General information


Most of us are aware of our heart and know that it pumps blood around our bodies, keeping us alive. The continuous and smooth flow of blood around our body is essential; formation of a blood clot in the wrong place—known medically as thrombosis—can cause interruption of normal blood flow and result in illness, which in some cases can be life threatening.

A blood clot within the veins or arteries, which are the vessels that form our ‘cardiovascular system’, causes a number of common and well-known cardiovascular illnesses. Blood clots forming in veins can cause deep vein thrombosis (called DVT for short). A ‘pulmonary embolism’ (PE for short) results from a blood clot in the arterial blood supply to the lungs. Blood clots in arteries supplying blood to the heart cause a heart attack and clots in arteries supplying blood to the brain cause a stroke.

In this section, we will explore the biology of blood clotting and examine how our bodies form blood clots and why thrombosis occurs.

This information is not intended as a substitute for consultation with your healthcare professional.

Normal blood clotting

We all know that if we cut our skin, a clot forms that stops blood loss from the injury. This process is known as ‘haemostasis’ and is the body’s normal response to injury. Clotting is the first step in the process, in which blood changes from a liquid to a gel. This is a complicated process that involves firstly the activation, adhesion, and aggregation of platelets and secondly the formation of fibrin. Fibrin is a fibrous protein that is formed through the interaction of a large number of clotting factors in a process known as the ‘coagulation cascade’.

The figure shows platelets and fibrin combining to form a clot and seal a wound.

The third component of this clotting system is a process known as ‘fibrinolysis’, which is the breakdown and removal of the clot once the vessel has repaired. Another factor that contributes to ensuring excessive blood clot or ‘thrombus’ formation does not occur is the normal functioning of the internal lining of blood vessels (endothelium). Blood clots tend not to form unnecessarily as long as blood flows smoothly over normal functioning endothelium.

This whole process exists in a delicate balance to ensure excessive bleeding or excessive clotting does not occur. An imbalance in the blood clotting system can result in excessive bleeding (haemorrhage) or the formation of ‘pathological’ blood clots (thrombosis).


Thrombosis is the formation of a blood clot or ‘thrombus’ inside a blood vessel that significantly reduces or stops the flow of blood to or from the heart.

There are three broad categories of factors that disturb the normal clotting process and contribute to thrombosis. These are:

  • Hypercoagulability—a rare tendency for blood to clot more easily than usual, caused by changes to the contents of the blood; for example, some people with cancer experience changes in the levels of proteins in their blood that are part of the clotting process
  • Changes in blood flow—alterations to normal blood flow, such as venous stasis in which blood flows slowly in the legs, for example after an operation when people are immobile
  • Endothelial dysfunction—injury to the lining of blood vessels, for example caused by surgery, or abnormal endothelial changes, caused by some metabolic illnesses such as diabetes

Doctors often refer to these categories as ‘Virchow’s triad’ after the doctor who first described them more than 100 years ago. These principles are still used today by physicians diagnosing and treating patients with thrombosis.

The figure shows an older lady bed-bound because of an acute medical illness; these factors—advanced age, acute illness, and immobility—can disturb the normal clotting process and contribute to an increased risk of thrombosis.

Venous thrombosis

Venous thrombosis is a blood clot that develops in a vein. Veins are the blood vessels that carry blood from tissues back to the heart. Blood is not under high pressure in veins as it is in arteries, and therefore veins have valves and require muscular movement to move blood through them to prevent it flowing back in the opposite direction. It is the valves of veins that are the common site of thrombus initiation.

Because of the way blood flows through a vein, the area at the base of the valve has reduced blood flow. If the body’s normal blood clotting system is out of balance, platelets can begin to attach to the endothelium, triggering fibrin formation, and a clot begins to form.

Figure: step 1 shows a small clot forming at the base of a valve. Step 2 shows the clot growing in size and step 3 shows a part of the clot breaking off. This detached clot—called an ‘embolus’—can then travel further up the body. The formation of a thrombus that becomes embolic is called ‘thromboembolism’.

The most common form of venous thrombosis is a DVT, which is when a clot forms in the deep veins of the leg. If an embolus breaks off and moves up the leg, through the heart, and lodges in a lung artery, a person suffers a PE. These related blood clot problems—DVT and PE—are known as ‘venous thromboembolism’ (VTE for short).

Learn more about VTE on the next pages. This information is not intended as a substitute for consultation with your healthcare professional.