Blood is fluid when it flows through the arteries and veins. However it has to be able to clot in certain circumstances - for example to stop bleeding after a cut or graze. 

In some circumstances the blood can clot when it still inside a blood vessel. When blood does this, either within an artery or vein, it is called a Thrombosis.

A famous pathologist called Virchow once pointed out that there are 3 main causes for blood to clot when it is still in a blood vessel. These three factors are called - Virchow's Triad.

Changes in the vessel wall

Blood vessels are lined by a special sort of cell called an endothelial cell. This is a very flat sort of cell that has special properties, preventing normal blood clotting on it. Anything that damages the endothelial cell, can cause blood to clot on to it or to clot to the lining of the blood vessel underneath the endothelial cell. Things that damage the endothelial cell include smoking or low oxygen concentrations.

The vessel wall can also change by having scars on it - such as having previous deep vein thrombosis - or bulges and narrowings of the vein walls as in varicose veins.

Changes in the blood flow

The humans, like all animals, are really needs to be quite active. Unfortunately with modern day life, there are many instances where we are less active than they should be. These might be for reasons we are unable to avoid, such as illness or breaking a leg, but also our often due to our way of life such as sitting for long periods in front of a computer or television, travelling in cars, coaches or aeroplanes. By reducing the activity of the leg, the muscle pump is infused and so the blood flow becomes very sluggish in the deep veins.

Another cause for a change in the blood flow would be a change in the diameter or length of a vessel - such as found in varicose veins. Instead of the blood flowing smoothly up a narrow straight vessel, varicose veins with their bulges, narrowings, curves and kinks means the blood flow is altered and so clots are more likely to form.

Changes in the blood composition

The commonest cause of blood composition changes is dehydration. This often occurs because people drink alcohol or drinks with caffeine in them such as tea, coffee or some soft drinks. Unfortunately alcohol and caffeine act as diuretics, meaning that although fluid is being taken in, more is passed out in the form of urine. Therefore the blood becomes more concentrated and more likely to clot.

Females taking oestrogen either in the form of the oral contraceptive pill or as HRT, are also changing the blood composition in a way that makes thrombosis more likely. People with high blood fats (hyperlipidaemia) are also more likely to get clots due to an abnormal blood composition.