A
Activated Partial Thromboplastin Time (aPTT). Hemostatic
test measuring time to clot formation when a particulate activator
is added to citrated plasma. aPTT is a measure of clot formation
in secondary hemostasis via the instrinsic pathway in the coagulation
cascade.
Adenosine Diphosphate (ADP). An agonist
for platelets in hemostasis that recruits and activates new platelets
for aggregation.
Adenosine Triphosphate (ATP).
An agonist for cells other than platelets.
Aggregometry.
An in vitro test that uses spectrophotometry to measure
platelet aggregation with agonists common in the hemostatic process,
including ADP, epinephrine, collagen, and ristocetin.
Antiplatelet Agents. Agents that inhibit the ability of
platelets to form a thrombus by various means, including blocking
production of the platelet agonist thromboxane A2 (aspirin),
inhibiting the activity of the platelet agonist ADP (ticlopidine,
clopidogrel), or inhibiting binding to the glycoprotein IIb/IIIa
receptor (Abciximab).
Arterioles. The smallest
arteries. Precapillary vessels that are a major site of hemostasis.
B
Bleeding Time. Hemostatic
test involving making an incision, usually of standard length
and depth, and measuring the time it takes for bleeding to cease.
C
Capillary. Tiny thin-walled vessel that connects an
arteriole to a venule. The smallest blood vessels that
compose the largest surface area of all blood vessels.
Coagulation Cascade. The process by which plasma proteins,
known as coagulation factors, interact in a series of complex
enzymatic reactions to convert soluble fibrinogen to insoluble
fibrin, the substance that reinforces the primary hemostatic plug.
The coagulation cascade is activated when inactive coagulation
factors come in contact with subendothelial tissue at the site
of vessel injury.
Coagulation Factors. Soluble
inert plasma proteins that play a central role in the coagulation
cascade, interacting to form fibrin after injury to a blood vessel.
Collagen. Protein that resides in the basement
membrane of the subendothelium. With the help of vWF, platelets
adhere to subendothelial collagen at the site of vessel injury
during primary hemostasis.
Cyclooxygenase. Cellular
membrane enzyme in the platelet; essential for production of thromboxane
A2.
D
Demopressin Acetate (DDAVP). Synthetic analog of the
antidiuretic hormone L-arginine vasopressin. Intravenous DDAVP,
which results in an increase in vWF and factor VIII, is a common
therapy for vWD.
E
Endothelium. The continuous layer of flattened cells
that line the lumen (the central cavity) of the blood vessel.
Breach of the endothelium during vessel injury initiates the hemostatic
process.
Epinephrine. Agonist in platelet aggregation
that binds to platelet membrane receptors, causing an opening
in the membrane that allows extracellular calcium to enter the
cell, contributing to platelet activation.
Extrinsic
Pathway. One of the three interacting pathways in the coagulation
cascade, termed extrinsic because the pathway requires tissue
factor, which originates outside of the blood. Exposure of tissue
factor to blood forms a complex with factor VII. This complex
activates factor X. The extrinsic system initiates the clotting
mechanism.
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F
Fibrin. The
substance that forms a meshwork within and surrounding the primary
hemostatic plug, giving it strength and stability. Insoluble fibrin
is the product of the coagulation cascade in which plasma proteins,
known as coagulation factors, interact in a series of complex
enzymatic reactions to convert soluble fibrinogen to fibrin.
Fibrinogen. Soluble hemostatic protein that is converted
to fibrin during secondary hemostasis.
Fibrinolysis.
The process of breaking down fibrin that occurs at the end of
the hemostatic process.
G
Glanzmann Thrombasthenia. Rare inherited platelet disorder
involving a mutation in one of the gene's coding for glycoproteins
IIb/IIIa, resulting in compromised ability of platelets to aggregate.
Glycoprotein Ib. A glycoprotein located on the surface
of the platelet containing the receptor for vWF. Glycoprotein
Ib plays a crucial role in adhesion of platelets to von Willebrand
Factor after vessel injury.
Glycoprotein IIb/IIIa.
Complex of proteins on the surface of the platelet membrane that
plays an essential role in platelet aggregation during primary
hemostasis. They function as receptors for fibrinogen and vWF
when activated by agonists during hemostasis.
Granule.
One of several types of bodies within platelet cytoplasm that
serves as storage containers for substances essential for platelet
function. Granules contain agonists (e.g., ADP, ATP, serotonin)
that are secreted in the activated platelet to promote platelet
aggregation.
H
Hemostasis. The arrest of bleeding; the process by
which a barrier to blood loss is created at the site of blood
vessel injury and limited to that site.
I
Idiopathic Thrombocytopenic Purpura. A rare, acquired
platelet disorder in which immune mechanisms destroy platelets
faster than bone marrow can replace them.
Incidental
Thrombocytopenia of Pregnancy. Common cause of platelet dysfunction
in pregnancy.
Intrinsic Pathway. One of the three
interacting pathways in the coagulation cascade, termed intrinsic
because all of the factors involved are contained in the blood.
Exposure of coagulation factors, including factors XII, XI, prekallikrein,
and kininogen, to subendothelial tissue initiates the intrinsic
pathway.
J
K
L
M
Menorrhagia. Excessive or prolonged bleeding during
menstruation.
N
O
P
Platelet. Ovoid or round structure in peripheral blood
formed in the bone marrow. Platelets play a central role in primary
hemostasis by aggregating at the site of injury to form the primary
hemostatic plug. They also play a role in secondary hemostasis
by providing platelet Factor III, which activates coagulation
proteins.
Platelet Activation. The series of morphologic
and functional changes that occur to platelets during primary
hemostasis after adhesion to collagen fibers in the subendothelium.
Platelet Adhesion. The initial step in primary hemostasis
in which platelets adhere to collagen fibers in the subendothelium
with the help of vWF.
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Platelet Aggregation. The process that occurs during primary
hemostasis in which activated platelets aggregate at the site
of vessel injury.
Platelet Count. Quantitative hemostatic test in which platelets
are counted by manual or automated means.
Platelet
Secretion. The process in primary hemostasis following adhesion,
activation, and aggregation, during which platelets release granule
content into the surrounding area to further promote the hemostatic
process.
Pre-eclampsia. Common hypertensive disorder that may occur
during pregnancy and can result in thrombocytopenia.
Primary Hemostasis. The first phase of hemostasis in which
platelets form the primary hemostatic plug at the site of vessel
injury.
Prothrombin Time (PT). Hemostatic test measuring time to clot
formation when thromboplastin and calcium are added to citrated
blood. PT measures clot formation in secondary hemostasis via
the extrinsic pathway in the coagulation cascade.
Q
R
Ristocetin. An antibiotic known to cause thrombocytopenia.
S
Secondary Hemostasis. The second phase of hemostasis in
which the primary hemostatic plug is strengthened by the addition
of fibrin.
Serotonin. Component of granules in
platelet cytoplasm that is released during primary hemostasis,
causing vasoconstriction.
Shear Flow. The flow
dynamic that occurs at the site of vessel injury, promoting platelet
contact with subendothelial tissue, thereby initiating primary
hemostasis.
Subendothelium. The layers beneath
the abluminal surface of the endothelium.
T
Thrombocytopenia. A decrease in the number of platelets
in peripheral blood below normal range.
Thrombocytosis.
An increase in the number of platelets in peripheral blood above
the normal range.
Thrombosis. The process by
which a blood clot or thrombus is formed within the vascular system.
Thrombosis is usually considered to occur under abnormal conditions
within the vessel. The intravascular formation of a blood clot.
Thromboxane A2. The major cyclooxygenase
product in platelets and a major agonist in platelet aggregation.
Thrombus. An intravascular blood clot that forms within
the vascular system.
U
V
Venules. The smallest veins and a major site of hemostasis.
von Willebrand Disease. Hereditary bleeding disorder
in which vWF—the factor needed for platelet adhesion to collagen—is
lacking.
von Willebrand Factor. Plasma factor
required for platelet adhesion to collagen. During hemostasis,
vWF binds to the glycoprotein Ib receptor on the activated platelet.
W
X
Y
Z
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