Clot retraction and fibrinolysis

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Clot retraction and fibrinolysis


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USMLE® Step 1 questions

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Clot retraction and fibrinolysis

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USMLE® Step 1 style questions USMLE

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A group of investigators are studying the process of clot retraction and fibrinolysis. They discovered that the process of fibrinolysis starts approximately two days after the injury and involves the action of tissue plasminogen activator (tPA). Which of the following best describes the function of tPA?  

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Fibrinolysis p. 418

Fibrinolytic system p. 420

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In hemostasis, hemo referring to blood, and stasis meaning to stop—so hemostasis is the process where blood flow is stopped after there’s damage to a blood vessel.

Primary hemostasis involves the formation of a platelet plug at the site of an injured blood vessel, and secondary hemostasis involves the coagulation cascade which is where a protein net called a fibrin mesh forms over the platelet plug to reinforce it - forming a blood clot.

Now, anticoagulation occurs during primary and secondary hemostasis and helps regulate clot formation, whereas clot retraction and fibrinolysis occur after primary and secondary hemostasis are complete, and help a clot contract and degrade.

Anticoagulation prevents clots from growing too large and blocking blood flow to tissues supplied by the vessel.

It also prevents clots from getting so big that small parts of the growing clot break off in the form of emboli.

Depending on the location of the primary blood clot, these emboli may then cause a disruption in blood flow to organs like the heart or brain.

Now, the most important point of clot regulation is when a coagulation factor called thrombin is produced.

Thrombin, or factor II, is a very important clotting factor, because it has multiple pro-coagulative functions. Think of thrombin as the accelerator on a car--the pedal that takes secondary hemostasis from 20 miles per hour to 100 miles per hour!

First, thrombin binds to receptors on platelets causing them to activate.


After an injury to a blood vessel, primary and secondary hemostasis forms a blood clot to stop bleeding. After hemostasis, it follows another process called clot retraction, which stabilizes the clot by pulling together the wounded edges of the vessel. Next, fibrinolysis occurs, which is an enzymatic process during which blood clots are dissolved to clear the way for blood circulation.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Basic mechanisms and regulation of fibrinolysis" Journal of Thrombosis and Haemostasis (2015)
  6. "Insights into platelet-based control of coagulation" Thrombosis Research (2014)
  7. "Treating thrombosis in the 21st century" N Engl J Med (2003)

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