Cut yourself, and a cascade of proteins in your blood activates within seconds, forming a clot that plugs the wound and stops you from bleeding out — one of the body's most reliably fast-acting protective systems. That same clotting cascade, triggered in the wrong place at the wrong time, is also the direct mechanism behind some of the most common causes of death worldwide: heart attacks, strokes, and pulmonary embolisms are all, at their core, an inappropriately formed blood clot.
A precisely triggered chain reaction, meant to stay local
Normal clotting (haemostasis) begins when blood vessel damage exposes underlying tissue that platelets recognise and stick to, forming an initial plug, while a cascading sequence of clotting factor proteins activates each other in a tightly regulated chain reaction, ultimately converting a soluble protein called fibrinogen into fibrin — an insoluble mesh that reinforces the platelet plug into a stable clot. The entire system is built to be fast, localised, and self-limiting: it activates specifically at the site of vessel damage, and the body maintains a set of counterbalancing anticoagulant mechanisms that keep the clot confined to where it's actually needed and eventually break it down once the vessel has healed.
Thrombosis: when the same system activates where it shouldn't
Thrombosis is the formation of a clot inside a blood vessel where there's no injury actually requiring one — often triggered by damage to a vessel's inner lining (frequently from atherosclerosis, a build-up of fatty plaque), by abnormally slow or turbulent blood flow, or by an imbalance that tips the blood toward excessive clotting. A clot that forms in a coronary artery supplying the heart can block blood flow to heart muscle, causing a heart attack. A clot that forms in or travels to an artery supplying the brain causes a stroke. A clot that forms in a deep vein, often in the leg, can break loose and travel to the lungs, causing a pulmonary embolism. In every case, it's the identical fibrin-forming machinery that seals a cut on your finger — just switched on in a vessel that didn't need sealing, with the clot itself becoming the thing doing the damage rather than preventing it.
What we're still unsure about
The core clotting cascade and the major risk factors for pathological thrombosis are well characterised, and this isn't disputed medical science. What remains genuinely difficult is prediction at the individual level: two people with apparently similar risk factors — similar age, similar cholesterol levels, similar lifestyle — can have very different actual clotting risk, and medicine still can't fully explain why a dangerous clot forms in one person's artery on one particular day and not another's under seemingly comparable conditions. Balancing anticoagulant medication's benefit (preventing dangerous clots) against its cost (increasing the risk of dangerous bleeding, since the same mechanism runs in both directions) remains one of the genuinely difficult, individualised judgment calls in clinical medicine, without a single formula that gets it right for everyone.
This sits inside Haemodynamic Disorders & Thrombosis, one of seven topics in Pathology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.