primary hemostasis

what causes primary hemostasis to happen? what is the outcome?

cause: damage to vessel wall

outcome: formation of a platelet plug

what happens when there is damage to the vessel wall?

smooth muscle contracts

• vasoconstriction to minimize blood loss

injury exposes tissue factor (TF)

• starts clotting cascade (extrinsic pathway)

collagen exposed

• surface for platelet adhesion

what is tissue factor (TF)?

primary initiator of coagulation (extrinsic pathway)

where is tissue factor found?

stored in subendothelial tissue and fibroblasts

• localized predominantly in the t. media and adventitia of blood vessels and a smaller quantity as circulating TF on monocytes

• higher concentration in vital organs (heart, brain, lungs, uterus, testis, placenta)

when does tissue factor become active?

when exposed to blood via disruption of the vessel wall

what activates tissue factor?

physical injury (activation of vessel wall TF)

functional injury (inflammation activates circulating TF)

what are conditions that cause abnormal activation of TF even when the vessel is physically intact?

hypoxia

sepsis

malignancy

what happens if you have a lack of TF?

can lead to excessive bleeding

where is von Willebrand Factor (vWF) found?

synthesized in endothelial cells and megakaryocytes

• majority is continuously circulating in plasma and stored in platelets

• some stored in endothelium

when is vWF released?

injury to the blood vessel

what is vWF function?

join platelets together and promote production of clotting factor VII

after vascular injury, vWF acts as a bridge between endothelial collagen and platelet surface receptors to promote platelet adhesion

where are platelets (thrombocytes) produced?

bone marrow

smallest element formed in blood

what is thrombopoietin? where is it produced?

major regulator of platelet production

produced by liver and kidneys

how do platelets get made?

kidneys send the message to the bone marrow to make more platelets

are there more platelets in circulation or stored in organs?

75% in circulation and 25% stored in liver and spleen

what is thrombocytopenia?

low level of platelets

what is the functions of platelets?

transform to self-adhere in the presence of vWF

form mechanical plugs at site of vascular injury

as they change shape → release more clotting factors

what state do platelets spend most time in?

resting state (free floating)

• will not adhere to intact endothelium

• become active in the presence of exposed collagen and vWF

what are the 3 processes that lead to active state of platelets?

adhesion: platelet to vessel wall

aggregation: platelet to platelet

degranulation products: constriction and coagulation

• when they change shape and form a clot

what are the functions of platelet degranulation products?

trigger immediate vasoconstriction

promote aggregation → platelets call more platelets

promotes platelet adhesion via fibrin network

what elements are required for clotting cascade?

calcium and fibrinogen