Understanding Thrombosis

Definition

Thrombosis is the formation of a thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system

A thrombus is a structureless mass of red blood cells, platelets, leukocytes and fibrin.

Thrombosis results from an imbalance between antithrombotic and prothrombotic activities in which prothrombotic activities predominate.

Classically, thrombosis is caused by abnormalities in one or more of the following (Virchow’s triad):

The composition of the blood: hypercoagulability
The quality of the vessel wall: vessel wall injury
The nature of the blood flow: blood stasis

Blood clot versus thrombus

A blood clot is the final product of the blood coagulation step in haemostasis
A thrombus is a blood clot in a blood

There are two types of thrombus

White thrombus:

Composed mainly of platelets and fibrin
Formed in vessels with a rapid blood flow
Typically observed in arterial thrombosis

Red thrombus:

Composed mainly of red cells and fibrin
Formed in vessels with a slow blood flow
Typically observed in venous thrombosis

Arterial thrombosis

Arterial thrombosis corresponds to blockage of an artery by a thrombus, usually following rupture of an atherosclerotic plaque and leads to infarction (i.e. necrosis) of the tissues supplied by the artery.

Coronary artery blockage: acute myocardial infarction
Cerebral artery blockage: stroke
Leg artery blockage: peripheral arterial disease

Venous thrombosis

Venous thrombosis corresponds to formation of a thrombus in a vein. There are two types of venous thrombosis:

Superficial vein thrombosis or superficial thrombophlebitis: thrombosis in a superfi cial vein, i.e. near the surface of the
skin and visible to the eye
Deep-vein thrombosis: thrombosis in a vein that runs deep within muscle tissue

Venous thrombosis may have severe consequences: embolisation

Embolisation occurs when part of the thrombus breaks off and is transported by the blood flow further downstream to other tissues where it can lodge and block the blood supply, leading to tissue damage.

A transported thrombus fragment is called an embolus. Transport of an embolus into the lungs, via the right side of the heart, results in pulmonary embolism, potentially leading to the infarction (i.e. necrosis) of lung tissues. Embolisation is most typically the consequence of deepvein thrombosis.

Formation of venous thrombosis

1. Stasis

Venous thrombi mostly form in regions of slow or disturbed blood flow, typically the large sinuses and valve cusp pockets in the deep veins of the calf.