Platelet plug formation (primary hemostasis)

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Platelet plug formation (primary hemostasis)

Hematological system


Platelet plug formation (primary hemostasis)


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Platelet plug formation (primary hemostasis)

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An investigation is done on hemostasis to determine the steps involved in preventing blood loss after injury. During the experiment, the team determines that hemostasis typically has two stages: primary and secondary hemostasis. Which of the following is true regarding primary hemostasis?  

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Coagulation disorders p. 433

mixed platelet/coagulation p. 435

Fibrinogen p. 211

thrombocytes p. 415


thrombocytes p. 415

Hemolytic-uremic syndrome (HUS)

platelet disorders p. 434

Platelet disorders p. 434

transfusion for p. 435


thrombocytes in p. 415


liver markers p. 399

in wound healing p. 209

Thrombocytes (platelets) p. 415

aggregation inhibition p. 445


in thrombocytes p. 415


Platelet plug formation, also called primary hemostasis, is the first of two steps needed for hemostasis.

Hemostasis is how the body prevents blood loss a blood vessel is injured and broken.

Without hemostasis even a minor injury would be life-threatening - imagine dying from a nosebleed! During primary hemostasis, platelets clump up together and form a plug around the site of injury.

Then in the second stage, called secondary hemostasis, the platelet plug is reinforced by a protein mesh made up of fibrin.

You can think of it like a brick wall where the platelets make up the bricks and the fibrin makes up the mortar that goes between the bricks.

So going back to primary hemostasis, the clumping up of platelets - this step can be further divided into five steps: endothelial injury, exposure, adhesion, activation, and aggregation.

Let's imagine that you accidently slice a tiny artery in your finger while cutting fruit, ouch! When this happens, the knife cuts several layers of the artery.

The innermost layer of the artery is the endothelium, and it’s made up of endothelial cells.

Just outside of this layer are several layers of smooth muscle cells, which control the size of the lumen, or the inner diameter of the vessel by contraction and relaxation.

Outside of the smooth muscle there’s a layer of protein - specifically elastic fibers which give the blood vessel the ability to expand and contract.

Outside of the elastic fibers, there’s connective tissue made up of collagen, which is the major structural protein in humans.

This fibrous layer protects the vessel and anchors it to the surrounding tissues.


Platelet plug formation, also known as primary hemostasis, is the first step of hemostasis and is the process where a platelet plug forms to prevent further loss of blood from a damaged vessel. It is divided into five stages: endothelial injury, exposure, adhesion, activation, and aggregation. After the platelet plug is formed, it is not a strong enough barrier to stop the bleeding completely, it is further reinforced by secondary hemostasis, which is the process by which a protein called fibrin is used to create a more stable clot, also called clot retraction.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Primary Platelet Adhesion Receptors" IUBMB Life (International Union of Biochemistry and Molecular Biology: Life) (2005)
  6. "Glycoprotein VI-dependent and -independent pathways of thrombus formation in vivo" Blood (2006)
  7. "Differential Sensitivity of Various Markers of Platelet Activation with Adenosine Diphosphate" BioNanoScience (2018)

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