Platelets

Platelet facts

removed by phagocytes, mainly in the spleen

150k - 500k /mL whole blood

1/3 are stored in vascular organs like spleen - used during circulator crisis

platelet functions

release clotting chemicals

temporary patch in wall (platelet plug)

reduce size of break (actin + myosin fibers constrict to reduce clot)

What occurs in Thrombocytopoiesis

platelet production in red bone marrow

Megakaryocytes - giant cells of bone marrow that shed membrane-enclosed packets of cytoplasm (the platelets themselves)

Hormones that control platelet production

Thrombopoietin - peptide hormone fr. kidney; speeds platelet formation + stims megakaryocyte production

Interleukin 6 - stims platelet formation

Multi CSF - stims platelet production by promoting megakaryocyte formation and growth

what is hemostasis

stoppage of bleeding

vascular phase

platelet phase

coagulation phase

What occurs in the vascular phase

Vascular spasm - contraction to decrease vessel diameter 30 min

endothelial cells contract + expose basement membrane to bloodstream

release local hormones and chemical factors

What do endothelial cells release during vascular phase

ADP, tissue factor, + prostacyclin

Endothelins - promotes vascular spasm + cell division

Platelet aggregation does what

contributes to the platelet plug

What do activated platelets release when they arrive to the injury site

ADP - stims aggregation

Thromboxane A2 + serotonin - stims spasm

Clotting factors

Calcium Ions - required for aggregation

Platelet Derived Growth Factor - promotes vessel repair

Factors that limit the platelet plug

Prostacyclin inhibits aggregation

Inhibitory compounds released by WBCs

Circulating enzymes break down ADP

- feedback fr increasing serotonin

blood clot isolates area from general circulation

Components of coagulation pahse

depends on clotting factors (coagulants) - Ca2+ and 11 other proteins

Proenzymes (inactive enzymes) - converted into actives that direct reaction in clotting

What happens in the extrinsic pathway

damaged cells release Factor III (tissue factor) → combine with Factor VII and Ca2+ → enzyme complex to activate Factor X

What happens in the intrinsic pathway

begins with activation of proenzymes (usually XII) exposed to collagen fibers @ inj site

aggregating platelets release PF3 for assistance

Factors VIII + IX combine to activate Factor X

The common pathway

begins with activation of Factor X from either pathway

activates Prothrombin activator → converts prothrombin to thrombin → converts fibrinogen to fibrin - produces clot

Feedback ctrl of clotting

Anticoagulants - enzymes that inhibit clot (antithrombin III)

Heparin - blood thinner, speeds activation of antithrombin III

Thrombomodulin - binds to thrombin + converts an enzyme to activate protein C - inactivates clot factors + stims plasmin formation

Prostacyclin - inhibits plate aggregation + opposes clot factors

Why are calcium and Vit K important

calcium is required by all 3 pathways

vitamin K is required for synthesis of clotting factors in the liver

Thrombocytopenia

Hemophilia

Thrombophilia

DVT

Deficiency in platelets - causes frequent bleeding + slow clot

Clotting disorder - lacking of clot factors

Clot too easily

Clot in deep vein

What is clot retraction

pulls torn edges of vessel together - reduces bleeding + stabilizes injury site

reduces size of damaged area - easier for cells to repair

What is fibrinolysis

gradual clot dissolve

thrombin + tissue plasminogen activator (tPA) activate plasminogen → make plasmin - digests fibrin strands