6.2 Composition of Blood & Cellular Elements

6.2 Composition of Blood & Cellular Elements

Blood is a vital fluid connective tissue that circulates continuously through the human heart and vascular tree. It performs essential physiological functions, including the transportation of oxygen, nutrients, cellular signals, and metabolic wastes; regulation of body temperature, pH, and fluid balance; and defense against infectious pathogens and foreign substances. A comprehensive knowledge of blood volume, fluid fractions (plasma versus serum), and cellular formed elements is fundamental to clinical laboratory science and accurate phlebotomy practice.

Total Blood Volume and Gross Composition

In a healthy adult, total blood volume averages approximately 5 liters (ranging from 4.5 to 5.5 liters depending on body mass and sex), accounting for roughly 7% to 8% of total body weight. Pediatric total blood volume is proportionally higher relative to body weight (approximately 80 to 85 mL/kg in infants), making pediatric patients particularly vulnerable to iatrogenic anemia from excessive phlebotomy draw volumes.

When whole blood is collected with an anticoagulant and allowed to separate (or centrifuged), it divides into two major components:

1. Liquid Portion (Plasma): Represents approximately 55% of total blood volume.
2. Formed Elements (Cells): Represent approximately 45% of total blood volume (a proportion referred to in clinical hematology as the hematocrit or packed cell volume).

+-------------------------------------------------------------------+
|                        WHOLE BLOOD VOLUME                         |
+-------------------------------------------------------------------+
|               PLASMA (55%)               |  FORMED ELEMENTS (45%) |
| 90% Water | 10% Solutes & Proteins        | Erythrocytes (RBCs)    |
| (Albumin, Globulins, Fibrinogen, Salts)  | Leukocytes (WBCs)      |
|                                          | Thrombocytes (Platelets|
+-------------------------------------------------------------------+

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Liquid Portion: Plasma vs. Serum

Understanding the biological distinction between plasma and serum is a core requirement for phlebotomy technicians, as specimen collection tube selection directly depends on which fluid phase is required by the laboratory assay.

Plasma: The Unclotted Liquid Matrix

Plasma is the straw-colored liquid component of whole blood obtained when blood is drawn into a tube containing an anticoagulant (such as EDTA, sodium citrate, or heparin) and centrifuged. Because clotting is chemically inhibited, plasma retains all soluble coagulation factors.

• Chemical Composition of Plasma:
  • Water (90-92%): Serves as the primary solvent for transport.
  • Plasma Proteins (6-8%):
    • Albumin: The most abundant plasma protein, synthesized by the liver; maintains oncotic/osmotic pressure and transports hydrophobic molecules.
    • Globulins: Includes alpha, beta, and gamma globulins (immunoglobulins/antibodies), essential for immune defense and enzymatic transport.
    • Fibrinogen: A major soluble glycoprotein (Factor I) essential for blood coagulation.
  • Dissolved Solutes (1-2%): Electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻), nutrients (glucose, amino acids, lipids), metabolic waste products (urea, creatinine, bilirubin), gases (CO₂, O₂), and regulatory hormones.

Serum: The Clotted Liquid Matrix

Serum is the liquid portion remaining after whole blood has been allowed to undergo natural clot formation in a tube without anticoagulants (such as a plain red-top or gold-top Serum Separator Tube / SST).

• The Mechanism of Serum Formation: When blood clots, soluble fibrinogen is converted into insoluble fibrin strands, forming a gel-like meshwork that traps red cells, white cells, and platelets. As the clot retracts, liquid serum is squeezed out.
• Biochemical Difference: Serum lacks fibrinogen and coagulation factors V and VIII because they are consumed during the clotting cascade. Serum contains all other plasma proteins (albumin, globulins), electrolytes, and metabolic solutes.

Summary Comparison: Plasma vs. Serum

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Formed Elements: Erythrocytes, Leukocytes, and Thrombocytes

The cellular components of blood—known as formed elements—are produced in the red bone marrow through a continuous process called hematopoiesis.

+------------------------------------------------------------------------------------+
|                                  FORMED ELEMENTS                                   |
+------------------------------------+-----------------------------------------------+
| Cell Type                          | Normal Reference Range                        |
+------------------------------------+-----------------------------------------------+
| Erythrocytes (Red Blood Cells)     | 4.5 – 5.5 million / mm³ (or µL)              |
| Leukocytes (White Blood Cells)     | 5,000 – 10,000 / mm³ (or µL)                  |
| Thrombocytes (Platelets)           | 150,000 – 400,000 / mm³ (or µL)              |
+------------------------------------+-----------------------------------------------+

1. Erythrocytes (Red Blood Cells / RBCs)

• Structure & Reference Range: Normal adult count is 4.5 to 5.5 million cells/mm³. Mature RBCs are unique, non-nucleated biconcave discs approximately 7.8 micrometers in diameter. Their flexible biconcave shape maximizes surface-area-to-volume ratio for gas diffusion and allows deformation through narrow capillaries.
• Function: Packed with the iron-containing protein hemoglobin, erythrocytes transport oxygen from the lungs to peripheral tissues and return carbon dioxide from tissues to the lungs.
• Lifespan: Approximately 120 days, after which aged RBCs are removed from circulation by macrophages in the spleen and liver.

2. Thrombocytes (Platelets)

• Structure & Reference Range: Normal adult count is 150,000 to 400,000 cells/mm³. Thrombocytes are not complete cells; rather, they are small, non-nucleated cytoplasmic fragments shed from large bone marrow precursor cells called megakaryocytes.
• Function: Platelets are the key mediators of primary hemostasis. Upon vascular injury, platelets adhere to exposed subendothelial collagen, release chemical mediators (ADP, thromboxane A2), and aggregate to form a temporary hemostatic plug.
• Lifespan: Approximately 7 to 10 days.

3. Leukocytes (White Blood Cells / WBCs)

• Structure & Reference Range: Normal adult total WBC count is 5,000 to 10,000 cells/mm³. Unlike RBCs, leukocytes retain nuclei and organelles. They serve as the mobile defense system of the immune body.
• Classification: WBCs are broadly divided into granulocytes (cells containing prominent cytoplasmic granules visible under light microscopy) and agranulocytes (cells lacking distinct cytoplasmic granules).

                                  LEUKOCYTES (WBCs)
                                          |
                +-------------------------+-------------------------+
                |                                                   |
           GRANULOCYTES                                       AGRANULOCYTES
    (Neutrophils, Eosinophils, Basophils)                    (Lymphocytes, Monocytes)

Granulocytes

1. Neutrophils (55–70% of total WBCs):
   • Feature multi-lobed nuclei (3 to 5 lobes); also termed polymorphonuclear leukocytes (PMNs or "segs").
   • Primary first line of defense against acute bacterial infections. They perform active phagocytosis, engulfing and destroying bacteria. Immature neutrophils with unsegmented nuclei are called "bands"; a high band count ("shift to the left") indicates severe bacterial infection.
2. Eosinophils (1–4% of total WBCs):
   • Feature bilobed nuclei and distinct reddish-orange cytoplasmic granules.
   • Specialize in combating parasitic infections (e.g., helminth worms) and modulating allergic inflammatory responses.
3. Basophils (0.5–1% of total WBCs):
   • Feature dark blue/purple granules that often obscure the nucleus.
   • Release histamine (promotes vasodilation and vascular permeability during allergic reactions) and heparin (an endogenous anticoagulant).

Agranulocytes

1. Lymphocytes (20–40% of total WBCs):
   • Small cells with large, dark-staining spherical nuclei occupying most of the cytoplasm.
   • Primary mediators of adaptive immunity. Divided into T-lymphocytes (cell-mediated immunity) and B-lymphocytes (humoral immunity via antibody production). Increased during viral infections.
2. Monocytes (2–8% of total WBCs):
   • The largest leukocyte in peripheral circulation, featuring a characteristic kidney-bean-shaped nucleus.
   • Monocytes circulate briefly before migrating into body tissues to become tissue macrophages. They perform powerful phagocytosis of cellular debris, foreign pathogens, and dead cells during chronic infections.

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Clinical Scenarios, Specimen Integrity & Traps

• Specimen Hemolysis Trap: Rough specimen handling, drawing blood through a small needle (e.g., 25-gauge), or forcing blood into a vacuum tube can rupture delicate erythrocyte membranes (hemolysis). Hemolysis turns plasma or serum pink or red, causing falsely elevated potassium, lactate dehydrogenase (LDH), and iron values.
• Buffy Coat Verification: When centrifuging anticoagulated whole blood (e.g., green or lavender top), three distinct layers form: a bottom layer of dense RBCs (~45%), a clear top layer of plasma (~55%), and a very thin grey-white middle layer called the buffy coat (<1%), which contains the leukocytes and thrombocytes.
• Centrifugation Delay Failure: Failing to allow a red-top or SST tube to clot completely (30-60 minutes) before centrifugation leads to incomplete clot separation, resulting in fibrin strands contaminating the serum supernatant and clogging automated chemistry analyzers.