The Lab / Core medicine / Lesson 12
A torn blood vessel seals itself in three moves: it squeezes, plugs the hole, then knits the plug into a tough patch. Coagulation means clotting; the cascade is the chain reaction that builds the clot. Build it one picture at a time, then see where blood thinners and clot-busting drugs step in.
Educational, not for clinical use
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Blood flows through tubes called vessels. It must stay liquid to keep flowing, yet stop leaking the moment a vessel tears. Hemostasis is the body’s way of sealing that leak. It has three steps, built up here one at a time.
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Muscle in the vessel wall clamps down around the tear. This vascular spasm narrows the tube so less blood escapes, buying time. It is set off by chemicals from the damaged lining and by pain sensors.
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Platelets are tiny fragments of cells, much smaller than red blood cells, carried in the blood in huge numbers. Normally they slide past the smooth vessel lining without sticking to anything.
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A tear exposes collagen, tough fibres under the smooth lining. Platelets that touch it turn spiky and sticky and cling on, helped by von Willebrand factor, a blood protein that acts like double-sided tape between platelet and collagen.
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Stuck platelets release chemicals that call in more platelets, which pile on. Together they form a platelet plug: a soft, temporary seal over a small tear, like the wooden bung a ship’s crew hammers into a leak.
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A plug of platelets alone is soft. Clotting knits it into a tough patch. Blood carries fibrinogen, a dissolved protein. Turned into fibrin, long sticky threads, it weaves a net through the plug. Something has to flip that switch.
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Clotting factors are proteins, most made by the liver, that float in blood switched off. One switched on turns on the next, and that one the next: a cascade, like water dropping down a waterfall. Each step turns on more.
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Damaged tissue outside the vessel releases tissue factor, starting the fast extrinsic pathway. Contact inside the vessel starts the slower intrinsic pathway. Both switch on factor ten, the first step of a shared common pathway.
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Switched-on factor ten, written Xa (say “ten-a”), works with calcium on the platelets’ surface to turn prothrombin, an idle blood protein, into thrombin: an enzyme, a protein that speeds up a chemical reaction.
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Thrombin turns fibrinogen into fibrin threads. They weave through the platelet plug and trap red blood cells, and factor thirteen ties the threads together into a firm net. Platelets then pull the threads tight, like lacing a shoe.
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Antithrombin is a blood protein that acts as a brake. It switches off factor ten and holds back the making of thrombin, so the clot stays at the wound instead of spreading along the vessel.
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As the wall heals, the clot must go. Plasminogen, an idle blood protein, is switched on into plasmin, an enzyme that gradually breaks down the fibrin threads. This slow clean-up is called fibrinolysis.
Build the idea
Squeeze, plug, net. Platelets plug the tear; the cascade switches on factor ten, then thrombin, which spins fibrin through the plug. Two things to watch: how many platelets there are, the platelet count, and how much thrombin gets made.
What changes it
Heparins are anticoagulants, drugs that oppose clotting. The body makes a natural heparin too, found on the vessel lining. Heparin opposes prothrombin, so less of it is turned into thrombin, and with less thrombin the fibrin net thins.
What changes it
The liver needs vitamin K to finish several clotting factors. Vitamin K antagonists such as warfarin block that last step, so the liver releases factors that cannot work. With fewer working switches, less thrombin and less fibrin are made.
What changes it
Factor Xa inhibitors act on switched-on factor ten. The shared path stalls at its first step, so less thrombin is generated and the fibrin net stays thin.
What changes it
Direct thrombin inhibitors act on thrombin itself. Factor ten still switches on, but thrombin’s work is held back, so fibrinogen is not turned into fibrin and the net barely forms. The platelet plug is left without its net.
What changes it
Antiplatelet drugs, such as aspirin, stop platelets clumping together. Fewer pile on, so the plug is smaller and looser, and the cascade has less platelet surface to work on, so a little less thrombin is made.
What changes it
Thrombolytics are clot-busting drugs. One, tissue plasminogen activator, is an enzyme the vessel lining releases naturally; given as a drug, it turns plasminogen into plasmin, which speeds up the breakdown of an unwanted clot blocking a vessel.
Start with Next, or use the arrow keys. Pictures are illustrative, not measurements. For how blood flow is driven, see Lesson 4.
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Normal teaching baseline.
The blue line on the bar is an illustrative teaching model, not a patient measurement. The shaded band is the range quoted from the source under the row; the bar’s end points are an illustrative choice. Arrows come from the quoted source for the selected state.
An illustrative teaching model of the wound and the cascade. Change how many platelets there are, then add drugs one at a time or together. The faint dashed shapes and ticks are the normal clot.
Everything is relative to a normal clot, which scores 1 on every part. The platelet plug grows with the platelet count but levels off (count × 1.5 ÷ (count + 0.5)), and antiplatelet drugs shrink it. Factor ten’s activity falls when vitamin K antagonists leave fewer working factors and when a factor Xa inhibitor jams it. Thrombin follows factor ten, scaled by how much platelet surface the plug offers, and falls further with heparins (which oppose prothrombin) and direct thrombin inhibitors. The fibrin net follows thrombin, and thrombolytics then cut most of it away.
The sizes of every effect are illustrative choices; only their directions are meant to teach. Left out: vascular spasm’s timing, factor thirteen, the protein C system and tissue factor pathway inhibitor (two more natural brakes), time, and every drug amount.