Blood components Knowledge-Based Study

Study notes

1. Blood is a specialized fluid connective tissue that circulates throughout the cardiovascular system, performing vital functions such as transport, regulation, and protection.

   When a sample of whole blood is treated with an anticoagulant and separated using a centrifuge, it resolves into three distinct components based on density:

   • Plasma: The yellowish liquid supernatant, accounting for approximately 55% of the total blood volume.
   • Buffy Coat: A very thin, whitish layer directly above the red cells, comprising less than 1% of total volume, which contains white blood cells and platelets.
   • Erythrocytes: The densest fraction at the bottom of the tube, making up approximately 45% of total blood volume. This percentage is clinically termed the hematocrit.

   Blood plasma serves as the extracellular fluid matrix of blood. It is composed of roughly 90% to 92% water, which acts as a solvent for numerous essential substances.

   The remaining 8% to 10% consists of dissolved solutes, primarily plasma proteins synthesized mainly by the liver:

   • Albumin: The most abundant plasma protein, responsible for maintaining colloid osmotic (oncotic) pressure and transporting hydrophobic substances.
   • Globulins: Includes transport proteins (alpha and beta globulins) and immunoglobulins (antibodies produced by plasma cells during immune responses).
   • Fibrinogen: A soluble precursor essential for the coagulation cascade, converted into insoluble fibrin strands during clot formation.

   Plasma also transports dissolved gases, glucose, amino acids, lipid carriers, electrolytes (such as sodium, potassium, and calcium), metabolic wastes like urea, and regulatory hormones.

   Erythrocytes are the most numerous formed elements, specialized almost exclusively for respiratory gas transport.

   Key structural and functional features include:

   • Biconcave Disc Geometry: This morphology increases the surface-area-to-volume ratio, facilitating rapid gas exchange across the cell membrane, and provides flexibility to squeeze through narrow capillary beds.
   • Absence of Nucleus and Organelles: Mature human erythrocytes lack nuclei, ribosomes, and mitochondria. This maximizes intracellular space for hemoglobin and ensures the cell consumes none of the oxygen it carries (generating ATP entirely via anaerobic glycolysis).
   • Hemoglobin: A quaternary protein composed of four globin chains, each carrying an iron-containing heme group capable of reversibly binding a molecule of oxygen ($O_2$). Hemoglobin also aids in buffering blood pH and carrying a portion of carbon dioxide.

   Erythrocytes have an average lifespan of about 120 days before being engulfed and degraded by macrophages in the spleen and liver.

   The remaining formed elements—leukocytes and thrombocytes—are primarily involved in immunological defense and hemostasis, respectively.

   Leukocytes (White Blood Cells)

   Unlike erythrocytes, leukocytes possess a nucleus and complete cellular organelles. They function primarily outside the bloodstream in tissues, using diapedesis to leave capillaries. They are categorized based on the presence of cytoplasmic granules:

   • Granulocytes: Possess visible granules and lobed nuclei. These include neutrophils (phagocytosis of bacteria), eosinophils (combating parasites and modulating allergic reactions), and basophils (secreting histamine and heparin).
   • Agranulocytes: Lack visible specific granules. These include lymphocytes (B cells, T cells, and natural killer cells mediating adaptive immunity) and monocytes (which differentiate into tissue macrophages).

   Thrombocytes (Platelets)

   Platelets are not complete cells but small, membrane-bound cytoplasmic fragments shed from giant bone marrow cells called megakaryocytes.

   Platelets contain active enzymes and granules that are crucial for hemostasis (the arrest of bleeding). Upon vascular injury, platelets adhere to exposed collagen, aggregate to form a primary platelet plug, and release chemical factors that catalyze blood clotting.