Ch 11.4A -- Platelets and Hemostasis -- VTPP 435

Ch 11.4A -- Platelets and Hemostasis -- VTPP 435

Platelets (Thrombocytes)

-shed from outer edges of megakaryocytes
-function for about 10 days then removed by macrophages of spleen and liver

Megakaryocytes

-large bone-marrow bound cell that sheds off blood-borne platelets from its outer edges
-one makes about 1000 platelets
-derived from stem cells of RBCs and WBCs

Thrombopoietin

-hormone that increases number of megakaryocytes in bone marrow
-stimulates each megakaryocytes to produce more platelets
-made by liver

Platelets Structure

-no nuclei
-organelles and cytosolic enzyme systems to make energy and secretory products
-have high concentration of myosin and actin

Hemostasis

-the stopping of bleeding from an injured vessel
-more serious if in an artery than in an vein

3 Major Steps of Hemostasis

1) Vascular Spasms
2) Formation of a Platelet Plug
3) Blood Coagulation (clotting)

Vascular Spasm

-constriction due to torn blood vessel
-slows blood flow thru the defect
-minimizes blood loss
-endothelial surfaces become sticky and adhere to each other

Von Willebrand Factor (vWF)

-a plasma protein
-secreted by megakaryocytes, platelets, and endothelial cells
-adheres to exposed collagen
-prevents platelets from being swept into circulation to form a platelets plug at defect site

Collagen

-activates the bound platelets
-helps platelets activate and reorganize

Adenosine Diphosphate (ADP)

-makes nearby platelets in circulation to adhere to platelets aggregated layer and activate them
-new platelets release ADP to build a plug in positive feedback

Thromboxane A2

-an eicosanoid
-reinforces platelet aggregation and triggers release of more ADP from platelet granules

Prostacyclin and Nitric Oxide

-stimulated by ADP discharge
-inhibit platelet aggregation
released from endothelium

3 Other Functions of Platelet Plug

1) Actin-Myosin complex contract to compact and strengthen the loose plug
2) Platelet plug releases vasoconstrictors to reinforce vascular spasm
3) Releases other chemicals that enhance blood coagulation

Blood Coagulation or Clotting

-transformation of blood form a liquid into a solid gel
-if blood solidifies can no longer flow

Fibrinogen

-is converted into Fibrin (ultimate step of clot formation)
-produced by liver
-always present in plasma

Fibrin

-insoluble, threadlike molecule
-highly elastic
-sticks to damaged surface to make a meshwork to trap blood cells

Thrombin

-enzyme that catalyzes fibrinogen to fibrin conversion at site of injury

Clot

-mass of activated fibrin threadlike molecules that form a net like meshwork that traps blood cells
-transforms blood in the vicinity from a flowing liquid into a nonflowing gel
-appears due to fibrin trap
-can take place in absence of all blood cells except platelets

Factor XIII (Fibrin-stabilizing factor)

-catalyzes cross-linkage of fibrin to strengthen and stabilize meshwork
-normally inactive in plasma
-a clotting factor

Thrombin other roles

-activates Factor XIII
-acts in a + feedback for its own formation
-enhances platelet aggregation

Prothrombin

-inactive precursor of thrombin
-converted into thrombin by clotting cascade when blood clotting needed

Factor X

-an activated plasma clotting factor
-converts prothrombin to thrombin
-usually inactive and needs to be activated by another factor

Clotting Cascade

-sequence of clotting factors activating each other until thrombin activates fibrinogen
-12 plasma clotting factors to start conversion of fibrinogen to fibrin