Primary hemostasis

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Definition: Primary hemostasis is defined as the formation of the primary platelet plug. This serves to plug off small injuries especially in microvessels (< 100 μm) in mucosal tissues (respiratory, gastrointestinal, genitorurinary tracts). Platelets are not only involved in platelet plug formation but are also crucial for formation of fibrin (secondary hemostasis). Activated platelets express a negatively charged phospholipid, phosphatidylserine (PS), on their surfaces, which is a binding site for the assembly of coagulation factor complexes. Assembly of these complexes on this surface markedly amplifies fibrin formation (called the propagation phase of secondary hemostasis). In addition, activated platelets also activate neutrophils, leading to NETosis or extrusion of nuclear material (Carestia et al 2016), which is strongly procoagulant and antifibrinolytic (Liaw et al 2015).

Constituents: Platelets, von Willebrand factor (vWf), and the vessel wall.

  • Cells: Endothelial cells (source of vWf), platelets
  • Adhesive proteins: vWf, subendothelial matrix protein (e.g. collagen)
  • Facilitators: Thrombin (platelet agonist)
Primary hemostasis

Primary hemostasis

Sequence of events: Normally, the intact endothelium is a physical barrier separating circulating platelets from thrombogenic substances (such as extracellular matrix proteins) in the extravascular space. When the endothelium is injured, the procoagulant subendothelial matrix (consisting of proteins such as collagen, laminin, and fibronectin) is exposed and immediately initiates primary hemostasis, which consists of three main events:

  • Platelet adhesion: Platelets adhere to the exposed subendothelial matrix (directly or indirectly via vWf).
  • Platelet activation: Once platelets adhere, they then become activated and recruit (and activate) additional platelets to the injured site. Also, thrombin generated by the coagulation cascade is an extremely powerful platelet activator.
  • Platelet plug formation: Fibrinogen forms bridges between activated platelets to form the platelet plug.

More detail on these three events is given below.

Note: Secondary hemostasis (formation of fibrin by coagulation factors) is usually initiated simultaneously (through exposure of tissue factor on subendothelial fibroblasts). Activated platelets promote fibrin formation and provide a physical scaffold on which fibrin formation proceeds.

Sequence of events

Platelet adhesion

Upon endothelial injury, platelets bind to exposed subendothelial matrix proteins with the aid of adhesion molecules or receptors on their surfaces. These receptors are transmembrane glycoproteins (GP) and are either integrins or non-integrin receptors. Platelets adhere to exposed collagen in the subendothelial matrix when cell surface receptors (that are constitutively expressed) encounter exposed thrombogenic subendothelial matrix proteins. There are three main receptors that mediate platelet adhesion:

  1. GPVI and α2β1 (an integrin): These bind directly to collagen.
  2. GP1b-IX-V: This binds to vWf, which acts as a bridge or glue between extracellular matrix proteins and the receptor complex of  on the platelet surface. vWf is produced by endothelial cells (where it is stored in Weibel-Palade bodies) and is secreted into the subendothelial matrix and plasma (thus it can be measured in plasma). GPIb-IX-V/vWf interactions are important for mediating platelet adhesion to vessels with high shear rates. vWf is actually a multimeric molecule that is quite large. Studies have shown that under high shear, the molecule unfurls and can act as a sling to capture platelets streaming by the injury. Abnormalities in either GP1b-IX-V complex or vWf can result in a hemorrhagic diathesis. These are usually inherited disorders known as Bernard-Soulier syndrome (GP1b deficiency) or von Willebrand disease (vWD;  abnormality or deficiency in vWf). Of these conditions, only vWD has been reported in animals.

Platelet activation

The adhesion of platelets to subendothelial matrix proteins activates the platelets (increasing intracellular calcium and inducing cell signaling) and causes a variety of changes in platelets:

  • Shape change: Platelets change from their normal discoid shape to elongated cells with cytoplasmic extensions, which increases their surface area.
  • The release reaction (degranulation): Platelets release the contents of preformed cytoplasmic granules (α-granules and dense or δ-bodies).
    • α-granules contain vWf and coagulation factors, including fibrinogen, Factor V (FV) and FXIII. Once released, these factors can participate in platelet plug formation (vWf, fibrinogen) or fibrin formation (FV, FXIII). Of these factors, only vWf is produced in megakaryocytes; the remaining proteins are endocytosed from plasma. There are also species differences in the amount of vWf within platelets, with dog platelets having little vWf, whereas cat platelets can contain up to 20% vWf. These granules also contain adhesion molecules, such as P-selectin, which is expressed on the platelet surface after activation. Detection of P selectin is actually used as an in vitro  marker of platelet activation in the laboratory. In vivo, P selectin binds to its ligand, PSGL-1, on endothelial cells and leukocytes. This helps firmly platelets bind to the endothelium and allows leukocytes to be incorporated into developing clots (leukocytes, like platelets, also provide a surface scaffold for fibrin formation).
    • Dense granules are rich in ADP and serotonin, both of which are platelet agonists (i.e. they recruit and activate additional platelets). They  store calcium which is needed for secondary hemostasis and platelet activation. Recent studies have shown that they are also stores for short-chain polyphosphate (chains of phosphate molecules). These polyphosphates have been shown to have multiple roles in hemostasis and are both procoagulant (they promote the cofactor activity of Factor V in the common pathway and also serve as a cofactor for thrombin activation of FXI in the amplification phase of secondary hemostasis) and antifibrinolytic, i.e. they help form dense fibrin fibrils which are more resistant to fibrinolysis. Dense granule secretion can be measured by detecting the amount of 14C-serotonin or ATP released during aggregation.
    • Note that the release reaction (via platelet factor IV, von Willebrand factor and P-selectin) also promote NET formation (release of nuclear material) from neutrophils (Carestia et al 2016), which are enmeshed in the developing clot, which is procoagulant and antifibrinolytic (Liaw et al 2015), providing a link between primary and secondary hemostasis.
  • Phospholipid metabolism (membrane metabolism): Activation of phospholipase A2 hydrolyzes membrane phospholipids (phosphatidylcholine) to arachadonic acid. This is the initial substrate for subsequent reactions involving several enzymes (e.g. cyclo-oxygenase, COX-1) which produce eicosanoids, particularly thromboxane A2, which are powerful platelet agonists. Pharmacologic inhibition of COX-1 can be accomplished by aspirin and non-steroidal anti-inflammatory agents. These inhibitors are used to prevent thrombotic events in both people and animals (principally dogs).
  • Phosphatidylserine exposure (membrane flipping): The normal platelet membrane is asymmetrical with the more negatively charged phospholipid, phosphatidylserine (PS), being preferentially and actively maintained on the inner membrane leaflet. Phosphatidylserine is very important, because it is the membrane surface on which the coagulation cascade occurs, i.e. it is the binding site for coagulation factors and actually amplifies their activity, thus promoting fibrin formation. When platelets are activated, PS is flipped to the outside of the platelet membrane, allowing coagulation factors to bind to the activated platelet. Phosphatidylserine used to be called platelet factor 3 (PF3). An inherited defect in PS exposure (Scott syndrome) has been reported in German Shepherd dogs and results in excessive hemorrhage in affected dogs. This has been shown to be due to a genetic defect in a calcium channel called TMEM16F in platelets, which is required for membrane flipping (Brooks et al 2002Brooks et al 2016).
  • Release of procoagulant microparticles (microvesiculation): Activated platelets shed tiny microparticles, that are enriched in PS, from their membrane surfaces. These microparticles extend the surface area of the platelet, thus providing a larger surface on which fibrin formation can proceed. Dogs with Scott syndrome also have a defect in microvesiculation. Phosphatidylserine exposure and microvesiculation can be assessed in the laboratory with flow cytometry (Brooks et al 2002).
  • Recruitment of additional platelets (outside-in signaling): Additional platelets are recruited to the site of vessel injury by agonists that are released by activated platelets themselves (ADP, thromboxane A2) or that are formed from activated coagulation factors (thrombin). These agonists bind to surface receptors on platelets and trigger outside-in signaling which activates the recruited platelets. This outside-in signaling involves G proteins and other facilitator molecules, such as guanine exchange factors (GEF). An important GEF is CalDAG-GEF1 (calcium diacylglycerol-regulated guanine exchange factor-1) which is downstream of the ADP receptor and has shown to be defective in Basset Hounds and Eskimo Spitz with thrombopathia. Important platelet receptors for agonists are:
    • P2Y12: This is the receptor for ADP. Clopidogrel is a P2Y12 antagonist that is also used as an antithrombotic drugs in humans, dogs and cats. Efficacy in horses is less certain.
    • Protease-activated receptors (PAR): They are unique G-protein coupled receptors expressed on several different cells. They carry their own ligand, which is only expressed after the receptor is cleaved to expose the ligand, which then activates the receptor. Once activated, the receptor cannot be re-activated even though the cleavage enzyme dissociates from the receptor. These receptors bind several agonists, but the main agonist is thrombin, which binds to PAR-1 and PAR-4 expressed on human platelets, with PAR-1 being the main activator.
  • Activation of the fibrinogen receptor (inside-out signaling): An integrin, αIIbβ3, also known as GPIIb/IIIa or CD41/61, is the fibrinogen receptor on platelets. It is normally present on platelet surfaces (indeed it is a platelet-specific marker) but cannot bind fibrinogen on unstimulated platelets. Once platelets are activated, the fibrinogen receptor changes conformation on the outside of the platelet due to intracellular signaling (inside-out), which allows fibrinogen to bind adjacent platelets, forming platelet aggregates. Binding of fibrinogen to activated platelets can be used as a laboratory marker of platelet activation. A genetic defect in GPIIb/IIIa has been detected in various breeds of dogs (Otterhound, Great Pyrenees) and cattle. This results in a bleeding disorder called Glanzmann’s thrombasthenia. A protein called kindlin is required for correct functioning of integrins in platelets and there has been a report of a dog with kindlin deficiency. The dog had combined leukocytosis, due to a leukocyte adhesion deficiency (integrins are required for leukocytes to migrate into tissue), and hemorrhagic defect due to defective platelet aggregation. Platelet aggregation can be analyzed using platelet agonists in aggregometers. Platelet adhesion and aggregation in response to agonists can also be evaluated using platelet function analyzers (PFA).

Platelet aggregation

This is mediated primarily by fibrinogen, which binds to the activated fibrinogen receptor (GPIIb/IIIa) on platelets. This links platelets together forming the primary plug. The platelet plug is sufficient to stop bleeding from most small blood vessels in response to every day trauma (e.g. eating) or venipuncture, however in larger vessels or with more severe damage, the primary platelet plug must be stabilized to cease the hemorrhage. This is accomplished through secondary hemostasis or fibrin production by coagulation factors. Also note, that platelets have roles beyond what has been described for hemostasis. For instance, it is now known that platelets are essential for vascular integrity. It is possible that hemorrhage in patients with thrombocytopenia is not only due to an absence of platelets but also to a loss of vascular integrity.

Platelet heterogeneity

It is now known that platelets are not a homogeneous population. There are subsets of platelets that are either procoagulant (express PS, assemble coagulation factor complexes and propagate thrombin generation) or those that are adhesive and contractile and more likely to bind to extracellular matrix proteins and cause clot retraction (Agbani and Pool 2017). Even within the procoagulant category, there are subtypes of different platelets, such as ballooned or “coated” platelets, the latter of which are formed from the dual activation of collagen and thrombin and have serotonin-conjugated proteins on their surface, including fibrinogen and thrombospondin (Dale 2005). Procoagulant platelets require robust sustained internal calcium-mediated signaling and strong agonists, such as collagen which binds to the collagen receptor, GPVI, or thrombin, which binds to PARs. Calcium signaling activates calpain and chloride entry via the chloride channel, TMEM16F (which is defective in Scott syndrome), which are required for microvesiculation and irreversible ballooning, and PS exposure (Agbani and Pool 2017). Platelet subsets are also located in different parts of the developing thrombus, with the adhesive platelets being found closer to the vessel wall and the procoagulant platelets forming an overlying cap.


There are physiologic, pathologic and pharmacologic inhibitors of primary hemostasis.

  • Physiologic: Endothelial cells release prostacyclin (PGE12) and nitric oxide, both of which inhibit platelet activation. They also form a physical barrier, preventing platelets from being exposed to thrombogenic subendothelial matrix proteins.
  • Pathologic: High concentrations of fibrin(ogen) degradation products can inhibit platelet aggregation (fragments D and E have a high affinity for platelet membranes and compete with fibrinogen for platelet receptors, thus impairing aggregation) as can paraproteins (high concentrations of monoclonal immunoglobulins with multiple myeloma – the monoclonal protein coats platelets, interfering with platelet aggregation, adhesion and phospholipid exposure).
  • Pharmacologic: These inhibit the following platelet activation events. Note: Drugs that inhibit platelet function can precipitate or worsen hemorrhage in an animal with a mild hemostatic disorder.
    • Arachidonic acid metabolism: Aspirin (irreversibly) and non-steroidal anti-inflammatory drugs (reversibly) inhibit cyclo-oxgenase, which is needed for thromboxane A2 production.
    • ADP agonist activity: Clopidogrel is an ADP receptor antagonist.

Clinical signs


Epistaxis in a Bassett with inherited thrombopathia (photograph courtesy of Dr. Marjory Brooks)

A defect in any aspect of primary hemostasis can manifest as hemorrhage, typically from mucosal surfaces (the sites where the primary platelet plug is important due to rapid fibrinolysis). Hyperactivity in primary hemostatic components is a cause of hypercoagulability and a risk factor for thrombosis.

  • Defective primary hemostasis: Platelet plug formation is inadequate with a defect in any primary hemostasis event, including thrombocytopenia. This usually manifests as hemorrhage from small blood vessels subjected to daily minor trauma, i.e. those supplying mucuosa (e.g. epistaxis, hematuria). Hemorrhage can be spontaneous with severe defects or induced by trauma or surgery with milder defects. Since platelets plug small holes in vessels, thrombocytopenia or abnormal platelet function (thrombopathia) can result in pinpoint mucosal or cutaneous hemorrhages (petechiae). Petechiae are not typical in vWD. Bruises (ecchymoses) can also occur.
  • Hyperactive primary hemostasis: Platelet hyper-reactivity and increased vWf concentrations contribute to hypercoagulability, i.e. are risk factors for thrombosis, but are unlikely to induce thrombosis alone. Thrombocytosis in animals is not usually associated with abnormal thrombotic events.

Sample collection

Two samples are required to evaluate primary hemostasis:

  • EDTA-anticoagulated blood: Platelet count, genetic testing (if needed).
  • Citrate-anticoagulated plasma: All other tests.

Please refer to the sample collection page for additional guidelines on how to collect samples appropriately to optimize hemostasis test results.


It is imperative that a platelet count is done in all animals presenting with clinical signs of excessive hemorrhage. Platelet counts can be quantified using automated analyzers or can be semi-quantified (estimated) from a well-prepared fresh blood smear. Additional platelet testing requires fresh blood or, preferably, referral of the patient to a testing center that is capable of performing advanced hemostasis testing. Few tests of primary hemostasis are available to general practitioners. These include a platelet count, a buccal mucosal bleeding time (BMBT), genetic testing for specific inherited disorders, and measurement of vWf concentrations.

  • Screening tests: Platelet count, BMBT, platelet function analyzer testing.
  • Specific tests: von Willebrand factor antigen (vWf:Ag), genetic testing for vWD and defined inherited thrombopathias (e.g. CalDAG-GEF1 defect in Bassett Hounds and Eskimo Spitz, GPIIb/IIIa deficiency or Glanzman’s thrombasthaenia in Otterhounds and Great Pyrenees), TMEM16F genetic defect in Scott Syndrome (Brooks et al 2016).
  • Specialized testing: Flow cytometric tests for platelet activation (P selectin expression), function in primary hemostasis (platelet aggregation, release reaction), procoagulant activity (PS exteriorization, microparticle quantification, thrombin generation assays), platelet-associated antibodies, reticulated platelets.


Defects arise in any aspect of the hemostatic pathway. As indicated above, these defects can result in deficient hemostasis (hypocoagulability), which can result in excessive hemorrhage, or accelerated hemostasis (hypercoagulability), which can result in thrombosis. Disorders can also be inherited or acquired. Inherited conditions should be suspected in young animals presenting with episodes of recurrent hemorrhage or thrombosis. Acquired disorders are more likely in older animals with underlying disease. The most common inherited defect in primary hemostasis is von Willebrand Disease and thrombocytopenia is the most common acquired defect (there are multiple causes of thrombocytopenia).

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