Hemostasis and clotting

Cut your finger and it bleeds for a minute or two, then stops. Within that time, three overlapping processes have closed the leak: the vessel narrowed, platelets plugged the hole, and a mesh of fibrin set around them. This page explains hemostasis step by step, with the coagulation cascade simplified into the cell-based model your body actually uses and the intrinsic, extrinsic and common pathways that lab tests and exams still use. It then covers how clots shrink and dissolve, how anticoagulants work, and what happens when clotting fails or goes too far.

Hemostasis: three overlapping steps

Hemostasis (hemo- = blood, -stasis = standing still) is the stopping of bleeding from a damaged vessel. It is not the same as clotting: clotting is only its third step. The three steps start within seconds of each other and overlap (Figure 1):

1. Vascular spasm: the damaged vessel constricts, which slows the flow through it.
2. Platelet plug: platelets stick to the damaged wall and to each other, forming a soft plug.
3. Coagulation: clotting proteins in the plasma build a mesh of fibrin threads through and around the plug, turning it into a firm clot.

For a small cut, the first two steps can be enough. For anything larger, only the fibrin mesh holds against the pressure of the blood.

Vascular spasm

When a vessel is cut or crushed, the smooth muscle in its wall contracts. This vascular spasm is a local vasoconstriction, and it has three causes:

• Direct injury to the smooth muscle makes it contract.
• Chemicals released at the site: platelets release thromboxane A2 and serotonin, and damaged lining cells release their own constricting signals.
• Local nerve reflexes triggered by pain fibers in the vessel wall.

A narrower vessel carries less blood, so less leaks out, and slower flow lets platelets and clotting proteins build up at the site instead of being washed away. The spasm lasts up to about half an hour. In a small artery it can nearly close the vessel; this is part of why a cleanly cut artery may bleed more than a crushed one, whose muscle constricts harder.

The platelet plug

Inside an intact vessel, platelets drift past the smooth lining without sticking. The lining cells release nitric oxide and prostacyclin, which keep platelets quiet, and they carry an enzyme on their surface that destroys ADP. A tear exposes what lies under the lining, above all collagen fibers. The plug then forms in three stages:

1. Platelet adhesion. A large plasma protein, von Willebrand factor (vWF, named for the Finnish physician Erik von Willebrand), sticks to exposed collagen. Platelets grab vWF with a receptor protein on their surface, which catches them even in fast-flowing arterial blood. Other receptor proteins then bind collagen directly.
2. Platelet activation. Binding sets off signals inside the platelet. It swells, grows spiky projections and releases its granules: ADP, serotonin and calcium, among others. It also makes thromboxane A2, a chemical that spreads to nearby platelets.
3. Aggregation. ADP and thromboxane A2 activate passing platelets, which switch on a receptor protein that binds fibrinogen. One fibrinogen molecule can bind two platelets, so fibrinogen bridges platelet to platelet and the pile grows.

This is positive feedback: activated platelets release chemicals that activate more platelets, which release more chemicals. The plug stays local because the healthy lining on either side keeps releasing nitric oxide and prostacyclin and destroying ADP.

A platelet plug is soft. It can seal a pinprick in a capillary but is easily washed away from a larger hole. Its other role is to give coagulation a surface to work on.

Coagulation

Coagulation (Latin coagulare = to curdle) is the conversion of liquid blood into a gel. Its end point is simple: the enzyme thrombin cuts soluble fibrinogen into fibrin, which links up into long, insoluble threads. Everything else in coagulation is about making a large burst of thrombin, in the right place, at the right time.

Clotting factors

The clotting factors are the plasma proteins and ions that coagulation needs. Most are made by the liver. Many circulate as inactive enzymes. When one is activated, it cuts the next one in line and activates it: a cascade (the clotting cascade). Because each activated enzyme activates many molecules of the next, the signal is amplified at every step, just as you saw with second messengers.

The factors are numbered with Roman numerals in the order they were discovered, not the order in which they act. An activated factor gets an "a": factor X becomes Xa. The ones to know:

There is no factor VI: it turned out to be activated factor V. Four of the factors, II, VII, IX and X, need vitamin K; that comes up below.

How clotting happens in the body: the cell-based model

Clotting in the body happens on cell surfaces: first on the cells that carry tissue factor, then on activated platelets. Researchers describe it in three overlapping phases (Figure 2).

1. Initiation, on a tissue factor cell

Tissue factor sits on the membranes of fibroblasts and smooth muscle cells in the vessel wall, which blood never touches while the lining is intact. A tear brings plasma into contact with it. A small amount of factor VII circulates already active (VIIa), and it binds tissue factor. The pair activates factor X and factor IX. Factor Xa, with its cofactor Va, turns a little prothrombin into a little thrombin. This first trickle of thrombin is too small to make a clot.

2. Amplification, on platelets

The trickle of thrombin reaches the platelets already stuck in the plug. Thrombin is a strong platelet activator, and it also activates factors V, VIII and XI on the platelet surface. Activated platelets flip a negatively charged phospholipid to the outer face of their membrane, where calcium ions anchor the clotting factors.

3. Propagation, on the activated platelet surface

On the platelet surface, IXa pairs with VIIIa and activates X at a high rate. Xa pairs with Va and converts prothrombin to thrombin, now in a large burst. The burst of thrombin cuts fibrinogen to fibrin. Fibrin threads stick to each other side to side and end to end, and factor XIIIa, activated by thrombin, cross-links them. The mesh traps red blood cells and more platelets: a red, firm clot.

Why this explains hemophilia

Within seconds of starting, the tissue factor–VIIa pair is switched off by an inhibitor protein in plasma. From then on, factor X on the platelet surface is activated mainly by IXa with VIIIa. A person who lacks factor VIII or IX can start clotting on the tissue factor cell, but cannot make the burst on the platelet surface, and so bleeds. Any model of clotting has to explain that, and the cell-based model does.

Positive feedback, and what limits it

Thrombin makes more thrombin. It activates platelets, whose surface hosts the burst, and it activates factors V, VIII and XI, which speed its own production. This is one of the body's few positive feedback loops: the response, more thrombin, strengthens the stimulus that produced it. The loop runs until the hole is sealed.

What keeps a clot from spreading through the whole circulation is that everything around the clot works against it:

• Flow carries activated factors away from the injury, diluting them.
• Healthy lining cells release nitric oxide and prostacyclin, destroy ADP and carry a protein, thrombomodulin, that binds thrombin. Thrombin bound to it stops cutting fibrinogen and instead activates protein C, which destroys factors Va and VIIIa.
• Antithrombin, a plasma protein made by the liver, inactivates thrombin and factor Xa. Heparin-like molecules on the lining speed it up.

So clotting is fast where tissue factor and activated platelets are, and stopped where there is healthy lining.

The lab model: intrinsic, extrinsic and common pathways

Before the cell-based model, clotting was described as two routes that meet (Figure 3). This picture came from test tubes, and it is still how clotting tests are read:

• The extrinsic pathway (extrinsic = from outside: it needs tissue factor, from outside the blood): tissue factor and factor VIIa activate factor X.
• The intrinsic pathway (intrinsic = within: everything it needs is in the blood): contact with a negatively charged surface, such as glass, activates factor XII; XIIa activates XI; XIa activates IX; IXa, with VIIIa, activates X.
• The common pathway: Xa, with Va and calcium, converts prothrombin to thrombin; thrombin converts fibrinogen to fibrin; XIIIa cross-links the fibrin.

The two clotting tests

• Prothrombin time (PT): tissue factor and calcium are added to a plasma sample, and the time to clot is measured, normally about 11–13.5 seconds. It tests the extrinsic and common pathways. Because labs use different reagents, the result is also given as the INR (international normalized ratio), which is about 1.0 in a healthy person.
• Activated partial thromboplastin time (aPTT): a contact activator, phospholipid and calcium are added, normally clotting in about 25–35 seconds. It tests the intrinsic and common pathways.

The plasma for both tests is collected in a tube containing citrate, which binds calcium and so stops the sample from clotting until calcium is added back.

Where the lab model breaks down

People who lack factor XII have a very long aPTT, yet they do not bleed. In the body, factor XII is not needed to stop bleeding. And people with hemophilia lack a factor of the intrinsic pathway, yet their extrinsic pathway, which the lab model says should be enough, cannot stop their bleeding. The cell-based model explains both: tissue factor starts clotting in the body, and the IXa–VIIIa pair on platelets does the heavy lifting.

Vitamin K

Vitamin K (K for the German Koagulation) is a vitamin, one that dissolves in fat, that the liver needs to finish making factors II, VII, IX and X, and the anticoagulant protein C. The liver uses it to add extra acid groups to these proteins; the extra groups bind calcium, which lets the factors attach to phospholipid surfaces. Without vitamin K, the liver still makes the proteins, but they cannot bind calcium and do not work.

• Sources: leafy green vegetables, some oils, and bacteria in the intestine, although how much the bacteria contribute in humans is uncertain.
• Newborns have very little: little vitamin K reaches the fetus, breast milk is low in it, and the newborn gut is only beginning to gain bacteria. A single vitamin K injection at birth prevents a rare but serious bleeding disease of newborns.
• Adults run short with poor diet, poor fat absorption, or long courses of antibiotics.

Liver disease causes bleeding for a related reason: a failing liver cannot make enough clotting factors, with or without vitamin K.

Clot retraction and fibrinolysis

Clot retraction

Within 30–60 minutes, a clot starts to shrink. Platelets caught in the mesh use their actin and myosin to pull on the fibrin threads they are attached to. The clot tightens, squeezes out serum, and pulls the torn edges of the vessel closer together. You saw the same process in a clotted blood tube: the fluid squeezed out is serum. Platelets also release growth factors that make smooth muscle cells and fibroblasts divide and repair the wall.

Fibrinolysis

Once the wall is repaired, the clot is removed. Fibrinolysis (fibrin + -lysis = loosening, breaking down) is the breakdown of fibrin:

1. Plasminogen, an inactive plasma protein made by the liver, binds fibrin and is built into the clot as it forms.
2. Endothelial cells slowly release tissue plasminogen activator (tPA), which also binds fibrin.
3. On the fibrin, tPA converts plasminogen into the enzyme plasmin.
4. Plasmin cuts fibrin into fragments, and the clot dissolves over days.

Because both tPA and plasminogen bind fibrin, plasmin is made on the clot and not throughout the blood. One of the fragments, D-dimer, is measured in blood: a normal D-dimer makes an active clot in the deep veins unlikely. Manufactured tPA is given as a clot-dissolving drug in some strokes and heart attacks.

Anticoagulants

Anticoagulants (anti- = against) are substances that slow or stop coagulation. Your body makes its own: antithrombin, protein C, the tissue factor inhibitor, and heparin-like molecules on the vessel lining that speed antithrombin. Heparin itself is stored in the granules of mast cells and basophils, where it helps pack their contents. It is not normally found in plasma. The heparin drug is extracted from animal tissue. Several drugs work by boosting or copying the body's anticoagulants.

Neither drug dissolves a clot that has already formed. They stop it from growing and stop new clots forming, while fibrinolysis clears the old one. Newer tablets block thrombin or factor Xa directly.

Aspirin works differently: it is an antiplatelet drug. It permanently blocks the platelet enzyme cyclooxygenase, which makes thromboxane A2, so platelets activate less. A platelet has no nucleus and can make almost no new enzyme, so one dose affects each platelet for the rest of its 7–10 day life. That is why aspirin is often stopped a week before surgery.

Clotting disorders

Too little clotting

• Thrombocytopenia (-penia = shortage): a platelet count below about 150,000 per µL. Bleeding after injury becomes a risk below about 50,000, and bleeding without injury below about 10,000–20,000. Causes include marrow failure, leukemia, chemotherapy, an enlarged spleen, and antibodies that destroy platelets.
• Hemophilia (-philia = tendency): an inherited lack of a clotting factor, factor VIII in hemophilia A and factor IX in hemophilia B. The genes for both lie on the X chromosome. Males have one X, so one faulty copy causes the disease; females have two, so they are usually carriers without symptoms. People with hemophilia bleed into joints and muscles, often after minor knocks. It is treated with replacement factor, other drugs that restore thrombin generation, and now gene therapy.
• Von Willebrand disease: too little or faulty vWF, the most common inherited bleeding disorder. Platelets adhere poorly, and factor VIII, which vWF carries and protects in plasma, is low too.
• Vitamin K deficiency and liver disease: too little working II, VII, IX and X.

The pattern of bleeding points to the problem. Platelet problems cause bleeding from small vessels in the skin and mucous membranes: petechiae, bruising, nosebleeds, bleeding gums, heavy periods. Clotting factor problems cause deep bleeding into joints and muscles, and bleeding that starts again hours after an injury, because the platelet plug forms but no fibrin reinforces it.

Too much clotting

• A thrombus (Greek = clot) is a clot that forms inside an unbroken vessel and stays attached to its wall. In an artery it can block flow to the tissue beyond, as in most heart attacks and many strokes. Thrombosis is the forming of a thrombus.
• An embolus (Greek embolos = plug) is a mass carried along in the blood until it lodges in a vessel too small to pass: most often a piece of thrombus, but it can be fat, air or other material. The blockage it causes is an embolism. A thrombus in a deep leg vein (deep vein thrombosis) can break off, travel through the right side of the heart and lodge in the lungs as a pulmonary embolism.
• Thrombocytosis: a platelet count above about 450,000 per µL. Most cases are reactive, following infection, inflammation or iron deficiency, and carry little risk; a marrow disorder that makes too many platelets raises the risk of both thrombosis and bleeding.

Three conditions make a thrombus likely, often called Virchow's triad: slow or stagnant flow (long immobility, a long flight, after surgery), damage to the vessel lining, and blood that clots more readily than normal (some inherited factor variants, cancer, pregnancy, estrogen-containing drugs).