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Hemostasis
Heme= blood stasis= to halt Process of retaining blood within the
vascular system Repairs injury to blood vessels Stops or prevents blood loss
Balance of Hemostasis
*Balance of bleeding (hemorrhaging) and clotting (thrombosis)
*Imbalance in one direction can lead to:
bleeding : hypocoagulable state OR
thrombosis: hypercoagulable state
Procoagulant Factors
Regulatory Factors
Fibrin
Hemostasis Components
Vascular System Controls rate of blood flow
Platelet System Interaction of vasculature and platelets form a temporary plug
Coagulation System Forms a stable insoluble plug (i.e) fibrin forming
Fibrinolytic System Fibrin lysing
Coagulation Inhibition System Natural inhibitors Control fibrin formation and fibrin lysis
Hemostasis
Failure or deficiencies in any of the five systems involved with hemostasis can leads to varying degrees of uncontrolled hemorrhaging or clotting
Hemostasis
The hemostatic components remain inert in the presence of intact vascular tissue or endothelium
Following injury, each component must function optimally.
Hemostasis: OverviewConsists of three stages
Primary Hemostasis Process of blood clotting in response to injury or desquamation
of dying /damaged endothelial cells Blood vessels (vasculature) and platelets are the main “players.” Primary Hemostatic plug is formed Platelet plug temporarily arrests bleeding. Insoluble fibrin
strands deposit on the initial plug to reinforce and stabilize. The fibrin originates from soluble plasma proteins.
Secondary Hemostasis Actions of the protein coagulation factors form fibrin in response
to injury Fibrin is stabilized by Factor XIII At this time, blood has changed into a solid state
Fibrinolysis Clot is removed following healing of wound
Vascular System
Components Arteries/ Arterioles
Carry blood from the heart to capillaries Thickest walls of the vasculature
Veins/Venules Return blood from capillaries to the heart Thinnest walls of vasculature
Capillaries No vessel wall Do not contribute to hemostasis
Vascular System:Anatomy of the Blood Vessels Structure
Endothelium Single layer of endothelial cells, lining vessels Coated by glycocalyx (protein and
mucopolysaccarides) Protects basement membrane Negatively charged, repels circulating proteins and
platelets Secretes substances to keep the blood vessel in a
nonreactive environment
Vascular System: Anatomy of the Blood Vessels Structure con’t
Subendothelium Smooth muscle and connective tissue with
collagen fibers Basement membrane
Collagen– stimulates platelets Tissue Factor (TF)- activates coagulation & fibrin
formation Connective tissue
Elastic fibers- provide support around vessels
Vascular System: Blood Vessels Daily Function
Endothelium Controls vessel permeability Controls blood flow rate Produces and releases substances that
inhibit OR stimulate platelets, coagulation and fibrinolysis
Hemostatic Trigger
Once vessel damage occurs, action begins!Arteries and arterioles vasoconstrictSmooth muscle cells contract to reduce blood
flowThe endothelium becomes thrombogenic
Platelets and coagulation proteins are activated VWF is secreted Fibrinolysis initiated
How Does the Endothelium Become Thrombogenic?
Actions Von Willebrand’s Factor is made and released to
assist the platelets in primary hemostasis Produce tissue factor needed for secondary
hemostasis Collagen is exposed which secretes platelet
activating factor which in turn activates platelets Subendothelium promote the binding of leukocytes Plaminogen activator inhibitor is released to inhibit
fibrinolysis
Vascular System: Function Following Injury Initiate hemostasis: FIRST RESPONSE
Vasoconstriction of the arterioles Minimizes blood flow to injured area Prevents blood loss Delivers platelets and plasma proteins to the
vessel wall Immediate Short-lived
Vasoconstriction
MechanismNeurogenic factorsRegulatory substances
Prolong vasoconstriction Serotonin ( product of platelet activation &
endothelium) Thromboxane A2 ( product of platelet activation &
endothelium) Endothelin-1 (product of damaged endothelial
cells)
Roles of the Healthy Endothelium
Prostaglandin (PGI2)/ Prostacyclin and Nitric Oxide Vasodilates to increase blood flow to bring
fresh supplies of clotting substances Inhibits platelet aggregation & recruitment Causes redness at the injury site
Additional Roles
Contraction of venulesCauses gaps between them which
pushes fluids causing edema or swelling
Inflammatory response