Blood: Plasma, Formed Elements, and Blood Typing Review

Whole Blood Composition

  • Whole blood consists of Plasma (55\%) and Formed Elements (45\%).
    • Plasma components: Water (\approx 91\%), Proteins (\approx 8\%), and Other Solutes (\approx 2\%).
    • Major plasma proteins: Albumins (57\%), Globulins (38\%), Fibrinogen (4\%), Prothrombin (1\%).
    • Formed Elements include Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets.
    • RBCs constitute \approx 99.1\% of formed elements; WBCs and platelets form the 'buffy coat' (\approx 0.9\%).

Blood Components & WBC Types

  • Blood Components: Plasma (Water, Ions, Organic molecules, Gases) and Cellular Elements (RBCs, WBCs, Platelets).
  • WBC Types: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils. Fibrinogen is a key plasma protein.

Plasma Functions & Cellular Elements

  • Plasma is the solvent for crucial ions (e.g., \text{Na}^+, \text{K}^+, \text{Ca}^{2+}), nutrients, waste products, respiratory gases, and hormones.
  • Plasma proteins (Albumin, Fibrinogen, Immunoglobulins) regulate osmotic pressure, aid transport, and are vital for clotting and immunity.
  • Typical Cell Counts:
    • Erythrocytes (RBCs): 5-6 \times 10^6/\text{mm}^3
    • Leukocytes (WBCs): 5,000-10,000/\text{mm}^3
    • Platelets: 250,000-400,000/\text{mm}^3
  • Hematocrit: Measures the percentage of RBCs in blood. Normal ranges: Males \approx 42-52\%, Females \approx 37-47\%. Depressed hematocrit indicates anemia; elevated indicates polycythemia.

Solutes & Formed Element Proportions

  • Plasma solutes include essential electrolytes, nutrients (lipids, carbohydrates, amino acids), and waste products (urea, creatinine).
  • Plasma proteins primarily include Albumins (\approx 60\%$), Globulins (\approx 35\%$), and Fibrinogen (\approx 4\%$).
  • Formed Elements:
    • RBCs are the most abundant.
    • WBCs are categorized into Granulocytes (Neutrophils, Eosinophils, Basophils) and Agranulocytes (Lymphocytes, Monocytes).
    • Platelets are anucleated cell fragments.

Blood Smear Staining & Cell Types

  • Wright's stain uses Methylene blue (basic, stains basophilic structures like nuclei) and Eosin (acid, stains eosinophilic structures like cytoplasm proteins).
  • Common cell types observed on a smear include Basophil, Eosinophil, Neutrophil, Erythrocyte, Monocyte, and Lymphocytes.

Erythrocytes (RBCs)

  • Most numerous blood cells, transporting O2 and CO2 using hemoglobin.
  • Mature RBCs are anucleated and lack mitochondria, resulting in a lifespan of approximately 120 days before being destroyed by macrophages.

Platelets (Thrombocytes)

  • Anucleated cell fragments (\approx 2\,\mu\text{m}) with a \approx 10-day lifespan.
  • Essential for blood clotting, produced from megakaryocytes in the bone marrow.

Leukocytes (White Blood Cells)

  • Granulocytes (Neutrophils, Eosinophils, Basophils) have lobed nuclei and cytoplasmic granules.
    • Neutrophils: Multi-lobed nucleus; phagocytose bacteria.
    • Eosinophils: Bi-lobed nucleus, eosinophilic granules; combat parasites and modulate allergic reactions (histaminase).
    • Basophils: Bi-lobed nucleus, basophilic granules; secrete histamine and heparin in allergic/inflammatory responses.
  • Agranulocytes (Lymphocytes, Monocytes) have less prominent granules.
    • Monocytes: Largest WBC, kidney-shaped nucleus; differentiate into macrophages in tissues for phagocytosis.
    • Lymphocytes: Round nucleus; B cells produce antibodies, T cells mediate cell-based immunity.

Specific Leukocyte Types & Proportions

  • Neutrophils: 60-70\% of WBCs; multi-lobed nucleus; primary phagocytes against bacteria.
  • Lymphocytes: 20-25\%; single, round nucleus; key players in specific immunity (B cells: antibodies; T cells: cell-mediated).
  • Monocytes: 3-8\%; largest WBC, kidney-shaped nucleus; become macrophages in tissues.
  • Eosinophils: 2-4\%; bi-lobed nucleus, eosinophilic granules; active in allergic reactions and parasitic defense.
  • Basophils: <1\%; bi-lobed nucleus, basophilic granules; release histamine and heparin in allergic/inflammatory responses.

Differential White Blood Cell Count

  • Mnemonic: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).
  • Typical Ranges:
    • Neutrophils: 55-70\%
    • Lymphocytes: 20-33\%
    • Monocytes: 2-9\%
    • Eosinophils: 2-4\%
    • Basophils: 0.5-1\%

Hematologic Tests

  • Hemoglobin (Hb): Oxygen-transporting protein in RBCs. Normal: 12-18\,\text{g/dL} (Men 13-18\,\text{g/dL}, Women 12-16\,\text{g/dL}).
    • HbA1c: Glycosylated Hb, indicates average blood glucose over \approx 3 months for diabetics.
  • Hematocrit (PCV): Percentage of RBCs in total blood volume. Measured by centrifugation. Normal: Males 42-52\%, Females 37-48\%.
    • Depressed values suggest anemia; elevated values suggest polycythemia.

Hemostasis & Coagulation Tests

  • Bleeding Time assesses platelet function.
  • Coagulation Time measures the time for blood to clot (normal: 2-6\,\text{minutes}$$), involving clotting factors like fibrinogen.
  • Hemostasis occurs in phases:
    1. Vascular Phase: Vasoconstriction.
    2. Platelet Phase: Platelet plug formation.
    3. Coagulation Phase: Activation of clotting cascade (Extrinsic and Intrinsic pathways) leading to fibrin formation, stabilizing the clot.
    4. Clot Retraction: Compaction of the clot.

Blood Typing & Agglutination

  • Blood typing identifies surface antigens (A, B, Rh factor/D antigen) on RBCs using specific antibodies (agglutinins) in test serums.
  • Agglutination (clumping) indicates the presence of the corresponding antigen.
    • Anti-A, Anti-B, and Anti-Rh serums are used.
    • E.g., Type A blood clumps with Anti-A serum; Type B with Anti-B; Type AB with both; Type O with neither.
  • Rh factor is critical for transfusion compatibility and preventing Rh incompatibility in pregnancy.

ABO Blood System & Antigen-Antibody Principles

  • The ABO blood system defines types (A, B, AB, O) based on the presence or absence of A and B antigens (glycoproteins) on RBC surfaces.
    • Genotypes (AA, AO for A; BB, BO for B; AB for AB; OO for O) determine the phenotype.
    • Individuals naturally produce antibodies against the A or B antigens they lack (e.g., Type A has anti-B antibodies). Type AB has neither; Type O has both anti-A and anti-B.
  • This antibody-antigen interaction leads to agglutination, which is the basis for blood typing and transfusion compatibility.

Blood Typing Protocol & Interpretation

  • Blood typing involves adding specific anti-sera (Anti-A, Anti-B, Anti-Rh) to blood samples.
  • Agglutination (clumping) observed after mixing indicates the presence of the corresponding antigen, allowing inference of the blood type.

Clinical Significance of Blood Tests

  • Understanding blood types and performing accurate blood typing is crucial for safe blood transfusions and managing Rh incompatibility in pregnancy.
  • Hematologic tests (e.g., Hb, hematocrit, coagulation time) are vital for diagnosing and managing conditions like anemia, diabetes, and bleeding disorders.
  • Microscopic examination of blood smears with stains like Wright's stain aids in diagnosing various hematologic disorders.