GPVI

GPVI-the central receptor that facilitates direct contact with sub- endothelial collagen (other receptors also suggested to play a role in contacting platelets directly to collagen is α2β1-integrin and GPIa).

What is hemostasis?

Hemostasis is the cessation of blood loss from a damaged vessel (arrest of bleeding)

what is primary hemostasis

Immediately following vasoconstriction, primary hemostasis occurs, a process that requires the participation of platelets (they provide the initial hemostatic plug at sites of vascular injury).

VENOUS THROMBOSIS

In deep vein thrombus formation, factors like a) reduced blood flow and stasis (associated with surgery, hospitalization, paralysis, long-haul travel, pregnancy, etc.), b) thrombophilia or procoagulant changes in the blood (where the body makes too much off a blood-clotting protein like Prothrombin or a blood-clotting protein like Factor V does not function well-it cannot be inactivated by Protein C and S), and c) activation of the endothelium contribute to the formation of a thrombus. The presence of valves in veins leads to turbulent flow and reduced oxygenation of the valve endothelium that may activate the endothelium and allow a thrombus to form. This is because the valve pocket sinus, due to its tendency to become hypoxic, can lead to surface expression of adhesion proteins (P-selectin, E-selectin, vWF). Circulating leukocytes, platelets, and TF+ MVs bind to the activated endothelium (via the P-selectin glycoprotein ligand-1; PSGL- 1). The bound leukocytes become activated and express TF. By binding to factor VII (and activating it to VIIa), to form TF-FVIIa complex, there is activation of factor X to Xa. This initiates the coagulation process where large amounts of thrombin are generated. The local activation of the coagulation cascade overwhelms the protective anticoagulant pathways and triggers thrombosis. Once formed, the thrombus may extend along the vessel and embolize. Valve pocket thrombi contain mostly red cells and fibrin. Therefore, the management of venous thrombosis is mainly centered on anticoagulation strategies.

What happens during the vasoconstriction in thrombin?

In response to an injury to a blood vessel, local vasoconstriction occurs due to local spasm of the smooth muscle in the wall of the blood vessel as a result of secretion of vasoconstrictors such as endothelin-1.

coagulation cascade

In the coagulation cascade, each stage involves the conversion of a precursor protein (synthesized in the liver that is normally inactive) to an active protease (indicated by "a") by cleavage of one or more peptide bonds in the precursor molecule. At each stage, the usual components involved are a protease from the preceding stage, a precursor protein, a non- enzymatic protein cofactor (reaction accelerator), Ca2+, and an organizing surface (e.g., phospholipd surface of activated platelets in vivo). The coagulation proteases include HMWK (high molecular weight kallikrein), prekallikrein, and factors XII, XI, IX, X, VII and II (prothrombin). The non-enzymatic protein cofactors include factors V, VIII, and tissue factor (TF also called thromboplastin-a glycoprotein receptor found on the surface of a number of cells surrounding blood vessels-its present on extravascular tissue). The final events in the coagulation process are the conversion of prothrombin to thrombin (by the action of factor Xa) and fibrinogen (soluable) to fibrin (insoluable) monomer (in the presence of thrombin also called factor IIa).

major component of arterial thrombosis

In this case, clot formation is a dynamic process that requires platelet adhesion, activation and aggregation (major component of arterial thrombosis), followed by formation of thrombin.

Arterial thrombosis

Is usually a result of adherence of platelets to the arterial wall (i.e., largely a phenomena of platelet activation; the arterial thrombi are pale, granular and have a lower cell count).

hemostatic plug

Platelet-fibrinogen linkages rapidly enlarge the platelet plug; called the primary hemostatic plug. Primary hemostasis is the name given to this process of platelet-plug formation at sites of injury. It occurs within seconds of injury and is of prime importance in stopping blood loss.

how do plateles adhere to th collagen in cascular sub-endothelium

Platelets adhere to collagen in vascular sub-endothelium via a specific platelet collagen receptor made up of glycoproteins (Gp).

Platelets and their role is vascular system

Platelets are anucleate (do not have a nucleus of their own) cellular fragments that circulate as inactive, non-binding concave discs. Platelets perform a very important biologic role in the vascular system. Platelets perform a very important biologic role in the vascular system. Interaction between the platelet and vessel wall are of crucial importance in physiology and pathophysiology.

How does pulmonary embolism happen?

Should the venous thromboemboli dislodge from the deep veins of the legs, they travel to the right side of the heart to reach the lung where it causes pulmonary embolism which is potentially life threatening.

Major components of hemostasis?

Successful hemostasis depends on the vessel wall, circulating platelets, and plasma- coagulation proteins which are the three major components involved in the process of hemostasis.

The coagulation phase has 2 major pathways

The coagulation phase has 2 major pathways, divided into the intrinsic and extrinsic systems. Within the intrinsic system, all of the clotting factors are present within the blood vessels. Activation of Factor XII, on contact with a negatively charged surface (e.g., glass) or exposed collagen fibres (in the subendothelium of damaged blood vessels), initiates the intrinsic coagulation system - this pathway is generally slower (takes several minutes). In the extrinsic pathway, the initial stimulus is present outside the blood vessels - tissue factor (TF). Damage to the blood vessels exposes TF containing cells from underlying layers to the blood stream. TF (proposed to be a cell surface glycoprotein that is membrane bound) is then able to bind in the presence of calcium to Factor VII which circulates in the blood stream (TF acts as a high affinity receptor for Factor VII). This sets off sequential protease activations - clotting in this pathway is faster (within seconds). Under pathological conditions, TF is also expressed on circulating leukocytes and possibly activated endothelial cells. In addition, TF is present on microvesicles (MVs-fragments of plasma membrane from various cell types including platelets, and leukocytes), which are small membrane vesicles released from activated cells. These intravascular sources of TF may trigger the formation of venous clots.

critical for platelet aggregation

The fibrinogen:GPIIb/Illa interaction is critical for platelet aggregation.

why the process of secondary hemostasis begins

The platelet plug is not stable and can be dislodged. Hence, the process of secondary hemostasis begins. This usually requires several minutes for completion. It begins with the local activation of plasma coagulation factors (e.g., thrombin) and the eventual formation of a fibrin clot (that strengthens the primary hemostatic plug).

Platelet aggregation:

The second wave chemical mediators promote platelet aggregation (and thrombus growth) by exposing platelet surface receptors (that are normally inactive on resting platelets but undergo conformational transformation when there is an increase in calcium). The predominant receptor is GPIIb/Illa (also known as integrin αIIbβ3) which facilitates contact with circulating proteins, mainly fibrinogen (but also the von Willebrand factor); fibrinogen can then act as a bridge between two platelets, which are essential in the process of adhesion and aggregation (the RGD recognition sequence on the fibrinogen makes this possible-RGD is a one letter amino acid abbreviation for Arginine-Glycine-Aspartic acid and each fibrinogen has 2 RGD sequences).

Platelet activation:

This is the process where the initial adherent platelets undergo a process of activation (or degranulation). Thus, on binding of platelets to the underlying collagen (the most important stimulus for platelet activation) or vWF, there is calcium mobilization from intracellular stores (within the DTS-dense tubular system) into the cytoplasm thereby increasing the amount of calcium. Calcium can promote platelet shape change (platelets contain contractile proteins like actin and myosin), allowing for the movement of granules. During this process, prepackaged platelet granules that contain second wave agonists (like TXA2 and ADP) are released (degranulation). Released ADP and TxA2 can further enhance platelet activation by binding to their respective platelet receptors (TXA2 to the TP receptor; ADP to P2Y12 receptor) to increase intracellular calcium even more in these "frontline" platelets, or activate ADP and TxA2 receptors on other secondary platelets (and thus augment their intracellular calcium). This robust increase in calcium stimulates cyclooxygenase to promote the synthesis of thromboxane A2 [Calcium activates phospholipase A2, which cleaves platelet membrane phospholipids and liberates arachidonic acid. Arachidonic acid, in the presence of cyclooxygenase forms prostaglandin H2 (PGH2). Metabolism of PGH2 is facilitated by thromboxane synthase, to produce thromboxane A2 (TXA2)].

What is thrombin?

This latter enzyme catalyzes the production of fibrin which, when cross-linked, stabilizes the clot.

Thrombosis

Thrombosis, the formation of an unwanted clot, can happen within both arteries and veins.

Summary of platelet events:

Under normal circumstances, platelets do not adhere to healthy arterial walls nor are they activated by the vascular endothelium. This is because platelets have coated glycoproteins which carry negative charges and are thus repelled by the endothelial cell (EC) (which also has a glycoprotein coat of negative charge). Damage to the blood vessels exposes collagen and other underlying tissue (the sub endothelium) which have positive charges. Thus, at the site of injury (damaged vessel wall), such as that caused by a ruptured atherosclerotic plaque, platelets begin to adhere to the damaged site (within seconds). This is followed by platelet activation (release of intracellular granules) and finally platelet aggregation to form a platelet plug.

Venous thrombosis, where?

Venous thrombosis develops in areas of stagnated blood flow (especially in the deep leg veins of the leg, most commonly the femoral vein, near the venous valves).

Platelet adhesion:

Within seconds of vascular injury, platelets firmly adhere to collagen fibrils in the vascular sub-endothelium via ionic interaction and specific receptors. This process is provoked by the loss of an intact endothelial lining of the blood vessel as well as by the exposure of the platelet to activating sub-endothelial structures (e.g., collagen).

Arterial thrombosis can cause:

Within the arteries, Arterial thrombosis can lead to ischemia, myocardial infarction, angina, stroke or peripheral arterial disease. Is usually a result of adherence of platelets to the arterial wall (i.e., largely a phenomena of platelet activation; the arterial thrombi are pale, granular and have a lower cell count). The management of arterial thrombosis is mainly focused on antiplatelet strategies.

The management of venous thrombosis is mainly centered on

anticoagulation strategies

The management of arterial thrombosis is mainly focused on

antiplatelet strategies

steps of hemostasis

vascular spasm, platelet plug formation, coagulation - The process starts with constriction of the blood vessels to limit blood loss, followed by contribution from platelets and coagulation proteins to form a thrombus (a blood clot).