Chapter 5a. Comprehensive Hemostasis and Platelet Function in Blood Clotting

What is the definition of hemostasis?

The process by which the body spontaneously stops bleeding and maintains blood in a fluid state within the vascular compartment.

What are the five key components that regulate hemostasis?

Blood vessels, platelets, coagulation proteins, fibrinolysis, and serine protease inhibitors.

What clinical condition results from an imbalance in hemostatic factors?

Hemorrhage or thrombosis.

What is the primary function of primary hemostasis?

To form a temporary platelet plug at the site of vascular injury through platelet adhesion to exposed collagen.

What is the primary function of secondary hemostasis?

To enzymatically activate coagulation proteins to produce fibrin, stabilizing the platelet plug.

What is the process of breaking down and removing a clot after healing?

Fibrinolysis.

Which enzyme is responsible for the proteolytic digestion of fibrin and fibrinogen?

Plasmin.

What is the role of platelet-derived growth factor (PDGF)?

It promotes vascular healing and repair.

What type of bleeding is suggestive of qualitative or quantitative platelet disorders?

Mucocutaneous hemorrhage (e.g., ecchymoses, epistaxis, petechiae, gastrointestinal/genitourinary bleeding).

Bleeding into joint cavities is characteristic of what type of condition?

Factor deficiencies or warfarin (Coumadin) toxicity.

What are the three layers of a blood vessel?

Tunica adventitia, tunica media, and tunica intima.

Which vessel layer is responsible for vasoconstriction and vasodilation?

Tunica media.

Why is the endothelial surface of a blood vessel considered 'nonwettable'?

Its physical and biochemical characteristics render the surface thromboresistant.

Name two substances secreted by endothelial cells to maintain thromboresistance.

Prostacyclin (PGI2) and tissue plasminogen activator (t-PA).

What is the function of thrombomodulin?

It acts as a cofactor in the thrombin-dependent activation of protein C.

What is the principle mechanism of platelet adhesion?

Interaction between von Willebrand's factor (vWF), collagen fibers, and platelet membrane glycoprotein Ib (GPIb).

What substance is secreted by platelets to promote further vasoconstriction?

Thromboxane A2 (TXA2) and serotonin.

What is required for the initiation of the extrinsic pathway of coagulation?

Tissue factor, factor VII/VIIa, calcium, and a negatively charged phospholipid membrane.

What is the origin of platelets?

Bone marrow megakaryocytes.

What is the normal platelet count range in humans?

150,000 to 400,000 per microliter (µL).

What is the average lifespan of a platelet in circulation?

7 to 10 days.

Where are approximately 30% of platelets sequestered?

In the microvasculature of the spleen.

What platelet count is generally associated with spontaneous hemorrhage?

Less than 10,000 per microliter (µL).

What platelet count is typically sufficient to minimize hemorrhage during surgery?

Greater than 50,000 per microliter (µL).

What is the role of activated platelets in coagulation?

They provide a negatively charged phospholipid surface that enhances the activation of coagulation proteins.

What happens to blood flow when a vessel is injured and undergoes vasoconstriction?

Blood flow is diverted around the damaged vessel, enhancing contact activation of platelets and coagulation factors.

What is the primary function of the hemostatic platelet plug?

Primary hemostasis, involving vasoconstriction, platelet aggregation, and coagulation.

Which glycoprotein receptor is essential for platelet adhesion to collagen via vWF?

GPIb

Which glycoprotein receptor is required for platelet-to-platelet interaction via fibrinogen?

GPIIb/IIIa

What are the three distinct zones of a platelet?

Peripheral zone, sol-gel zone, and organelle zone.

What is the function of the open canalicular system (OCS)?

It serves as a conduit for releasing granule contents to the exterior and increases surface absorptive area.

What structure maintains the discoid shape of a platelet?

The circumferential band of microtubules.

What are the primary contractile proteins found in the platelet sol-gel zone?

Actin and myosin.

What is the role of platelet factor 3 (PF3) on the platelet membrane?

It moves to the outer surface to serve as a cofactor for the assembly of vitamin K-dependent coagulation factors.

How long after vascular injury does platelet adhesion typically occur?

1 to 2 seconds.

What triggers the shape change in platelets during aggregation?

The release of ADP and ATP from intracellular granules.

Which two substances are identified as potent platelet aggregators?

ADP and TXA2.

What is the function of the dense tubular system (DTS)?

It is the site of prostaglandin and thromboxane synthesis and sequesters calcium.

What triggers platelet contraction and internal activation?

The release of calcium from the dense tubular system (DTS).

What are the three types of storage granules found in platelets?

Dense granules, alpha granules, and lysosomes.

Which granules are the most numerous in platelets?

Alpha granules.

What is the function of Platelet-derived growth factor (PDGF)?

It promotes smooth muscle growth to aid in vessel wall repair.

What substance in dense granules promotes vasoconstriction?

Serotonin.

What is the primary role of fibrinogen stored in alpha granules?

Fibrin formation.

What is the function of Beta-thromboglobulin (β-TG)?

It inhibits heparin and acts as a chemotactic agent.

What is the role of von Willebrand factor (vWF) in platelets?

It promotes platelet adhesion.

What is the function of thrombospondin?

It promotes platelet-to-platelet interaction and mediates cell-to-cell interactions.

What is the role of plasminogen in platelets?

It is a precursor to plasmin and functions in fibrinolysis.

What is the primary energy source for platelet metabolism?

Glycogen.

How many mitochondria are typically present per platelet?

10 to 60.

What is the function of the circumferential band of microtubules during activation?

It contracts toward the center to facilitate organelle movement and the secretory process.

What is the role of divalent cations like calcium or magnesium in aggregation?

They are required for fibrinogen binding to the GPIIb/IIIa receptor.