Platelets, also known as thrombocytes, are blood component whose function is to respond to bleeding from bleed vessel injury through clumping, thereby starting a blood clot.

Platelets don’t have a cell nucleus; they are cytoplasm fragments which are come from bone marrow and then enter the flow. Flowing un-activated platelet is biconvex structures.

On a blemished blood mark, these platelets look red, purple spots, approximately 20 percent the size of the red blood cells. A blood smear is utilized to look at the shape, qualitative number, clumping as well as the size of the platelets. The percentage of platelets to red blood cells in a strong and fit grown-up individual ranges from 1:10 to 1:20.

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Also See: How many platelets are in a drop of blood

What is the Main Function of Platelets?

The main role of platelets is to contribute to hemostasis. It is the process of stopping the bleeding at the part of interrupted endothelium. Platelets gather at the affected area, and unless the break is big, they block the hole.  First and foremost, platelets attach to substance outside the broken endothelium. Next, they alter the shape, switch on receptors as well as ooze chemical messengers. After which, platelets connect to each other using receptor bridges. Platelet plug formation is related to coagulation cascade activation with resultant fibrin deposition.

These procedures might extend beyond. The range is from a mainly platelet plug or also known as white clot to a mainly red clot or fibrin clot, or the typical combination, which result in a blood clot. Some will add the succeeding retraction as well as platelet inhibition as the last steps to the end of the process.

Further Reading: What protein involved in coagulation provides the activation for the final step in clotting?

Platelet Adhesion: A Very Important Function

The adherence of platelet is a vital function in response to vascular injury. In general, it is viewed as the initial step in the binding of platelets in membrane receptors to extracellular and cellular matrix constituents of the tissues and vessel wall. This reaction starts the formation of thrombus which arrests hemorrhage and allows wound to heal. Pathological cases which cause blood flow disturbances and vascular alterations might turn this helpful process into a disordered mechanism which leads to arterial occlusion, most often in brand and heart’s atherosclerotic vessels.

Aside from their vital role in thrombosis and hemostasis, the properties of platelet adhesive are vital to an array of pathophysiology procedures, which extend from swelling of immune-mediated host defense, metastasis as well as pathogenic mechanisms.  These actions depend on the capability of platelets to flow in your blood as lookouts of vascular integrity, hold on where changes are seen, and signal the irregularity to blood cells and other platelets. So the adhesions of platelet to the vascular wall, to other blood cells, or one another, show diverse factors of the same basic biological procedures.

Thorough and comprehensive researches by a lot of researchers over the previous years have been intended to dissect the intricacy of these functions. The results gathered now allow an effort to incorporate all the information available into a picture which highlights the synergy of different platelet adhesive interactions and balanced diversity.

Read: Can dehydration cause high platelet count – Our answer

Receptors and Substrates for Platelet Adherence to the Injured Vessel Wall

The hemostatic reaction to vascular wall injury is reliant on the kind of lesion. It depends on hemodynamic conditions and the matrix protein exposed to blood, the adhesion of platelet needs the synergistic function of diverse platelet receptors, eventually resulting in the aggregation and activation of the platelet. The extracellular matrix parts which response with platelets includes diverse kinds of collagen, fibronectin, VWF,  thrombospondin, fibulin, and laminin. Vitronectin and fibrinogen aren’t synthesized by the cells of your vascular wall but should be regarded as potentially applicable thrombogenic substrates as they become powerless onto extracellular matrix at spots of injury. You can assume that all components of tissue can interact with platelet can add to the start of the formation of thrombus once exposed to blood, even if just a few might have important roles.


It is the thinning of your blood vessels resulting from tightening of the muscle wall of your vessels, specifically in the small arterioles and large arteries. Vasoconstriction is the reverse of vasodilation. During hemostasis, a short vasoconstriction spasm happens, which allows the flow of blood to the injured site while the blood clot forms.

Vasoconstriction Mechanisms

The response of vasoconstriction is triggered by many factors such as direct wound to smooth muscle of your vascular muscle, signaling molecules released by wounded endothelial cells, nervous system reflexes started by pain receptors as well as activated platelets.

The response of spasm becomes more effective as the level of harm is increased. Vascular spasm is effective at reducing the blood flow in smaller blood vessels. This condition also causes an augment in blood pressure for blood vessels that are affected by the injury.

Vessel wall smooth muscle goes in intense contractions, which constrict the blood vessel. When the vessel is small, spasm compresses the inner walls mutually and might be capable of stopping the bleeding fully. In case the vessel is medium to large sized, spasm slows down the instant flow of blood, reducing the damage, however still preparing it for the last methods of hemostasis. In a serious wound, the response of spasm is stronger as well as lasts longer. Medications called vasopressin might induce vasoconstriction. This increase blood pressure, as well as help, treat specific conditions.

Further Reading: How many platelets are in the human body

Swelling and Injury

During wound, vasoconstriction is short, only last for a couple of minutes while the coagulation cascade and platelet plug happen. That is due to the reason that as skin tissues are harmed during a wound or injury, inflammation happens as an outcome of inflammatory mediator release from your immune system cells like the NK cells or Mast cells, which get cell stress cytokines from harmed endothelial cells or serotonin which are produced by platelets that are activated. During swelling, vasodilation happens, together with augmented vascular permeability as well as leukocyte chemotaxis, stopping the hemostasis and vasoconstriction spasm as the healing of wound starts.

The Formation of Platelet Plug

At the location of vessel injury, platelet sticks mutually to make a plug that is the start of the formation of blood clot.  The second important stage in hemostasis that follows vasoconstriction is the formation of a platelet plug.

The Adherence of Platelet

Usually, the endothelial cells convey molecules which inhibit the activation and adherence of platelet while it flows in the blood vessels. This molecule takes accounts of endothelial, prostacyclin and nitric oxide.

During an injury, the subendothelial collagen that comes from extracellular matrix under the endothelial cell is exposed on epithelium as the healthy epithelial cells are harmed and eliminated, which generates VWF or Von Willebrand Factor. That causes your platelets to alter form with adhesive filaments which stay to the subendothelial collagen on an endothelial wall.

The Activation of Platelet

After the occurrence of platelet adherence, subendothelial collagen unites to platelet receptors that set it off. During the activation of platelet, it generates some essential chemical mediators and cytokines through degranulation. The generated chemicals include VWF, ADP, PDGF or platelet-derived growth factor, VEGF or Vascular Endothelial Growth Factor, coagulation and serotonin factors as well as generate more VWF, ADP, and other substances. The formation of a platelet plug is considered a good process as VWF and ADP levels are increased successively as more numbers of platelets activate to make the plug.

Other factors released during the activation of platelet perform other essential roles. Thromboxane is an arachidonic acid which sets off other platelets at the same time maintains vasoconstriction. On the other hand, serotonin is a short-lived provocative mediator that has a vasoconstrictive effect, which contributes to the changes in vascular-related to inflammation during a wound or injury. VEGF and PDGF play a very important role in angiogenesis, the development of new blood vessel as well as cell cycle division following injury. Coagulation factors take account of factor V and factor VIII that is responsible for coagulation cascade which converts fibrin into fibrin mesh following the formation of a platelet plug.

Usually, the endothelial cells convey molecules which inhibit the activation and adherence of platelet while it flows in the blood vessels.

The Aggregation of Platelet

The last step in the formation of platelet plug is its aggregation into a barrier. Platelet receptors connect to fibrinogen molecules and VWF that hold the platelets as one. Platelets might also connect to subendothelia VWF to secure or fasten to the wounded and damages endothelium. The finished plug will cover the wounded parts of the endothelium and end blood from flowing out of it. On the other hand, if the wound is big, blood will not clot until the fibrin mesh from the coagulation cascade is generated that strengthens the plug of the platelet. In case the wound is small or minor, platelet plug might enough to end the bleeding with no coagulation cascade.

What is Coagulation?

It is the procedure by which a blood clot develops to minimize blood loss after a wound or injury to a vessel wall. A lot of coagulation cascade components, which takes account of cellular and protein parts, are involved in the repair of a blood vessel. The function of a protein and cellular components can be classified as primary and secondary hemostasis. Classically, coagulation cascade is separated into three pathways: intrinsic or contact pathway, tissue factor or popularly known as an extrinsic pathway as well as the common pathway. The tissue factor and contact pathway feed into and set the common pathway off.

Coagulation Issues

While the coagulation flow plays an essential role in hemostasis as well as in healing wound, it can also lead to various issues. An embolism is a blood clot, which breaks away without being softened and travels in your bloodstream to another location. Once it hinders an artery which supplies blood to organ or tissue, it can lead to infarction and ischemia to body tissues, resulting in a heart attack, stroke or pulmonary embolism.

Coagulation can take place even without wound or injury because blood is pooling from extended stillness can cause clotting factors to build up and set off a coagulation flow independently. What is more, endothelia injure due to immune system factors such as hypersensitivity and inflammation might also lead to embolism and unnecessary thrombosis. Like for instance, during serious bacterial infections, swelled-induced tissue injury and the unconstructively charged bacteria molecules set off coagulation cascade pathways and lead to DIC or disseminated intravascular coagulation, wherein lots of clots develop and breakaway, resulting in immense organ failure.


A lot of anticoagulants avoid unnecessary coagulation as well as those who cannot genetically generate these molecules will be at risk to coagulation. These systems include:

  • Protein C:  this is vitamin K-dependent serine protease enzyme which degrades Factor V and VIII.
  • Antithrombin: This is an inhibitor of serine protease which degrades thrombin, Factor Xa, Factor IXa, Factor XIa, as well as Factor XIIa.
  • TFPI or Tissue Factor Pathway Inhibitor: This restricts TF or Tissue Factor action as well as the factors it generates.
  • Plasmin:  released by proteolytic cleavage, a potent fibrinolytic which degrades fibrin at the same time destroys clots.
  • Prostacyclin: Produced in the endothelium, this also slows down platelet activation.
  • Thrombomodulin: Generated in the endothelium, this changes  thrombin into a stationary form


Decades ago have witnessed remarkable developments in people’s understanding of the mechanism which support the formation of platelet thrombus in flowing red blood cells. The outcomes of ex vivo flow studies, as well as intravital microscopy experiments, have shed new light on the procedures underlying thrombosis and hemostasis.

Animal samples with targeted gene mutations or deletions have very much contributed to these developments. That will surely give more insights in the coming years. An important development in proteomics and genomics has generated data relevant to clarifying the integrated procedures which connect the interactions of platelet substrate and signaling pathways to the growth of thrombus as well as the stability of it.

To conclude, the introduction of enhanced medication development systems is likely to allow the conversion of this underlying skill into well-organized therapeutic methods to avoid too much bleeding as well as thrombosis.

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