When a vessel is injured, the body seals it in steps: the vessel narrows, platelets plug the hole and a fibrin mesh makes the plug solid. Once the vessel heals, the clot is slowly dissolved.
Platelets
Platelets are small cell fragments made in the bone marrow that live about 7 to 10 days. At an injury they stick to the exposed vessel wall, clump together and release signals that call in more platelets.
Clotting factors
Clotting factors are proteins, most made in the liver, that activate one another in a cascade. The end product is thrombin, which turns fibrinogen into fibrin strands that hold the clot together. Several factors need vitamin K, which is why warfarin, a vitamin K blocker, slows clotting.
Breaking clots down
Plasmin slowly dissolves fibrin once healing begins. D-dimer is a fragment released in this process, so a high level means a clot has formed and is being broken down somewhere.
When it goes wrong
Too few platelets or missing factors cause bleeding, as in liver disease, hemophilia or anticoagulant use. Too much clotting causes deep vein thrombosis, pulmonary embolism, heart attack and stroke.
| Test | What it checks | Normal |
|---|---|---|
| Platelet count | Number of platelets | 150,000 to 400,000 per microliter |
| PT and INR | Extrinsic pathway, warfarin effect | INR about 1.0 (2 to 3 on warfarin for most uses) |
| aPTT | Intrinsic pathway, heparin effect | About 25 to 35 seconds |
At the bedside
Before an injection, IV start or procedure, check the platelet count and any anticoagulant use. Hold pressure longer on every puncture in a patient with a bleeding risk.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).