Hematology

Definition and Scope

Hematology is the study of the formed elements of the blood (cells) and blood-forming tissues. It encompasses both the quantitative and qualitative analysis of blood cells, which is essential for diagnosing and monitoring various health conditions.

• Formed elements: Red blood cells (RBC), white blood cells (WBC), and platelets (PLT).

• Quantitative analysis: Measures the number of each type of cell.

• Qualitative analysis: Examines the appearance and morphology of each cell type.

Laboratory tests in hematology are used to detect and monitor:

• General well-being

• Anemias

• Leukemias

• Inherited blood disorders

• Cancer treatment monitoring

The Circulatory System

Functions of the Circulatory System

The circulatory system is responsible for transporting substances throughout the body and maintaining homeostasis.

• Transport: Delivers oxygen (O2), nutrients, water, and hormones to tissues and cells.

• Removal: Removes carbon dioxide (CO2) and waste products from tissues and cells.

• Regulation: Helps regulate body temperature.

• Protection: Protects against infections.

Cardiopulmonary and Systemic Circulation

• Cardiopulmonary circulation: Blood circulates from the heart to the lungs and back. In the lungs, O2 is picked up and CO2 is released.

• Systemic circulation: Blood is carried from the heart to the tissues and back. O2 is delivered to tissues in exchange for CO2.

Blood Vessels

• Arteries: Thick-walled, elastic, and muscular vessels that carry oxygenated blood away from the heart.

• Capillaries: The smallest blood vessels, connecting arterioles and venules. Their thin walls allow for the exchange of gases, nutrients, and waste products.

• Veins: Carry deoxygenated blood from capillaries back to the heart. They are less thick, elastic, or muscular than arteries and contain valves to ensure one-way blood flow.

• Fact: The average human body contains approximately 60,000 miles of blood vessels.

Composition of Blood

Blood Components

Blood is a specialized body fluid with two main components: plasma and formed elements.

• Plasma: Makes up 50-60% of blood volume. It is a complex solution, over 90% water, with dissolved proteins, lipids, hormones, antibodies, and other solutes.

• Formed elements: Comprise the remaining 40-50% and include RBCs, WBCs, and platelets.

Cellular Elements of Blood

Erythrocytes (Red Blood Cells, RBCs)

• Most numerous blood cell type.

• Lifespan: Approximately 120 days.

• Function: Transport O2 to tissues and CO2 to the lungs, primarily via the protein hemoglobin.

Leukocytes (White Blood Cells, WBCs)

• Least numerous blood cell type.

• Types: Neutrophils, basophils, eosinophils, lymphocytes, and monocytes.

• Lifespan: Varies by cell type.

• Function: Immunity and defense against infection.

Thrombocytes (Platelets)

• Cell fragments released from megakaryocytes in the bone marrow.

• Lifespan: About 10 days.

• Function: Help stop bleeding by forming plugs in injured vessel walls (hemostasis).

Hematopoiesis

Blood Cell Production

Hematopoiesis is the process of blood cell formation, beginning in embryonic development and continuing throughout life.

• Starts in the yolk sac, progresses to the fetal liver, and then to the bone marrow before birth.

• Bone marrow stores immature blood cells and releases mature cells as needed.

• All blood cell types mature gradually; stages of maturation are continuous.

• In adults, the total blood volume is about 6 liters (7-8% of body weight), with 45% cells and 55% plasma.

Erythrocyte Function and Maturation

Function of Red Blood Cells

• Main function: Carry oxygen to body cells and remove carbon dioxide.

• Oxygen is transported by binding to hemoglobin (Hb) within RBCs.

• The concentration of Hb in blood is a measure of its oxygen-carrying capacity.

Maturation of Erythrocytes

• RBCs begin as nucleated cells in the bone marrow.

• As they mature, they decrease in size, the nucleus condenses and is eventually extruded, resulting in a biconcave, anucleate cell.

Aging and Removal of Erythrocytes

• With age, RBC membranes become less flexible, hemoglobin concentration increases, and enzyme activity decreases.

• Old RBCs are removed by phagocytosis; proteins and iron are recycled, and waste is excreted as bilirubin in urine and feces.

Hemoglobin: Structure and Function

Molecular Structure of Hemoglobin

• Hemoglobin is a protein in RBCs responsible for oxygen transport.

• Composed of four globin protein chains, each with a heme group containing iron.

• Each hemoglobin molecule can bind four oxygen molecules.

• Iron is recycled as old RBCs are destroyed; insufficient iron leads to anemia.

Types of Hemoglobin

• Hemoglobin A (HbA): 95-98% of adult hemoglobin; 2 alpha and 2 beta chains.

• Hemoglobin A2 (HbA2): 2-3% of adult hemoglobin; 2 alpha and 2 delta chains.

• Hemoglobin F (HbF): Fetal hemoglobin; 2 alpha and 2 gamma chains.

• Hemoglobin S: Variant found in sickle cell anemia (different protein structure).

Specimens for Hematology Testing

Types of Blood Samples

• Capillary puncture: Yields a small blood sample; no chemicals added, so cell morphology is unaltered.

• Venous blood: Obtained by venipuncture for larger volumes; often treated with anticoagulants such as EDTA.

Clinical Hematology Procedures

Common Laboratory Tests

• Complete Blood Count (CBC): Includes hemoglobin (Hb), hematocrit (Hct), RBC count, WBC count with differential, and platelet estimate.

• Red Blood Cell Indices: Calculated values to assess hemoglobin content, concentration, and average RBC size.

Anticoagulants

• EDTA (Ethylenediaminetetraacetic acid): Prevents clotting by chelating calcium ions.

Standard Operating Procedures

• All laboratory procedures must have documented protocols for equipment, supplies, and reagents.

Hemoglobin Tests

Methods and Reference Ranges

• Hemoglobin can be measured separately or as part of a CBC, often using automated instruments.

• Cyanmethemoglobin method: Uses Drabkin's reagent to form a stable colored product measured photometrically.

• Point-of-care assay: HemoCue Hemoglobin Analyzer forms hemiglobinazide for rapid measurement.

• Reference ranges: Female: 12.3–15.3 g/dL; Male: 14.0–17.5 g/dL.

Hematocrit Tests

Principle and Methods

• Hematocrit (Packed Cell Volume): Measures the proportion of blood volume occupied by RBCs.

• Determines the oxygen-carrying capacity of blood.

• Methods: Microhematocrit (manual, spun in capillary tubes) and automated analyzers (part of CBC).

• Reference ranges: Female: 36–45%; Male: 42–50%.

Procedure for Spun Hematocrit

1. Clean finger with alcohol.

2. Puncture skin and collect blood in a microhematocrit tube.

3. Seal tube and centrifuge.

4. Read the packed cell volume as a percentage of total blood volume.

Red Blood Cell Indices

Definitions and Calculations

• Mean Corpuscular Volume (MCV): Average volume of an RBC in femtoliters (fL).

Formula:

• Reference range: 80–96 fL.

• Mean Corpuscular Hemoglobin (MCH): Average amount (weight) of hemoglobin per RBC in picograms (pg).

Formula:

• Reference range: 27–33 pg.

• Mean Corpuscular Hemoglobin Concentration (MCHC): Average concentration of hemoglobin in a given volume of packed RBCs (g/dL).

Formula:

• Reference range: 33–36 g/dL.

Summary Table: Red Blood Cell Indices

Example Calculation: If Hct = 38% and RBC = 4.0 x 106/μL, then:

fL

Additional info: These indices are essential for classifying anemias and other blood disorders.