Hemostasis is the physiological process by which bleeding ceases. Hemostasis involves three basic steps: vascular spasm, the formation of a platelet plug, and coagulation, in which clotting factors promote the formation of a fibrin clot. Fibrinolysis is the process in which a clot is degraded in a healing vessel. Anticoagulants are substances that oppose coagulation. They are important in limiting the extent and duration of clotting. Inadequate clotting can result from too few platelets, or inadequate production of clotting factors, for instance, in the genetic disorder hemophilia. Excessive clotting, called thrombosis, can be caused by excessive numbers of platelets. A thrombus is a collection of fibrin, platelets, and erythrocytes that has accumulated along the lining of a blood vessel, whereas an embolus is a thrombus that has broken free from the vessel wall and is circulating in the bloodstream.
anticoagulant
substance such as heparin that opposes coagulation
antithrombin
anticoagulant that inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin in the common pathway
clotting factors
group of 12 identified substances active in coagulation
coagulation
formation of a blood clot; part of the process of hemostasis
common pathway
final coagulation pathway activated either by the intrinsic or the extrinsic pathway, and ending in the formation of a blood clot
embolus
thrombus that has broken free from the blood vessel wall and entered the circulation
extrinsic pathway
initial coagulation pathway that begins with tissue damage and results in the activation of the common pathway
fibrin
insoluble, filamentous protein that forms the structure of a blood clot
fibrinolysis
gradual degradation of a blood clot
hemophilia
genetic disorder characterized by inadequate synthesis of clotting factors
hemorrhage
excessive bleeding
hemostasis
physiological process by which bleeding ceases
heparin
short-acting anticoagulant stored in mast cells and released when tissues are injured, opposes prothrombin
intrinsic pathway
initial coagulation pathway that begins with vascular damage or contact with foreign substances, and results in the activation of the common pathway
plasmin
blood protein active in fibrinolysis
platelet plug
accumulation and adhesion of platelets at the site of blood vessel injury
serum
blood plasma that does not contain clotting factors
thrombin
enzyme essential for the final steps in formation of a fibrin clot
thrombosis
excessive clot formation
thrombus
aggregation of fibrin, platelets, and erythrocytes in an intact artery or vein
tissue factor
protein thromboplastin, which initiates the extrinsic pathway when released in response to tissue damage
vascular spasm
initial step in hemostasis, in which the smooth muscle in the walls of the ruptured or damaged blood vessel contracts
View these animations to explore the intrinsic, extrinsic, and common pathways that are involved the process of coagulation. The coagulation cascade restores hemostasis by activating coagulation factors in the presence of an injury. How does the endothelium of the blood vessel walls prevent the blood from coagulating as it flows through the blood vessels?
Clotting factors flow through the blood vessels in their inactive state. The endothelium does not have thrombogenic tissue factor to activate clotting factors.
1. The first step in hemostasis is ________.
A) vascular spasm
B) conversion of fibrinogen to fibrin
C) activation of the intrinsic pathway
D) activation of the common pathway
A
2. Prothrombin is converted to thrombin during the ________.
A) intrinsic pathway
B) extrinsic pathway
C) common pathway
D) formation of the platelet plug
C
3. Hemophilia is characterized by ________.
A) inadequate production of heparin
B) inadequate production of clotting factors
C) excessive production of fibrinogen
D) excessive production of platelets
B
1. A lab technician collects a blood sample in a glass tube. After about an hour, she harvests serum to continue her blood analysis. Explain what has happened during the hour that the sample was in the glass tube.
When blood contacts glass, the intrinsic coagulation pathway is initiated. This leads to the common pathway, and the blood clots. Within about 30 minutes, the clot begins to shrink. After an hour, it is about half its original size. Its heavier weight will cause it to fall to the bottom of the tube during centrifugation, allowing the lab technician to harvest the serum remaining at the top.
2. Explain why administration of a thrombolytic agent is a first intervention for someone who has suffered a thrombotic stroke.
In a thrombotic stroke, a blood vessel to the brain has been blocked by a thrombus, an aggregation of platelets and erythrocytes within a blood vessel. A thrombolytic agent is a medication that promotes the breakup of thrombi.