4.2 Haemostasis and its abnormalities Flashcards
(29 cards)
FORMATION OF HAEMOSTATIC PLUGS
The formation of haemostatic plugs is central to the haemostatic process, which is a response to injury to the endothelial cell lining (through vasoconstriction)
• Primary haemostasis: formation of _____________ (by platelet adhesion and aggregation)
• Secondary haemostasis: stabilisation of plug with _______ (via blood coagulation)
• Fibrinolysis: __________ and physiological dissolution of clot (resolution of injury)
Unstable platelet plug;
fibrin;
vessel repair
INITIAL RESPONSE TO VESSEL INJURY
When injury to the endothelial cell lining is detected, vasoconstriction occurs as vascular smooth muscle cells contract locally, decreasing vessel radius and limiting blood flow to injured vessel
• Local contractile response to injury mainly important in the _______________ (sufficient to prevent blood loss)
• Smooth muscle cells underlie the single layer of endothelial cells in the ________________________
o Endothelial cell monolayer has anticoagulant properties via ___________ (on endothelial cell surface) and _______________ (secreted by endothelial cells) prevent coagulation process
▪ Important barrier preventing contact with subendothelium, which has procoagulant properties
o Subendothelial layers (includes basement membrane) include collagen (platelet response), tissue factors (on smooth muscle cells and fibroblast surfaces)
small blood vessels;
subendothelial space (tunica media);
thrombomodulin;
tissue factor pathway inhibitor
PRIMARY HAEMOSTASIS
The formation of an unstable platelet plug involves platelet adhesion and platelet aggregation to limit blood loss and provide surface for coagulation
• Platelets (thrombocytes) are derived from _______________ in the bone marrow (from haematopoietic stem cells)
o Megakaryocytes have multi-lobular nuclei, and ____________ cytoplasm (development during differentiation and maturation)
o Fragmentation of cytoplasm occurs, from which platelets are formed (no nucleus)
megakaryocytes;
granulated
What is the function of the phospholipid membrane?
Defines the boundaries of the platelet and separates contents from external environment (maintaining constant internal environment
What is the open canalicular system for?
Invaginations in phospholipid membrane (sponge-like structure)
• Pathway for transport of substances into the cells
• Conduit for discharge of α granule products secreted during platelet release section
What is cell surface receptors for?
Binds to thrombin, ADP and prostaglandins (activation of platelets)
What are surface glycoproteins for?
Important in platelet adhesive and aggregation reactions (GpIa, GpIb, GpIIb, GpIIIa)
What are dense granules for?
Storage granules for ADP (important for activation of platelets), ATP, serotonin and calcium
What are a granules for?
Storage granules for proteins (growth factors, fibrinogen, factor V, von Willebrand factor)
Platelet adhesion: recruitment of platelets from circulating blood to the site of injury
• Platelets and important proteins (e.g. VWF produced by endothelial cells and stored in platelets) circulate in plasma in a globular concentration
o Circulates as __________ without interaction with platelets (binding sites hidden from platelet GpIb)
• When vascular injury occurs, endothelium is damaged, exposing the ____________, which results in VWF binding
• Tethered VWF unravelled by rheological shear forces of flowing blood, exposing platelet binding sites (GpIb) tethering of platelets
o Binding of VWF to platelet GpIb recruits platelets to the site of vessel damage
o Platelets also bind directly to subendothelial collagen via __________ (under low shear forces insufficient to unravel VWF), recruiting platelets to site of vessel damage
Platelet activation: conversion of platelets from passive to active, interactive cells:
• Change in shape, membrane composition (phospholipid changes nature to attract activated coagulation factors), and presentation of new/activated surface proteins (____________ normally quiescent on surface, but responds in aggregation)
• When platelets bind to VWF or directly to collagen, they become activated, releasing stored ADP from α granules, and synthesising thromboxane (platelet activation)
o ____________________ can also directly activate platelets
• Membrane phospholipid composition changes to attract activated coagulation factors (more active GpIIb and GpIIIa)
Platelet aggregation: formation of an unstable platelet plug
• GpIIb and GpIIIa are activated, binding more tightly to collagen and VWF
o Also binds to ___________ which circulates in high concentrations in the blood to form the unstable haemostatic plug
o Helps to slow bleeding and provides surface for coagulation
globular protein;
subendothelial collagen;
GpIa;
GpIIb and GpIIIa;
Collagen and thrombin;
fibrinogen
SECONDARY HAEMOSTASIS
The stabilisation of the plug with fibrin results in blood coagulation to stop blood loss
• Coagulation involves many haemostatic proteins, mainly synthesised in the liver
o Some proteins are synthesised by the endothelial cells (________________________), and megakaryocytes (____________-) for local use
• Clotting factors circulate as inactive precursors (serine protease zymogens or procofactors), activated by specific proteolysis
Coagulation system is a cascade (amplification) system, converting zymogens (inactive) to proteinases (active)
• Cofactors like TF initiate coagulation, while others like FVa and FVIIIa localise and accelerate (10000-fold) reactions on phospholipid surfaces
• Surface may be platelets formed during primary haemostasis
• Trigger for coagulation in vivo is tissue factor
• Activation of FXII to FXIIa is mainly an _______________ used in some important diagnostic tests
VWF, thrombomodulin, tissue factor pathway inhibitor;
VWF, factor V;
in vitro reaction
Which factors contain vitamin K dependent carboxylation domains?
Prothrombin, factor 7+9+10
Intrinsic pathway: blood coagulation is initiated by the activation of factor XII to factor XIIa, which in turn activates factor XI to XIa, then IX to IXa, and finally X to Xa
• Cofactor VIIIa is involved in activation by factor IXa (requiring phospholipids provided by platelet membrane) –> phospholipids change to provide _________ when platelet is activated
o Activated coagulation factors bind to ________________, and factor VIIIa accelerates conversion of factor X to Xa
• Factor VIIIa is activated by factor VIII by trace amounts of ___________
Extrinsic pathway: tissue factor released from cells of the _______________ exposed by vessel damage binds to factor VIIa to fully activate it
• Factor VIIa results in activation of factor X to Xa OR activation of intrinsic pathway by activating _______________
• Factor VII circulates in a partially activated form (VIIa) which complexes with TF to become fully active
Common pathway: factor Xa activates prothrombin to thrombin (IIa), requiring cofactor Va acting on phospholipid surface of platelets
• Cofactor Va is activated from factor V by trace amounts of thrombin positive feedback loop resulting in activation of more cofactors and thus more thrombin
• Soluble fibrinogen (structural) is converted to insoluble fibrin by thrombin via proteolysis, which spontaneously assembles to form a clot (traps __________________)
o Stabilised mechanically by covalent crosslinking by action of the enzyme factor XIIIa
o Factor XIIIa is activated from factor XIII by ________________
surface;
activated platelets (surface);
thrombin;
subendothelial matrix;
factor IX to IXa;
RBC and leukocytes;
trace amounts of thrombin
INHIBITORY MECHANISMS TO COAGULATION
A large amount of thrombin can be produced from a small amount of factor VIIa (amplification), but blood does not clot completely/excessively whenever it is initiated by vessel injury inhibitory mechanisms
TISSUE FACTOR PATHWAY INHIBITOR (TFPI)
TFPI is synthesised by endothelial cells, and recognises ___________________
• Complex inhibits factor Xa and binds with ______________________, thus inactivating the TF/FVIIa complex
• Important physiological reaction, though circulating concentrations of TFPI are low
o Extensive vessel damage resulting in extensive exposure of TF may result in overwhelming of TFPI mechanism
PROTEIN C
Thrombin can bind to ____________________ anticoagulant protein thrombomodulin, redirecting its activity towards activating protein C to APC
• Activated protein C can act with cofactor protein S to target and inactivate ____________________
• Negative feedback loop preventing over-acceleration of thrombin production and amount of thrombin generated
ANTITHROMBIN
Antithrombin is an inhibitor protein circulating in high concentrations, and directly inhibits thrombin (irreversible reaction)
• Antithrombin-thrombin complexes are quickly cleared from the circulation
• Broad spectrum inhibitor able to inhibit ______________– along with thrombin
factor Xa (forms complex Xa/TFPI);
tissue factor and factor VIIa;
endothelial cell surface
factor VIIIa and factor Va;
factors Xa and IXa
FAILURE OF MECHANISMS
Coagulation inhibitory mechanisms are essential in regulating the cascade amplification from occurring and clotting blood completely:
• May fail due to deficiencies in ___________________(congenital deficiencies) or presence of _________(mutated factor V causing hypercoagulability)
o Congenital deficiencies of anticoagulant proteins lead to 50% reduction in concentration and increased risk of venous thrombosis
o FV Leiden avoids normal binding from __________, resulting in increased tendency for patient to form abnormal and potentially dangerous blood clots (present particularly in _____________________)
• These are all risk factors for thrombosis
antithrombin, protein C, protein S;
factor V Leiden ;
APC;
5% of white skinned population
DISSOLUTION OF CLOT
Dissolution of the blood clot is caused by fibrinolysis and cell proliferation, and initiates vessel repair to restore the vessel integrity
• Tissue plasminogen activator (tPA) is synthesised by endothelial cells and circulates in partially activated form in the blood
o Activates the zymogen plasminogen to plasmin
• Under normal circumstances, tPA is unable to convert plasminogen to plasmin; when a fibrin clot forms, binding of ________________ to surface of clot occurs
o Brings plasminogen near partially active tPA
o Conformational change occurs in tPA, to activate plasminogen to plasmin via cleavage
• Plasmin breaks down the fibrin clot to generate _________________ –> diagnostic test for widespread clot formation breakdown in disseminated intravascular coagulation (DIC)
o tPA and streptokinase (bacterial activator) are used in therapeutic thrombolysis for myocardial infarction, ischaemic stroke etc. (clot busters)
plasminogen and tPA;
fibrin degradation products (FDP)
ALTERATION OF HAEMOSTATIC BALANCE
Normal haemostasis occurs in a state of equilibrium between anticoagulation (fibrinolytic factors and anticoagulant proteins) and coagulation (coagulation factors and platelets)
• Bleeding occurs with decreased coagulation factor concentration or circulating platelet number, or increased fibrinolytic factor and anticoagulant protein concentration
o von Willebrand disease (platelets and adhesion reactions), thrombocytopenia (decrease in _____________ ), or haemophilia ( __________ deficiency)
• Thrombosis occurs with decreased fibrinolytic factor and anticoagulant protein concentration, or increased coagulation factor concentration or circulating platelet number
The type of bleeding experienced in different conditions are very different:
platelet count;
FIX or FVIII
ABNORMAL BLEEDING
Bleeding that is ____________ (without any obvious precipitant, like in haemophilia), out of proportion to the trauma/injury, unduly prolonged, and restarts after appearing to stop (delayed bleeding)
• Bleeding no longer proceeds according to expectation as the haemostatic mechanisms are out of equilibrium
• However, about 12% of the population experiences “easy bruising” even though they would not be considered as having any major haemostatic disorder
o System by which bleeding history is taken must be refined
ABNORMAL BLEEDING HISTORY
• Epistaxis (bleeding from the nostril or nasal cavity/“nosebleeds”) not stopped by _______________, requiring medical attention or blood transfusion.
• Cutaneous haemorrhage or bruising without apparent trauma, especially if large (>5cm) or on multiple parts of the body.
• Prolonged (>15 min) bleeding from trivial wounds, or in oral cavity, or recurring spontaneously in 7 days after the wound. Spontaneous gastrointestinal bleeding leading to ___________ (indicating severity).
• ____________ (menstrual periods with abnormally heavy or prolonged bleeding) requiring treatment or leading to anaemia, and not due to structural lesions of the uterus.
• Heavy, prolonged or recurrent bleeding after surgery or dental procedures (if patient has been through major surgery without any significant problems in bleeding, it is unlikely to be a coagulation system problem at the time of the surgery).
spontaneous;
10 minutes of compression;
anaemia;
Menorrhagia;
Primary haemostasis requires platelets, von Willebrand factor (VWF) (bind to collagen) and collagen (in subendothelial matrix)
• Deficiency or defects in collagen or the vessel walls may result from ____________ or age (acquired disorders of collagen), or _____________ (rare, inherited disorders of collagen)
• Deficiency of VWF or presence of abnormal VWF molecules may lead to von Willebrand disease (more common genetic disorder)
• Deficiency of platelets could result from _____________ (low platelet count), while impaired platelet function could result from drugs like _________________
With vessel damage, the two key proteins (____________________) triggering blood coagulation are exposed
• Globular VWF binds to collagen and unravels due to high shear forces, exposing binding sites for platelets activation and degranulation of platelets
• More VWF released from platelets results in another layer of the clot forming (primary platelet plug)
o Primary haemostasis is sufficient for ______________, but the coagulation system needs to be activated (secondary haemostasis for larger vessels or injuries)
In patients with von Willebrand disease (VWF deficiency), no VWF is present to bind to collagen, so only a few platelets will bind to collagen directly (most will flow past)
• Primary haemostasis is not achieved, and the patient continues to bleed
steroid therapy ;
Ehlers-Danlos syndrome;
thrombocytopenia;
aspirin or clopidogrel;
collagen and tissue factor;
small blood vessels and small injuries
PATTERN OF BLEEDING FOR PRIMARY HAEMOSTATIC DEFECTS
Defects of primary haemostasis tend to give rise to a characteristic pattern of bleeding:
• Immediate (no period of stopping because primary haemostatic mechanisms do not start)
• Easy bruising/cutaneous bleeding (bleeding from smaller, peripheral or mucosal blood vessels high shear stress)
• ________/nosebleeds (prolonged >20 min)
• Gum bleeding (prolonged)
• Menorrhagia (abnormal bleeding during menstrual periods)
• Bleeding after trauma or surgery
• ______________ (specific for thrombocytopenia): appear like small blood spots
o Platelets are constantly plugging small vessel breaks in the circulation (primary haemostasis), so they are very essential; without them, small vessel breaks are left to occur, resulting in petechiae
Epistaxis;
Petechiae
SECONDARY HAEMOSTATIC DEFECTS
The process of coagulation (secondary haemostasis) can be measured via _____________ (net product of coagulation) generation over time (thrombogram):
• Coagulation system is designed to produce a large
amount of thrombin to convert fibrinogen to
fibrin (mesh of clot) later crosslinked by factor
XIIIa
• After coagulation system is triggered by ______________ (extrinsic pathway) exposed by endothelial damage, there is a time lag (taken for pathways to be activated and initial generation of thrombin)
o Rapid, high level production of thrombin, which is then gradually suppressed by regulatory proteins (antithrombin, protein C)
• In patients with factor VIII deficiency (_____________) <1%, the burst of thrombin does not occur
o Time lag for thrombin production still occurs, and when thrombin is produced, there is only a weak and feeble amount (inadequate for clotting)
o Fibrin is required for binding and stabilisation of the platelet plug and other cells (RBC, leukocytes)
thrombin;
tissue factor;
haemophilia
Patients with defects of secondary haemostasis (fibrin mesh formation) may have deficiencies or defects of coagulation factors (I – XIII):
• Haemophilia (hereditary): factor VIII or IX deficiency due to genetic defect
• Liver disease (acquired disease): most coagulation factors are made in liver lack one or more factors (most people with advanced liver disease have bleeding disorders)
• Drugs like warfarin (anticoagulant drug) inhibits the synthesis of coagulation factors (II, VII, IX, X) –> ___________ is involved in these reactions
• Dilution of blood: patients with major haemorrhage are often resuscitated using factors (e.g. RBC, crystalloid solutions/volume expanders like Ringer’s lactate/Hartmann’s solution) –> at risk of dilution effect reducing serum concentration of coagulation factors (____________________)
• ______________________ is an acquired disease and a consumptive process which uses up or depletes coagulation factors in the blood, increasing tendency for bleeding
o Generalised activation of coagulation around the body by tissue factor (cells in the blood like monocytes can express TF in response to inflammation) exposed throughout the circulation
o Associated with ________________ (sepsis, major tissue damage, incompatible blood transfusion)
o Consumes and depletes coagulation factors and platelets
o Activation of fibrinolysis depletes fibrinogen in the body
o Leads to widespread bleeding from __________, internal bleeding, widespread bruising, organ failure (deposition of fibrin in vessels of major organs like _______________)
calcium;
dilutional coagulopathy;
Disseminated intravascular coagulation (DIC);
major inflammatory stimuli;
IV lines;
kidneys and lungs
In patients with haemophilia, the primary haemostatic mechanism is normal, but their deficiency in factor VIII or IX results in failure to generate thrombin and thus fibrin to stabilise the platelet plug
• Platelets are only held loosely together by VWF _____________ tends to fall apart, causing delayed bleeding
primary haemostatic plug
PATTERN OF BLEEDING FOR SECONDARY HAEMOSTATIC DEFECTS
• Bleeding is often delayed, occurring only after _____________ (normal) has occurred secondary haemostasis fails to stabilise the primary plug
• Occurs deeper in the body (in joints and muscles) because it is dependent on tissue factor (particularly in areas with low TF concentration) ____________ (bleeding into joints) causes extreme pain (held in flex position), and is spontaneous, causing severe damage to the joint (requiring joint replacement)
• Does not occur from small cuts (primary haemostasis still sufficient)
• _____________ are rare (primary haemostasis still sufficient)
• Bleeding after trauma or surgery
• Bleeding after intramuscular injections (tend to produce very __________)
primary haemostasis;
haemarthrosis ;
Nosebleeds/epistaxis;
large haematomas
