Blood clotting occurs when a blood vessel is damaged. It prevents exit of blood and entry of pathogens. It involves a cascade of reactions where the product of the first reaction is an enzyme for the second reaction and so on. It therefore is a very rapid process.

A temporary platelet plug first forms at the site of the damage. Screen Shot 2016-04-16 at 08.08.15.png

This initial temporary platelet plug is followed rapidly by the formation of a fibrin blood clot. This traps erythrocytes and plugs the cut in a more permanent fashion. A similar cascade reaction occurs in a coronary artery in response to damage to the inner endothelium layer and this brings about a heart attack.

An atheroma/plaque in an artery (the hardening of the plaque with calcium based substances gives rise to a condition called atherosclerosis - the "sclerosis"means "hardening").

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This animation goes animation goes through the process of how the plaque initially builds up and the subsequent clot formation. You do not have to know about foam cells or the details of how the plaque/atheroma develops.

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