Clot retraction and fibrinolysis

In hemostasis, hemo referring to blood, and stasis meaning to stop—so hemostasis is the process where blood flow is stopped after there’s damage to a blood vessel.Primary hemostasis involves the formation of a platelet plug at the site of an injured blood vessel, and secondary hemostasis involves the coagulation cascade which is where a protein net called a fibrin mesh forms over the platelet plug to reinforce it - forming a blood clot.Now, anticoagulation occurs during primary and secondary hemostasis and helps regulate clot formation, whereas clot retraction and fibrinolysis occur after primary and secondary hemostasis are complete, and help a clot contract and degrade.Anticoagulation prevents clots from growing too large and blocking blood flow to tissues supplied by the vessel.

It also prevents clots from getting so big that small parts of the growing clot break off in the form of emboli. Depending on the location of the primary blood clot, these emboli may then cause a disruption in blood flow to organs like the heart or brain.Now, the most important point of clot regulation is when a coagulation factor called thrombin is produced.

Thrombin, or factor II, is a very important clotting factor, because it has multiple pro-coagulative functions. Think of thrombin as the accelerator on a car--the pedal that takes secondary hemostasis from 20 miles per hour to 100 miles per hour!First, thrombin binds to receptors on platelets causing them to activate.

Activated platelets change their shape to form tentacle-like arms that allow them to stick to other platelets. Second, thrombin activates two cofactors; factor V used in the common pathway, and factor VIII used in the intrinsic pathway.Third, thrombin proteolytically cleaves fibrinogen or factor I, into fibrin or factor Ia which binds with other fibrin proteins to form a fibrin mesh.

And finally, thrombin proteolytically cleaves stabilizing factor or factor XIII into factor XIIIa.Factor XIIIa combines with a calcium ion cofactor to form cross links between the fibrin chains, further reinforcing the fibrin mesh.

Since thrombin has so many jobs, it makes sense that it is the main target of two proteins that help with anticoagulation- protein C and antithrombin III.Protein C is a circulating plasma protein produced in the liver along with a cofactor called protein S.

Now both protein C and S interact with a protein called thrombomodulin, which is on the surface of intact endothelial cells, which line our blood vessels.Now - let’s go back to an existing clot.

When there’s a lot of thrombin around a damaged blood vessel, excess thrombin binds to thrombomodulin and it can no longer participate in the coagulation cascade.So in a sense, the undamaged cells help ensure that the coagulation process is limited to the injury site.

Furthermore, the thrombin-thrombomodulin complex binds to and activates protein C and S. The whole thing forms a complex that includes protein C, protein S, and thrombin-thrombomodulin.

This protein complex proteolytically cleaves and inactivates active factor V, a cofactor for factor X in the common pathway, and VIII, a cofactor for factor IX in the intrinsic pathway.

By inhibiting both the intrinsic and common pathway, coagulation slows down dramatically.Now, a second anticoagulant is antithrombin III, sometimes just called antithrombin.

Antithrombin is a protein made by the liver and released into the blood, and it binds both thrombin and factor X, both of which are in the common pathway.

Excess thrombin can bind to antithrombin--similar to how it binds to thrombomodulin, and become unavailable.Antithrombin also binds to excess active factor X, which is a pivotal coagulation protein that converts prothrombin into thrombin.

Antithrombin also inhibits factors VII, IX, XI and XII--although with much less affinity. Antithrombin is also the target of an effective medication called heparin.Heparin binds to antithrombin and increases its affinity for its target proteins, thus increasing its anticoagulant effects.

So when individuals are given heparin the balance between coagulation and anticoagulation - tips in favor of anticoagulation.Now, there are also some factors that prevent platelets from adhering to one another or to endothelial cells during primary hemostasis.Healthy undamaged endothelial cells release nitric oxide and prostacyclin.

These tiny molecules make activated platelets produce less thromboxane A2 - which is a molecule that activates platelets secrete to activate surrounding platelets.Specifically, thromboxane A2 makes platelets express GPIIB/IIIA proteins on their membrane, and GPIIB/IIIA proteins bind to fibrinogen which ultimately makes platelets stick to one another.

Now, within an hour or so after primary and secondary hemostasis create a blood clot, platelets begin the process of clot retraction.Activated platelets have proteins within them called actin and myosin - the same proteins found in muscle cells.

These proteins are arranged in bundles called lamellipodia, and from the outside of the platelet they look like little fingers poking through the platelet membrane - a little creepy.Anyway, these lamellipodia are really important because they increase the surface area of the platelet and that makes it easier for them to bind to the fibrin mesh as well as the endothelial lining of the blood vessel.Now, platelets that are part of a clot typically have a surface protein called integrin αIIbβ3, and when these receptors bind to fibrin in the presence of thrombin, it activates actin and myosin.

Actin and myosin slide along one another and it causes the lamellipodia to contract and since the cell surface integrins are bound to fibrin - it pulls in on the fibrin mesh.The fibrin mesh tightens down in all of the platelets in the clot - making the whole thing more rigid and contracted down.

When the clot contracts, it brings the edges of the wound closer together. When there’s an open wound, the clot retraction pulls the two wound edges close together, and the endothelial cells can divide and replace the lost tissue.

If the gap is too wide, it can get filled in with collagen but that process takes longer and typically results in scarring.Now, approximately two days after an injury, the blood clot is no longer needed since substantial healing has taken place.

At that point, it’s time for the body to dissolve the blood clot through a process called fibrinolysis, which is the gradual degradation of the fibrin mesh.

To do this, a circulating protein produced by the liver called plasminogen, gets converted by an enzyme called tissue plasminogen activator or tPA into its active form called plasmin.Normally, healthy endothelial cells release only tiny amounts of tPA but when they’re exposed to coagulation factors, mainly factor X and thrombin-- they start making lots of tPA.

But it’s important that plasmin activity doesn’t get out of hand, either.So the endothelial cells also release plasminogen activator inhibitor 1 and antiplasmin which are proteins that bind and sequestering plasminogen and plasmin, respectively.

It’s all about always reaching that zen balance of coagulation and anticoagulation.Now, during an injury more and more coagulation factors get activated, and also more and more tPA gets released by the endothelial cells converting plasminogen it to plasmin.

Plasmin then acts as a protease-- literally cutting the fibrin into smaller pieces--allowing the trapped red blood cells and platelets to float away, letting the clot dissolve.Now - although tPA is the main activator of plasmin - a few other proteins activate plasmin as well, including coagulation factors IXa, XIIa, kallikrein, and protein C.

tPA is also used clinically - it’s sometimes called a “clot buster” because it’s used to dissolve blood clots that form in the lungs or in the brain.Alright, as a quick recap, anticoagulation with protein C and S and antithrombin III, prevents clots from getting too big during primary and secondary hemostasis.

Within an hour or so after primary and secondary hemostasis have created a blood clot, clot retraction begins to stabilize the clot and starts pulling together wound edges.

Finally, there’s the process of fibrinolysis which is dependent upon the tPA which converts plasminogen into plasmin - an enzyme that cleaves fibrin and helps dissolve clots.

begins to stabilize the clot and starts pulling together the wound edges Finally there's the process of fibrin lysis which is dependent upon the TPA which converts plasminogen into plasmin an enzyme that cleaves fibrin and helps to dissolve the