White Blood Cells Lifespan and Distribution

Lifespan and Distribution of White Blood Cells (WBCs)

• WBC lifespan varies from days to months to years.
• Normal WBC distribution: 2% in blood, 98% in lymphatic fluid, organs (skin, lungs, spleen, lymph nodes).
• Tissues exposed to toxins/microbes exhibit variable WBC levels (e.g., skin, mucous membranes).
• Normal WBC count: 4000 – 10,000 cells per mm³ of blood.
• RBC to WBC ratio is approximately 700:1.
• Abnormal WBC counts:
  • Leukocytosis: Over 10,000 per mm³ (high WBC count).
  • Leukocytopenia: Below 3000 per mm³ (low WBC count).

Platelet Characteristics and Function

• Platelets are cytoplasmic fragments of megakaryocytes, without a nucleus but contain mitochondria.
• Platelets have two types of granules:
  • Alpha granules: Contain fibrinogen, thrombospondin, and clotting factors.
  • Dense granules: Contain calcium, serotonin, ADP, and ATP.
• Platelet shapes:
  • Stimulated form: Discoid shape (2-4 µm).
  • Unstimulated state: Spiny spherical shape.
• Function: Essential to stop bleeding in damaged vessels.

Hematopoiesis Overview

• Hematopoiesis: Process of blood cell formation, originates from Greek "haima" (blood) and "poiein" (to make).
• Average adult bone marrow produces approximately 5 x 10¹¹ cells daily.
• Controlled and balanced production of blood cells.
• Types of hematopoiesis:
  • Erythropoiesis: Formation of red blood cells (RBCs).
  • Leukopoiesis: Formation of WBCs, includes:
    • Myelopoiesis: Formation of granulocytes and monocytes.
    • Lymphopoiesis: Formation of lymphocytes.
  • Thrombopoiesis: Formation of platelets.

Embryonic Hematopoiesis

• Hematopoiesis starts in the yolk sac during the first month of embryogenesis and shifts to the liver and spleen.

Erythropoietin Mechanism

• Normal blood oxygen levels are essential.
• Hypoxia (low O2): Triggers erythropoietin release from kidneys (also from liver) to stimulate RBC production.
• Increases RBC count, enhancing oxygen-carrying ability of blood.

Leukocyte Classification

• Leukocytes are complete cells classified by:
  • Cytoplasmic granulation:
    • Granulocytes: Neutrophils, eosinophils, basophils.
    • Agranulocytes: Monocytes, lymphocytes.
  • Function:
    • Phagocytes: Granulocytes and monocytes.
    • Non-phagocytes: Lymphocytes.

Granulocyte Characteristics

• Neutrophils account for 40-80% of total WBCs, possess granules that stain with acidic and basic dyes, appearing pink-blue or gray-blue.

Leukopoiesis Regulation

• Leukopoiesis is hormonally induced by cytokines: interleukins (numbered like IL-1, IL-2) and colony-stimulating factors (CSFs).
• Key sources of cytokines: macrophages and T cells.
• Hematopoietic hormones are clinically used to stimulate bone marrow.

Control of Granulopoiesis

• Growth factors involved in granulocyte maturation include IL-1, IL-3, IL-5 (for eosinophils), IL-6, IL-11, GM-CSF, G-CSF, and M-CSF.
• These factors promote proliferation and differentiation of granulocyte precursors.

Platelet Production Process

• Platelets produced by fragmentation of megakaryocyte cytoplasm in bone marrow.
• Megakaryocytes arise from hematopoietic stem cells and undergo endomitotic synchronous replication.
• Each megakaryocyte produces 1000-5000 platelets; the process takes about 10 days.
• Normal platelet count ranges from 150-400 x 10⁹/L; lifespan is 7-10 days.
• Approximately one-third of platelets may be stored in the spleen, increasing to 90% during massive splenomegaly.

This quiz focuses on the lifespan, distribution, and normal counting of white blood cells (WBCs) in a healthy individual. Participants will explore topics such as normal WBC counts, their ratio with red blood cells, and abnormalities like leukocytosis. Test your knowledge on this essential aspect of human biology.