Blood: Structure, Function, and Laboratory Analysis

Blood: Structure and Function

Composition of Whole Blood

Blood is a fluid connective tissue composed of a liquid matrix (plasma) and suspended formed elements. Understanding its composition is essential for interpreting laboratory tests and physiological functions.

• Plasma: The liquid portion of blood, making up about 55% of whole blood. Contains water, proteins (albumins, globulins, fibrinogen), nutrients, hormones, and waste products.

• Formed Elements: Cellular components including erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Major Constituents of Plasma

Plasma is primarily water (about 92%), with dissolved proteins and other solutes.

• Plasma Proteins: Albumins (maintain osmotic pressure), globulins (immune function), fibrinogen (clotting).

• Other Solutes: Electrolytes, nutrients, gases, hormones, and waste products.

Formed Elements of Blood

Types and Functions of Blood Cells

The formed elements include erythrocytes, platelets, and five types of leukocytes. Each has distinct structural features and functions.

• Erythrocytes (RBCs): Biconcave, anucleate cells responsible for transporting oxygen and carbon dioxide.

• Platelets: Small cytoplasmic fragments involved in blood clotting.

• Leukocytes (WBCs): Immune cells classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

Classification and Identification of Leukocytes

Leukocytes are categorized based on their nuclear morphology and cytoplasmic granules. Their functions are vital for immunity and homeostasis.

• Neutrophils: Multi-lobed nucleus, most numerous (60-70%), phagocytose bacteria.

• Lymphocytes: Large round nucleus, 20-30%, specific immunity (B and T cells).

• Monocytes: Kidney-shaped nucleus, 3-8%, become macrophages in tissues.

• Eosinophils: Red granules, 2-4%, combat parasitic infections and modulate allergic responses.

• Basophils: Blue granules, 1%, release histamine, involved in inflammation and allergy.

Summary Table: Formed Elements of Blood

The following tables summarize the key features, normal counts, and functions of blood cells.

Cell Type	Description	Normal Count (cells/mm3)	Function
Erythrocytes	Biconcave, anucleate disc	4-6 million	Transport O2 and CO2
Neutrophils	Multi-lobed nucleus, pale granules	2500-7000	Phagocytosis of pathogens
Eosinophils	Bilobed nucleus, red granules	100-400	Kill parasites, modulate allergy
Basophils	Lobed nucleus, blue granules	20-50	Release histamine, inflammation
Lymphocytes	Round nucleus, scant cytoplasm	1500-3000	Specific immunity
Monocytes	Kidney-shaped nucleus	100-700	Phagocytosis, become macrophages
Platelets	Cytoplasmic fragments	150,000-400,000	Blood clotting

Blood Tests and Laboratory Procedures

Hematocrit Test

The hematocrit test measures the percentage of formed elements (mainly RBCs) in whole blood. It is a key indicator of blood health.

• Formula: $\text{Hematocrit} = \frac{\text{Volume of formed elements}}{\text{Total blood volume}} \times 100\%$

• Normal Values: Male: ~46%, Female: ~42%

• Procedure: Blood is placed in a capillary tube, sealed, and centrifuged. The proportion of RBCs is measured using a hematocrit reader.

Tallquist Hemoglobin Test

The Tallquist test estimates hemoglobin concentration by comparing the color of a blood stain to a standardized color scale.

• Hemoglobin: Protein in RBCs that binds and transports O2 and CO2.

• Normal Values: Adult male: 14–18 g/dL, Adult female: 12–16 g/dL

• Procedure: Place a drop of blood on Tallquist paper, match color to scale, determine hemoglobin concentration.

Blood Type Tests: ABO and Rh Typing

Blood typing identifies the presence of A, B, and Rh antigens on RBCs. Agglutination reactions with specific antisera determine blood type.

• Testing Materials: Blood sample, Anti-A, Anti-B, and Anti-D (Rh) sera.

• Procedure: Add each serum to separate spots, mix with blood, observe for agglutination (clumping).

• Interpretation: Agglutination in a spot indicates presence of the corresponding antigen.

• Example: Agglutination in A spot only = A- blood type.

Normal Values and Clinical Interpretation

Reference Ranges for Blood Cell Counts

• RBC Count: 4-6 million cells/mm3

• WBC Count: 4,000-10,800 cells/mm3

• Platelet Count: 150,000-400,000 cells/mm3

• WBC Differential: Neutrophils 60-70%, Lymphocytes 20-30%, Monocytes 3-8%, Eosinophils 2-4%, Basophils 1%

Clinical Significance

• Low Hematocrit: May indicate anemia.

• High Hematocrit: May indicate polycythemia.

• Abnormal WBC Counts: May indicate infection, inflammation, or hematological disorders.

Summary

Blood is a complex tissue with critical roles in transport, immunity, and homeostasis. Laboratory tests such as hematocrit, hemoglobin, and blood typing are essential for clinical assessment and diagnosis.