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a small disk or platelike structure, the smallest of the formed elements in blood. Blood platelets (called also thrombocytes) are disk-shaped, non-nucleated blood elements with a very fragile membrane; they tend to adhere to uneven or damaged surfaces. They average about 250,000 per cubic millimeter of blood and are formed in the red bone marrow by fragmentation of megakaryocytes, the largest of the bone marrow cells. Platelet production is controlled by a hormone, thrombopoietin, and regulatory lymphocytes acting at the stem cell level. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood.
The functions of platelets are related to the clotting of blood. Because of their adhesion and aggregation capabilities platelets can occlude small breaks in blood vessels and prevent the escape of blood. Platelets which have adhered to exposed collagen in damaged vessels release ADP in milliseconds which in turn initiates the synthesis of thromboxane A2, a very potent prostaglandin which causes platelet aggregation and localized vasoconstriction. Fibrinogen, factors V and VIII, calcium ions, platelet phospholipid (PF-3), associated with the platelet membrane are also released. Substances contained within the platelet granules such as thromboglobulin, heparin neutralizing activity (PF-4) mitogens such as platelet derived growth factor, thrombospondin, ADP, serotonin and calcium ions are also released by aggregated platelets.
platelet-activating factor (PAF)
see platelet-activating factor.
the adherence of platelets to any area with damaged blood vessels; an important component of hemostasis.
the progressive accumulation of platelets, attracted by other platelets once adhesion begins. Thromboxane A2 causes irreversible platelet aggregation.
platelet aggregation test
a known platelet aggregating factor such as collagen, ADP or thrombin is added to a suspension of the platelets under test and the degree of aggregation measured by decrease in turbidity of the suspension.
may be performed directly (in a hemocytometer chamber) or indirectly (estimating from the stained blood smear by number per field or in comparison to the number of white blood cells), expressed as number of cells per liter of blood.
platelet-derived growth factor
one of three growth factors released by platelets which undergo the release reaction; the growth factors stimulate endothelial cell proliferation.
platelet distribution width (PDW)
an indication of variation in platelet size which can be a sign of active platelet release.
platelet factor 3 (PF-3) test, platelet release test
test the antiplatelet activity of serum; used to detect circulating antiplatelet antibodies. Antibody-antigen reactions involving platelets cause the release of PF-3 from platelets which in turn shortens the contact-activated clotting time of platelet-rich plasma (PRP).
factors important in hemostasis which are contained in or attached to the platelets: platelet factor 1 is adsorbed clotting factor V from the plasma; platelet factor 2 is an accelerator of the thrombin-fibrinogen reaction; platelet factor 3 is a phospholipid with potent procoagulant activity; platelet factor 4 is capable of inhibiting the activity of heparin (heparin neutralizing activity).
mean platelet volume (MPV)
elevated level is an indication of increased megakaryocyte shedding of platelets and decreased level is seen in thrombocytopenia.
platelet plug formation
see platelet aggregation (above).
platelet release reaction
measured by the degree of secondary ADP-mediated aggregation that occurs. This is assessed by the amount of PF-4, PF-3 or serotonin, etc. released.
tested by testing the adhesiveness of a suspension of the subject platelets to a glass bead column or standard size filter.
platelet rich plasma
plasma prepared by centrifugation to separate out red blood cells but not platelets for transfusion.
platelet storage-pool disease
an inherited autosomal thrombopathia in American foxhounds and cats characterized by a deficiency of platelet storage granules.
transfusion of fresh, nonchilled whole blood is the usual method of transfusing platelets to an animal with thrombocytopenia.
platelet transfusionThe administration of platelets to ↑ platelet concentration in the circulation. See Platelet antigens.
Platelet transfusion guidelines
Platelet count–PC < 20 x 109/L–US: < 20 000/mm3
PC < 40 x 109/L in active hemorrhage
PC < 50 x 109/L in neonates, or Pts with documented coagulopathies, recurrent fever, severe infections or receiving drugs that cause platelet dysfunction
PC < 100 x 109/L before 'bloody' surgery, eg cardiopulmonary bypass or < 48 hrs after surgery
Bleeding time > twice upper limit of normal
For transfusing one unit of random-donor platelets/10 kg body weight/24 hrs