Hematocrit and Blood Cell Composition

Hematocrit

• Hematocrit, also called packed cell volume (PCV), is the percentage of red blood cells in a whole blood sample.
• A standard hematocrit is obtained by drawing blood from a vein (e.g., median cubital vein) and centrifuging it.
• Centrifuging blood separates it into three layers based on density:
  • Erythrocytes (Red Blood Cells): The densest layer, typically 45% of total blood volume.
    • Anything below 45% is considered anemia.
    • Anything above 45% is considered polycythemia.
  • Buffy Coat: A thin, middle layer containing white blood cells (leukocytes) and platelets.
    • Accounts for less than 1% of total blood volume.
  • Plasma: The less dense, uppermost liquid layer.
    • Accounts for approximately 55% of total blood volume.
• Buffy Coat
  • Platelets: Responsible for blood clotting, with a normal count of 150,000 to 450,000 per microliter.
    • Less than 150,000 platelets is called thrombocytopenia, which increases the risk of bleeding.
    • Greater than 450,000 platelets is called thrombocytosis, which increases the risk of clotting.
  • White Blood Cells (Leukocytes): Normal count is 4,800 to 11,000 per microliter.
    • Less than 4,800 leukocytes is called leukopenia, a sign of immune suppression.
    • Greater than 11,000 leukocytes is called leukocytosis, often indicating an infection.
• Plasma
  • Primarily composed of water (90-93%) acting as a universal solvent for transporting blood cells, solutes, nutrients, and waste products.
  • Contains plasma proteins (8%) which regulate blood volume and pressure.
    • Albumin (60%): Regulates water balance and osmotic pressure within blood vessels.
    • Globulins (30%): alpha and beta globulins function as transport proteins for insoluble substances like iron and hormones, while gamma globulins are antibodies produced by plasma cells (differentiated B cells) for fighting pathogens.
  • Also contains:
    • Respiratory Gases: Oxygen, carbon dioxide, and nitric oxide.
    • Electrolytes: Sodium, potassium, chloride, etc.
    • Nutrients: Glucose, amino acids, fatty acids.
    • Enzymes: Anti-microbial and functional enzymes.
    • Hormones: Hormones in the bloodstream are transported via plasma.
    • Metabolic Waste: Lactic acid, uric acid, creatinine, etc. Plasma transports these waste products to organs like the kidneys and liver for excretion.
• Normal red blood cell count is around 5 to 6 million per microliter.