COM 5855 Hemostasis I

• The lecture covers Hemostasis I for COM 5855 on March 11, 2025, from 10:10 AM to 11:00 AM, taught by Assistant Professor Anna Potter, PhD.

Learning Objectives

• Describe platelets, their composition, and activation
• List the four phases of hemostasis
• Describe blood coagulation

Hemostasis

• Hemostasis stops bleeding and prevents blood from leaving damaged vessels.
• Vessel wall damage is repaired by hemostasis, where "hemo" means blood and "stasis" means stop.
• The hemostatic system balances procoagulant and anticoagulant forces
• Proper blood circulation requires the integrity of blood vessels
• Unchecked blood loss reduces blood volume, compromising organ/tissue perfusion.
• Stages of hemostasis include vasoconstriction, platelet plug formation, coagulation cascade, and fibrinolysis.
• Vasoconstriction narrows the blood vessel
• Platelets form a plug to seal damaged area during the platelet plug formation stage
• Coagulation cascade involves a series of reactions that activate clotting factors
• The clot is broken down during fibrinolysis
• Primary hemostasis forms a weak platelet plug, mediated by interactions between platelets and the vessel wall.
• Secondary hemostasis stabilizes the platelet plug via the coagulation cascade.
• Major components of the hemostatic system are platelets, other formed elements like monocytes and red blood cells, plasma proteins (coagulation and fibrinolytic factors and inhibitors), and the vessel wall.
• Platelets ensure no blood is lost at capillary/venule junctions under normal conditions.
• Failure to prevent blood leakage may result in exercise-induced vasculitis (hiker’s rash), particularly after prolonged hikes or strenuous exercise in hot weather.

Platelets

• Without hemostasis, even minor injuries would be life-threatening
• Hemostasis involves a balance between coagulant and anticoagulant forces.
• Procoagulant forces lead to platelet adhesion, aggregation, and fibrin clot formation
• Anticoagulant forces lead to natural inhibitors of coagulation and fibrinolysis
• Hemostasis promotes blood flow under normal conditions
• Platelets, also known as thrombocytes, lack nuclei
• Platelets are granular cells derived from megakaryocytes
• They have a lifespan of 7-10 days.
• They circulate freely when inactive by molecules (nitric oxide, prostacyclin) secreted by endothelial cells lining blood vessels
• Platelets are essential for clotting when blood vessels are ruptured or the lining is injured
• They form a temporary plug at the damaged site
• They are primary cells responsible for hemostasis
• They contribute to hemostatic capacity via adhesion, activation, and aggregation, triggered by tissue injury
• Platelets stimulate coagulation factors and mediators to achieve hemostasis
• Platelets are the smallest blood component (2-4 μm)
• Normal platelet counts are between 150,000 and 400,000 per microliter of blood
• Up to one-third of circulating platelets are sequestered within the spleen at any time
• Platelets have a biconvex discoid shape, maintained by a circumferential bundle of microtubules.
• Platelets have two tubule systems: dense tubular & surface-opening canalicular systems
• The Open canalicular system provides space for platelet products to enter/exit.
• Glycogen granules provide energy
• Alpha granules contain fibrinogen (Fib), thrombospodin, Factor V, von Willebrand factor (vWF), beta-thromboglobuline (β-TG), & Factor IV.
• Dense granules store non-metabolic ATP, ADP, serotonin, and calcium, preventing cell stickiness and, when activated, release their granules.
• Lambda granules are lysosomes
• Platelets lack nuclei or other organelles besides mitochondria.
• Thrombopoietin, a hormone, regulates platelet formation
• Hematopoietic stem cells produce megakaryocytes
• The plasma membrane of megakaryocytes fragments, releasing platelets

Phases of Hemostasis

• Is fast, localized, and carefully controlled
• Involves clotting factors in plasma and substances released by platelets and injured tissue cells
• Rapid sequence includes vascular spasm, platelet plug formation, and blood clotting
• Clot retracts and dissolves, later replaced by fibrous tissue for permanent prevention of blood loss
• Vasoconstriction (vascular spasms) reduces blood loss for 20-30 minutes.
• Platelet plug (primary hemostasis) forms a framework for the clot and seals the vessel wall to minimize blood loss, measure clinically as "bleeding time."
• Coagulation (secondary hemostasis) occurs when circulating coagulation factors undergo a cascade of reactions to generate fibrin, which stabilizes platelets in the clot until damage repair.
• Fibrinolysis restores function by dismantling the fibrin clot

Blood Coagulation

• Vasoconstriction occurs when endothelial injury happens, causing smooth muscles to contract and reducing blood flow
• In the absence of injury, endothelial cells release nitric oxide (NO) and prostaglandins for vasodilation; with injury, they decrease production of vasodilators
• Intact endothelial cells release nitric oxide and prostacyclin (PGI2) to prevent platelets from sticking together or to blood vessels
• Underlying collagen fibers are exposed in response to injury
• Adhesion occurs when platelets adhere to collagen fibers
• Activation occurs when platelets swell, forming spiked processes
• Aggregation involves a positive feedback loop
• Activated platelets produce ADP, a potent aggregating agent and Serotonin and thromboxane A2 to enhance vascular spasm and platelet aggregation
• Damaged endothelial cells expose collagen at the injury site
• von Willebrand’s factor (vWF) released from endothelial cells binds to collagen
• Forming a protein bridge, linking platelets to exposed collagen
• Platelets can bind directly to collagen via α2β1 integrins
• Platelet receptor glycoprotein 1b (Gp1b) binds to vWF
• Glycoprotein 1b also binds to other platelet receptors and directly to subendothelial collagen
• Cytoskeletal alterations make the platelet "flat" and lead to form a podia "tube feet".
• When platelets bind to von Willebrand's factor (vWF) they get activated
• Platelets change to spiky structures with filopodia; morphological change increases platelet surface area
• Thrombin, ADP, and thromboxane A2 is secreted to promote platelet recruitment and activation.
• Calcium is secreted, this is useful in secondary hemostasis
• Bioactive contents are released when the release reaction occurs
• Alpha granules contain fibrinogen, von Willebrand factor, factors V, XI, & XIII
• Delta (dense) granules contain Ca2+, ATP, serotonin, histamine, & adrenaline
• Platelets synthesize thromboxane A2 (TXA2), a vasoconstrictor
• Activated platelets are expressing ADP and TXA2, they up-regulate (change conformation of) another surface receptor, the GPIIB/IIIA
• Platelets stimulate wound healing via secretion of platelet-derived growth factor to stimulate vascular smooth muscle cells and fibroblasts
• This promotes cell division and growth
• ADP and Thromboxane A2 both cause platelets to bind collagen
• GPIIB/IIIA receptors bind fibrinogen and clump platelets into the plug
• As more platelets get activated, they aggregate further, creating a positive cycle for more platelet plug formation
• The platelet plug is sufficient to seal wear and tear
• Larger vessel breaks need secondary hemostasis
• Secondary hemostasis reinforces the platelet plug with fibrin threads, which act as a "molecular glue" for aggregated platelets
• The procoagulants normally circulate in blood in inactive form
• Clotting factors activate fibrin, or Factor IA
• This forms a fibrin mesh
• Platelets are protected from inappropriate primary hemostasis when undamaged cells secrete prostaglandins and nitric oxide, preventing their activation and maintaining proper tissue perfusion
• Factor X activation leads to a common coagulation cascade
• Bleeding disorders involving hemostasis includes hemophilia, that can cause nosebleeds, bleeding gums, or wounds that are hard to heal
• Symptoms of hemophilia vary by severity
• Bleeding disorder treatments include blood thinners, surgery, or clot-dissolving medications.
• When damage to endothelial cells takes place, there’s local up-regulation of a ligand, von Willebrand’s factor, from the type-4 sub-endothelial collagen in the basement membrane
• Exposed collagen binds circulating von Willebrand’s factor (vWF) which then binds Glycoprotein 1b receptors, Gp-1b, on platelets
• Platelets also have Gp-2b/IIIa receptors (activated by ADP), which interact with fibrinogen (TXA2), allowing platelet aggregation and formation of a temporary hemostatic plug
• Fibrinogen is in precursor form (not yet cleaved to fibrin).
• For the next stage of coagulation, the enzyme thrombin cleaves fibrinogen into fibrin, forming a more stable platelet plug
• Clotting factors activate fibrin, or Factor IA and forms a fibrin mesh
• When Factor X gets activated, it proceeds to activate a common coagulation cascade
• Antiplatelet drugs prevent blood clots by making blood less sticky and are prescribed after heart attacks or strokes to prevent stent blockages
• Aspirin was the first antiplatelet drug which is a cyclooxygenase inhibitor
• Clopidogrel (Plavix), Prasugrel (Effient), Ticagrelor (Brilinta) are all are all ADP receptor inhibitors that makes platelets less sticky
• Dipyridamole is an adenosine reuptake inhibitor that blocks enzymes involved in clotting
• Cilostazol is a phosphodiesterase inhibitor that widens blood vessels and stops platelets from sticking together