Hemostasis and Fibrinolytic System Quiz

Platelets, Hemostasis, and Immunity

• Platelets are small, granulated, non-nucleated bodies, 2-4 µm in diameter.
• Normal platelet count: 300,000 /µL (mm³) (range 200,000 - 500,000).
• Platelets remain functional for an average of 10 days.
• Megakaryocytes in bone marrow create platelets by pinching off bits of cytoplasm.
• Platelets are crucial for hemostasis, which is the process stopping bleeding.
• Hemostasis occurs via vasoconstriction to minimize blood loss, forming a platelet plug (adhesion and aggregation), blood clotting (activating clotting factors to form fibrin threads), and releasing growth factors to heal the vessel wall.

Intended Learning Outcomes

• Students will understand the importance of platelets and their role in hemostasis.
• Students will be able to describe the mechanisms of hemostasis.
• Students will be able to describe mechanisms that prevent blood clots.
• Knowledge of the general functions of white blood cells will be outlined.

Platelet Plug Formation

• Blood vessel injury exposes underlying collagen.
• Platelets adhere to collagen (platelet adhesion).
• Platelet binding to collagen causes platelet activation.
• Activated platelets become sticky and start to aggregate (platelet aggregation).
• The plug physically seals the break in the vessel, preventing blood loss.

Fibrin Clot Formation

• Blood changes to a gel (called a clot or thrombus) around the platelet plug.
• A cascade of chemical reactions (involving many clotting factors) occurs in response to vessel rupture.

Clotting Factors

• Clotting factors are enzymes that work sequentially, activating each other.
• The result is the formation of complexes known as prothrombin activators. These catalyze the conversion of prothrombin into thrombin.
• Thrombin converts fibrinogen to fibrin.

Extrinsic and Intrinsic Pathways

• Two pathways lead to fibrin formation: extrinsic and intrinsic.
• The extrinsic pathway is initiated by tissue factor.
• The intrinsic pathway initiates from contact with exposed collagen.
• Both pathways converge at the common pathway.

Functions of Thrombin

• Thrombin converts fibrinogen to fibrin.
• Positive feedback loop for thrombin formation, stimulating factors VIII and V.
• Stimulates platelet aggregation.
• Activates factor XIII to stabilize fibrin meshwork.
• Activates plasminogen to plasmin for clot breakdown.

Vitamin K

• Fat-soluble vitamin, requires bile for small intestine absorption.
• Intestinal bacteria synthesize vitamin K naturally.
• Important for liver production of prothrombin, factors VII, IX, and X.
• Deficiency leads to bleeding tendencies.

Natural Anti-Clotting Mechanisms

• Rapid blood flow removes activated clotting factors and inactivates them in the liver.
• Intact endothelium prevents contact activation of the intrinsic clotting system and platelets.
• Glycocalyx layer on endothelium repels clotting factors and platelets.

Anticoagulant Proteins

• Protein C and protein S inactivate factors VIII and V.
• Elevated plasmin formation.
• Thrombomodulin binds thrombin to prevent clot formation.
• Antithrombin III inactivates factors IX, X, XI, and XII (enhanced by heparin).

The Fibrinolytic System

• Fibrin clot is temporary (until vessel repair).
• The fibrinolytic system breaks down clots.
• Restricts clotting to a limited area, preventing excessive intravascular coagulation.
• Plasminogen, activated by tissue plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), converts to plasmin.
• Plasmin digests fibrin into fibrin degradation products or fibrin split products.

White Blood Cells

• Mobile units of the body's defense system.
• Humoral response: secrete molecules to eliminate invaders.
• Cellular response: engulf and destroy invaders and cancer cells.
• Clean up debris from injured cells.

Types of White Blood Cells

• Granulocytes (e.g., neutrophils, eosinophils, basophils): contain granules
• Agranulocytes (e.g., monocytes, lymphocytes): lack visible granules

Test your knowledge on hemostasis and the fibrinolytic system with this quiz. Explore the roles of various components, such as the glycocalyx layer and anticoagulant proteins. Understand the conversion of plasminogen to plasmin and the functions of antithrombin III.