Blood: Structure, Function, and Laboratory Analysis

Blood: Structure, Function, and Laboratory Analysis

Formed Elements of Blood

Blood is a specialized connective tissue composed of plasma and formed elements. The formed elements include erythrocytes, leukocytes, and platelets, each with distinct functions and relative abundances.

• Erythrocytes (Red Blood Cells, RBCs): Most abundant formed element. Function: Transport oxygen and carbon dioxide via hemoglobin. Biconcave shape increases surface area for gas exchange.

• Neutrophils: Most numerous type of white blood cell (WBC). Function: Phagocytosis of bacteria and debris; first responders to infection.

• Eosinophils: Less abundant. Function: Combat parasitic infections and modulate allergic responses.

• Basophils: Rarest WBC. Function: Release histamine and heparin during inflammatory and allergic reactions.

• Lymphocytes: Second most common WBC. Function: Mediate immune responses; include B cells (produce antibodies) and T cells (cell-mediated immunity).

• Monocytes: Largest WBC. Function: Differentiate into macrophages for phagocytosis of pathogens and debris.

• Platelets (Thrombocytes): Small cell fragments. Function: Initiate blood clotting (hemostasis) by forming platelet plugs and releasing clotting factors.

• Plasma: Straw-colored liquid matrix. Function: Transports nutrients, hormones, waste products, and proteins (e.g., albumin, globulins, fibrinogen).

Physical Characteristics and Components of Plasma

Plasma is the liquid component of blood, making up about 55% of total blood volume. It is mostly water but contains dissolved proteins, electrolytes, nutrients, gases, and waste products.

• Physical Characteristics: Clear, pale yellow fluid; slightly alkaline (pH ~7.4); viscosity greater than water.

• Main Components:

  • Water (~92%)

  • Plasma proteins (~7%): albumin (maintains osmotic pressure), globulins (immune function), fibrinogen (clotting)

  • Other solutes (~1%): electrolytes (Na+, K+, Ca2+), nutrients (glucose, amino acids), gases (O2, CO2), waste products (urea, creatinine)

Laboratory Techniques in Hematology

Several laboratory techniques are used to analyze blood and its components for diagnostic and educational purposes.

• Blood Smear and Staining: A drop of blood is spread on a slide, air-dried, and stained (e.g., Wright's stain) to visualize formed elements under a microscope. This allows identification of cell types and assessment of morphology.

• Hematocrit (Packed Cell Volume, PCV): The percentage of blood volume occupied by erythrocytes. Determined by centrifuging a blood sample in a capillary tube.

  • Normal values: Adult males: 40–54%; Adult females: 37–47%

  • Interpretation: Low hematocrit indicates anemia; high hematocrit may indicate polycythemia or dehydration.

• Blood Typing (ABO and Rh): Determines the presence of specific antigens (agglutinogens) on RBCs and corresponding antibodies (agglutinins) in plasma.

  • ABO System: Four main blood types: A, B, AB, O, based on presence/absence of A and B antigens.

  • Rh System: Rh-positive (D antigen present) or Rh-negative (D antigen absent).

  • Blood Typing Test: Mixing blood with anti-A, anti-B, and anti-D sera to observe agglutination (clumping), indicating presence of specific antigens.

Table: ABO and Rh Blood Groups

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Receive From
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	A, B, AB, O
O	None	Anti-A, Anti-B	O
Rh+	D antigen	None	Rh+, Rh-
Rh-	None	Anti-D (if sensitized)	Rh-

Abnormal Blood Cell Counts and Disorders

Abnormalities in the number or function of blood cells can indicate various pathological conditions.

• Leukocytosis: Elevated white blood cell count; often indicates infection, inflammation, or leukemia.

• Leukopenia: Decreased white blood cell count; increases risk of infection, may result from bone marrow suppression or certain diseases.

• Polycythemia: Increased red blood cell count; may be due to chronic hypoxia, dehydration, or polycythemia vera (a bone marrow disorder).

• Anemia: Decreased red blood cell count or hemoglobin; leads to reduced oxygen-carrying capacity and symptoms like fatigue and pallor.

• Thrombocytosis: Elevated platelet count; increases risk of abnormal clotting (thrombosis).

• Thrombopenia (Thrombocytopenia): Decreased platelet count; increases risk of bleeding and bruising.

• Leukemia: Malignant proliferation of abnormal white blood cells; impairs normal blood cell production and function.

Key Equations

• Hematocrit Calculation:

$\text{Hematocrit (\%)} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

Example Applications

• Blood Smear: Used to diagnose anemia, infections, and blood disorders by examining cell morphology.

• Blood Typing: Essential for safe blood transfusions and organ transplantation.

• Hematocrit: Used to assess hydration status and diagnose anemia or polycythemia.

Additional info: Laboratory procedures such as blood smears and hematocrit determination are standard in clinical and educational settings to assess blood health and diagnose disorders.