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What Is Hemostasis & Thrombosis?

Hemostasis is clotting, a part of the healing process, and it can be exaggerated, which leads to thrombosis, or impaired, leading to bleeding/hemorrhage.

The hemostatic process is an interaction among three different phases:

  • Injury to the inner (endothelial) lining of a blood vessel
  • Initiation of the clotting process via the overlap of two different complex biochemical reactions; the clotting cascade
  • Clotting cessation and anti-thrombosis with removal of the clot

In thrombosis–abnormal clot occurrence–hemostasis has the disadvantage of reacting or overreacting to endothelial injury. When clot formation occurs, it can lead to two major developments:Thrombosis: The clot can grow to a size that causes obstruction of blood flow through the vessel.

  • Thromboembolic disease (embolism): The clot can break off and be pushed by the force of blood flow until it lodges in a distal vessel


Thrombosis can occur in either veins or arteries, but the more vulnerable are the veins, since they are a low-pressure conduit of blood flow.

Alterations in blood flow, endothelial injury, and hypercoagulable changes in the clotting process provoke thrombus formation in veins. This is a mass effect, and if large enough will either partially or completely obstruct venous blood flow leading to venous stasis, which alters the flow even more. Venous stasis can lead to venous ulcers and when the clots get infected, thrombophlebitis (inflammatory veins). Thrombophlebitis is very painful and can affect walking, but its biggest risk is if a thrombus breaks off and travels as an embolus.

In the arteries, thrombosis is associated with plaque formation which results from hypertension, smoking, obesity, and  dyslipidemia (abnormal cholesterol and triglyceride levels). Obstruction threatens the arterial blood flow to organs, their perfusion, and their oxygenation, which jeopardizes their tissue survival. An example is in the coronary arteries, where obstruction to blood flow can lead to cardiac cell death (ischemia and infarction).

Embolic Disease

When a clot partially or completely breaks away from its thrombotic bed, it can migrate with blood flow, in the veins, upward toward the right side of the heart; in the arteries, distally to the end-organs perfused by the respective artery.

In veins, all blood flow is directed toward the right atrium of the heart, which by way of the right ventricle enters the pulmonary artery with deoxygenated blood going to the lungs to exchange its carbon dioxide for oxygen. A traveling clot can enter the pulmonary artery and result in lung impairment distal to it as well as back up pressure to the right ventricle. Dyspnea can be severe enough to cause death, the biggest fear with pulmonary emboli.

In arteries, whatever tissue is perfused with a particular artery will be deprived of oxygenated blood and suffer hypoxia, risking tissue and organ death. If this were to involve the mesenteric artery, the intestines could die; if in the carotid arteries, stroke ensues; if in the coronary arteries, a heart attack can occur. Any thrombus in the arterial circulation can put one at risk, even thrombi (plural) in the left atrium that break off during atrial fibrillation.


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This information is provided by Vascular Health Clinics and is not intended to replace the medical advice of your doctor or healthcare provider. Please consult your healthcare provider for advice about a specific medical condition.

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