—low WBC count: below 5,000 WBCs/mL –Causes: radiation, poisons, infectious disease –Effects: elevated risk of infection Leukopenia —high WBC count: above 10,000 WBCs/mL  –Causes: infection, allergy, disease Leukocytosis —cancer of hemopoietic tissue that usually produces an extraordinary high number of circulating leukocytes and their precursors Leukemia What are the four kinds of leukemia? What are teh effects? –Myeloid leukemia: uncontrolled granulocyte production –Lymphoid leukemia: uncontrolled lymphocyte or monocyte production –Acute leukemia: appears suddenly, progresses rapidly, death within months –Chronic leukemia: undetected for months, survival time 3 years    –Effects: normal cell percentages disrupted; impaired clotting; opportunistic infections —the cessation of bleeding    –Stopping potentially fatal leaks    –Hemorrhage: excessive bleeding Hemostasis •Three hemostatic mechanisms –Vascular spasm –Platelet plug formation –Blood clotting (coagulation) —small fragments of megakaryocyte cells Platelets –Stem cells (that develop receptors for thrombopoietin) become megakaryoblasts •Thrombopoiesis –Repeatedly replicate DNA without dividing –Form gigantic cells called megakaryocytes with a multilobed nucleus    •100 mm in diameter, remains in bone      marrow •Megakaryoblasts —live in bone marrow adjacent to blood sinusoids –Long tendrils of cytoplasm (proplatelets) protrude into the blood sinusoids: blood flow splits off fragments called platelets –Circulate freely for 10 days –40% are stored in spleen Megakaryocytes •Clot retraction occurs within __ minutes 30 —dissolution of a clot    –Factor XII speeds up formation of kallikrein      enzyme    –Kallikrein converts plasminogen    into plasmin, a fibrin-dissolving enzyme that      breaks up the clot Fibrinolysis —abnormal clotting in unbroken vessel Thrombosis –clot  •Most likely to occur in leg veins   of inactive people Thrombus –clot may break free, travel from veins to lungs Pulmonary embolism —anything that can travel in the blood and block blood vessels Embolus (tissue death) may occur if clot blocks blood supply to an organ (MI or stroke) –650,000 Americans die annually of thromboembolism (traveling blood clots) Infarction