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The process of atherogenesis is initiated by disturbances in blood flow at so-called ‘predilection sites’, which lead to changes in the endothelial lining of the vascular system.1 These changes cause low-density lipoproteins (LDLs) to accumulate in the intima (the surface layer of the artery exposed to the blood), where they become modified by oxidation and aggregation.1
Increasing retention of LDL triggers the recruitment of inflammatory macrophages, which ingest the lipids to become foam cells.1,2 As the lesion progresses, these lipid-rich cells in the core of the plaque begin to die, releasing lipids and forming the necrotic core of the fibroatheroma.2 Meanwhile, smooth muscle cells migrate from the artery wall into the intima, where they proliferate to form a fibrous cap that covers the surface of the plaque.2
Arterial thrombosis explained.
Video explaining the formation of arterial thromboses
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The composition of an atherosclerotic plaque changes as it develops. Initially, the plaque consists of an accumulation of low-density lipoproteins (LDLs).1 Eventually, inflammatory macrophages enter the intima (the surface layer of the artery exposed to the blood) and, after ingesting the LDL, become foam cells – so-called because they are filled with cholesteryl ester droplets, giving them a foamy appearance.1-3
The next major change in plaque morphology takes place once the lipid-rich cells begin to die and release their lipid contents into the now necrotic core.2 Simultaneously, a fibrous cap of smooth muscle cells begins to form. Over time, infiltrating macrophages and the release of active proteases can cause thinning of the cap and make rupture more likely.2
Formation of an arterial blood clot.
How an arterial blood clot forms
Over time, loss of smooth muscle cells along with infiltrating macrophages and the release of active proteases can lead to thinning of a plaque’s fibrous cap, leading to vulnerable plaques that are at high short-term risk of rupture.1 The archetypal vulnerable plaque is characterized by a thin fibrous cap and an abundance of macrophage-derived foam cells beneath it.2 Eventually, the cap can no longer withstand the mechanical force of the blood pressure and the plaque may rupture.4
The majority of ischaemic events occur when a vulnerable plaque ruptures or is eroded, releasing procoagulant material into the bloodstream and leading to the formation of a thrombus – a process known as atherothrombosis.2 Alternatively, ischaemic events may be provoked by plaque erosion, a process that is poorly understood but results in the loss of the surface endothelium.1
A vulnerable plaque with a thin fibrous cap.
Structure of a vulnerable arterial plaque
When a vulnerable plaque ruptures, the contents of the plaque become exposed to the blood with important clinical consequences:5
The activation of these two pathways concurrently allows for the generation of a thrombus with both fibrin- and platelet-rich parts.5
The process of thrombus formation
Activation of the coagulation cascade and the platelet pathway results in thrombus formation
Consequences of atherothrombosis.
Plaque rupture and superimposed thrombosis can result in stroke, myocardial infarction or acute limb ischaemia
Arterial and venous thrombosis differ in terms of the triggers involved and thrombus composition.6
Arterial thrombosis develops as the result of plaque rupture and endothelial disruption (for more information, please see the Understanding arterial thrombosis section).6 However, for venous thrombosis, the endothelium remains intact and thrombus formation is a result of abnormal blood flow, increased coagulability and altered vessel walls.6
Because of the different mechanisms of formation, arterial and venous clots have different compositions. Arterial clots are rich in platelets, which are rapidly recruited to the site of vascular injury.6 However, as a clot begins to reduce the flow of blood, a fibrin-rich component can form.5 Venous clots, on the other hand, are always rich in fibrin, which holds together trapped red blood cells.5
Formation of venous thromboses
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Formation of arterial thromboses
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Venous thrombosis is different from arterial thrombosis. More information on venous thrombosis can be found at ThrombosisAdviser.com