What is hemostasis?
Why your body relies on a healthy balance of clotting and bleeding

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What is hemostasis?
Thrombosis and COVID-19

Blood is a fluid that is circulated around the body through blood vessels. It performs many vital functions within the body, including defense and protection mechanisms, supplying oxygen and nutrients and regulating body temperature. Injuries such as a cut or bruise can damage blood vessels, which causes bleeding. In a healthy person, bleeding will automatically stop after a short time to prevent extensive blood loss. This function is called hemostasis.


What is primary hemostasis?

As a first action to stop bleeding, the body sends proteins called collagen, which makes blood platelets travel to the wound and stick to each other.

Platelets also cause the vessels to contract, causing lower blood circulation around the injured area.

Platelets release various compounds that make even more platelets aggregate to help close the wound. Typically, within a few minutes, a seal is formed. This seal is a first reaction to stop the bleeding.

What is secondary hemostasis?

To stabilize the seal, substances released through the injury start another interlinked chain reaction, which is called secondary hemostasis.

This sequence of reactions can happen via two pathways. Both pathways lead to the production of cross-linked fibrin.

As a result, a clot made of platelets, fibrin, and blood cells is formed in the blood.

How are clots broken down?

Once the wound has healed underneath its stable seal and the clot is no longer needed, the clot is broken down by a process called fibrinolysis.

This process elevates a protein in the blood called D-dimer.

In a healthy person, clotting and fibrinolysis are balanced, meaning the blood appropriately develops clots and breaks them down only when needed in response to a wound or contusion.


What are thrombotic disorders?

A thrombotic disorder refers to conditions in which blood clots form in the blood vessels. This happens due to elevated clotting in the blood.

There are several conditions that may trigger elevated clotting activity in the blood. If the blood clots too much, a person could experience thrombosis.

If a clot becomes too large, it can block normal blood flow in arteries and veins, causing:

  • Ischemic stroke or heart attack
  • Deep vein thrombosis (DVT)
  • Pulmonary embolism (PE)

A clot can also wander through the body and may block blood vessels in the brain (stroke) or the lung (PE).

Learn more about DVT and PE, and the role of D-dimer testing.

How is thrombosis treated?

If a patient is at high risk for thrombosis, a physician will attempt to prevent excessive clot formation with medication, such as heparin or oral anticoagulants.

Acute thrombosis, like a heart attack or a stroke, requires immediate treatment. One of the standard therapeutic measures is treatment with fibrinolytic medications to dissolve the blocking clot (or thrombus) and reestablish blood flow to prevent damage or death.


How is COVID-19 related to hemostatic disorders?

COVID-19 patients are experiencing serious—and sometimes fatal—clotting abnormalities.

Studies show that many critically ill COVID-19 patients develop thrombotic complications such as confirmed venous thromboembolism (VTE) or pulmonary embolism (PE), sometimes despite medical treatment, e.g., heparin therapy.1-5

The risk of thrombosis in COVID-19 patients increases with disease severity and length of the hospital stay.4

Thrombosis and fibrin found as clots and/or microclots in the small blood vessels of the lung and other organs have been found to be a major cause of death in COVID-19 patients.5


What are bleeding disorders?

When the clotting rate in a person’s blood is too low, the patient may experience excessive bleeding.

Bleeding disorders are often inherited but can also be acquired via different diseases and conditions. Main inherited dysfunctions and acquired causes for bleeding disorders are:


  • Hemophilia A
  • Hemophilia B
  • Von Willebrand disease
  • Platelet dysfunction


  • Liver disorders
  • Disseminated intravascular coagulopathy (DIC)
  • Anticoagulant therapy
  • Platelet dysfunction

Bleeding disorders can be treated with medications that replace missing proteins to close the gaps in the hemostasis reaction cascade.

Lab Tests

What are hemostasis lab tests used for?

Hemostasis lab tests can help your doctor to identify hereditary or acquired blood clotting disorders and the effectiveness of therapy used to manage them.

These tests are used to determine how long it takes for your blood to clot, identifying any delay or amplification of time that may cause an imbalance between clotting or fibrinolysis.

Are you talking to your doctor about blood health? If you are at risk for a thrombotic complication or suspect a bleeding disorder, ask your physician about lab testing to check your coagulation status so he or she can identify appropriate therapy or refer you to a specialist.

We hope you learned something about hemostasis testing today!


Stay safe and healthy!

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The information provided herein represents a shortened and simplified explanation of hemostasis, hemostatic disorders and treatment. For more details, please consult scientific textbooks or publications.

General references for hemostasis:

Blanco A, Blanco G. Hemostasis. In: Blanco A, Blanco G, editors. Medical biochemistry. Academic Press; 2017. p. 781-9.

Reyes Gil M. Overview of the coagulation system. In: Shaz BH, Hillyer CD, Reyes Gil M, editors. Transfusion medicine and hemostasis (third edition). Elsevier; 2019. P. 559-64.

1. Cui, et al._JTH. 2020;18:1421. doi.org/10.1111/jth.14830

2. Klok FA, et al.Thromb Res. 2020;191:145. doi.org/10.1111/jth.14830

3. Llitjos, et al._JTH. 2020;18:1743. doi.org/10.1111/jth.14869

4. Middeldorp, et al. JTH. 2020. doi: 10.1111/jth.14888

5. Wichmann D, et al. Ann Internal Med. 2020;M20-2003. doi: 10.7326/M20-2003