Chapter 11 The Blood 333 Copyright Goodheart-Willcox Co., Inc. As the blood clot retracts, platelets pull the fibrin threads together. This, in turn, draws together the edges of the tear in the vessel. Meanwhile, the factors that started the hemostasis sequence are rapidly inactivated to prevent excessive clotting. In addition, coagulation inhibitors are released to prevent further clot formation. Eventually, other chemicals cut the fibrin strands and dissolve the blood clot completely. The blood clot is dissolved by a process called fibrinolysis, which occurs when fibrin is broken down by enzymatic action of plasmin. Plasmin is formed from its inactive form, plasminogen, which is normally found in circulating blood. Plasminogen is activated when tissue plasminogen activator (TPA) is formed by the injured tissue. 2. Platelet aggregation. Platelets stick to the collagen fibers and to the rough edges of the blood vessel wall (platelet adhesion). The platelets act like a spackling compound to repair the hole or tear. In addition, the platelets release chemicals that maintain constriction of the blood vessel and attract more platelets to the damaged wall. 3. Platelet plug formation and coagulation. This gathering of platelets forms a small, loose mass called a platelet plug at the site of the injury. To initiate coagulation, the injured blood vessel releases a chemical called tissue factor. Tissue factor activates eleven different clotting factors, or proteins, in the blood. These activated clotting factors produce prothrombin activator (PTA). In the presence of calcium and the platelet chemicals, prothrombin is activated to form thrombin. Thrombin activates the protein fibrinogen. The combination of thrombin and fibrinogen produces fibrin, a long, sticky, threadlike fiber. The fibrin strands weave in and around the platelet plug, forming a strong, tightly woven fibrin mesh (Figure 11.10). The process can be compared to throwing a fishing net around the platelet plug. 4. Blood clot formation and retraction. Red and white blood cells become trapped in the fibrin mesh, giving the blood clot a red color. When coagulation is complete—after about 2 to 15 minutes—the blood clot has formed. Shortly after the fibrin mesh is in place, the blood clot begins to retract, or shrink in size. This process normally takes 30 to 60 minutes. Susumu Nishinaga/Science Source Figure 11.10 Scanning electron microscopic photo of a blood clot. Human Umbilical Cord Blood Recent research shows that human umbilical cord blood (HUCB) contains hematopoietic cells, mesenchymal cells, and endothelial cells, which were previously thought to exist only in bone marrow. This relatively new discovery has important clinical applications for the treatment of hematologic and metabolic diseases, immune defi ciencies, and autoimmune disorders, as well as the creation of regenerative medical treatments. Collecting HUCB involves a relatively quick and simple process that does not pose any danger to the baby or mother. One drawback to HUCB is that there is only a small quantity of blood to be harvested, but this is alleviated by invitro expansion procedures, which allow the generation of blood cells in a petri dish. Clinical Application