Circulating platelets (thrombocytes) are anuclear, cytoplasmic disks that bud off from megakaryocytes, large multinucleated cells found in the bone marrow16,17 (see Fig. 1-2). Platelets survive in the circulation for about 10 days.
Regulation of platelet production is ascribed to thrombopoietin by analogy to erythropoietin (see Chapter 1 - Hematology and Tests of Hematopoietic Function), although no single substance has been specifically identified. With pronounced hemostatic stress or marrow stimulation, platelet production can increase to seven to eight times that of normal production. Newly generated platelets are larger and have greater hemostatic capacity than mature circulating platelets.18
Two thirds of the total number of platelets are in the systemic circulation, and the remaining third exists as a pool of platelets in the spleen. The pool exchanges freely with the general circulation.19 The spleen also aids in removing old or damaged platelets from the circulation. In disorders involving exaggerated splenic activity (hypersplenism), 90 percent of the body's platelets may be trapped in the enlarged spleen, and the client is predisposed to excessive bleeding. Hypersplenism is seen in certain acute infections (e.g., infectious mononucleosis, miliary tuberculosis), connective tissue diseases (e.g., rheumatoid arthritis, lupus erythematosus), myeloproliferative diseases (e.g., leukemias, lymphomas, hemolytic anemias), and chronic liver diseases (e.g., cirrhosis).20
The functions of platelets are discussed in the introduction to this chapter. In general, individuals with too few platelets or with platelets that function poorly experience numerous pinpoint-sized hemorrhages (petechiae) and multiple small, superficial bruises (ecchymoses). Frequently, there is generalized oozing from mucosal surfaces and from venipuncture sites or other small, localized injuries. Large, deep hematomas and bleeding into joints are not characteristic of platelet deficiency (thrombocytopenia).21
Platelet studies involve evaluating the number and function of circulating platelets. Platelet numbers are assessed by the platelet count (see the next section). Disorders of platelet function (thrombopathies) are less common than disorders of platelet number. An overview of the causes of altered platelet function is provided in Table 2-2.