INTRODUCTION TO HEMATOLOGY

1.1 What is Hematology?

Definition

Hematology is the branch of biomedical science and laboratory medicine concerned with the study of blood, blood-forming tissues, blood cells, and disorders of the hematopoietic system.

The term hematology is derived from:

Haema = blood
Logos/Logia = study

Thus, hematology literally means the study of blood.

However, modern hematology is much broader than the study of blood itself. It includes the study of:

Blood cells
Plasma
Bone marrow
Blood cell formation
Blood cell maturation
Hemostasis and coagulation
Blood cell disorders
Hematological malignancies
Laboratory investigation of hematological diseases

1.2 Scope of Hematology

Hematology can be considered from two major perspectives:

A. Normal Hematology

The study of:

Normal blood composition
Normal blood cell structure
Normal blood cell functions
Blood cell production
Blood cell maturation
Normal physiological variation

B. Clinical Hematology

The study of:

Disorders of blood cells
Disorders of bone marrow
Disorders of hemostasis and coagulation
Hemoglobin disorders
Hematological malignancies
Laboratory diagnosis and monitoring of hematological diseases

1.2.1 Major Areas of Hematology

Area	Main focus
Specimen collection	Proper collection and handling of blood and bone marrow
Hematopoiesis	Formation and development of blood cells
Erythrocytes	RBC structure, function and maturation
Hemoglobin	Structure, function and measurement
Erythrocyte studies	ESR, reticulocytes, indices and morphology
Leukocytes	WBC development, function and enumeration
Platelets	Platelet development and function
Hemostasis	Mechanisms preventing blood loss
Coagulation	Formation and regulation of blood clots
Hematological disorders	Diseases affecting blood and blood-forming tissues
Blood film examination	Morphological examination of peripheral blood
Special tests	Laboratory investigations used in specific hematological conditions

1.3 Importance of Hematology in Medical Laboratory Technology

Blood is one of the most frequently examined clinical specimens.
Laboratory examination of blood can provide information about:

Oxygen-carrying capacity
Immune status
Inflammation
Blood cell production
Hemostasis
Nutritional deficiencies
Infection
Hematological malignancy
Systemic diseases

For example:

Low hemoglobin; May indicate anemia.
Increased leukocyte count; May occur in infection, inflammation, or some hematological disorders.
Decreased platelet count; May be associated with an increased risk of bleeding.
Abnormal blood-cell morphology; May provide clues to underlying disease. Therefore, hematology laboratory testing contributes significantly to: Screening → Diagnosis → Differential diagnosis → Disease monitoring → Treatment monitoring

1.4 Overview of the Vascular System

The vascular system is the network of blood vessels through which blood circulates throughout the body.
The major blood vessels are:

Arteries
Arterioles
Capillaries
Venules
Veins

The basic pathway is:

Heart → Arteries → Arterioles → Capillaries → Venules → Veins → Heart

1.5 Heart and Circulation

The heart is a muscular organ that functions as the major pump of the circulatory system.
It maintains continuous movement of blood through the vascular system.
There are two major circulatory systems:

Pulmonary circulation
Heart → Lungs → Heart
Its major functions are:

Delivery of deoxygenated blood to the lungs
Removal of carbon dioxide
Oxygenation of blood

Systemic circulation

Heart → Body tissues → Heart

Its major functions are:

Delivery of oxygen and nutrients
Removal of metabolic waste products

1.6 Arteries

Arteries are vessels that carry blood away from the heart.
Most systemic arteries carry oxygenated blood.

Exception

Pulmonary arteries carry deoxygenated blood from the heart to the lungs.
Arteries generally have:

Thick walls
Elastic tissue
Smooth muscle
Relatively high pressure

The elastic and muscular components allow arteries to withstand and regulate pulsatile blood flow.

1.7 Arterioles

Arterioles are smaller branches of arteries.
They regulate the amount of blood entering capillary beds.
Their smooth muscle allows:

Vasoconstriction; Reduction in vessel diameter.

Vasodilation; Increase in vessel diameter.

Therefore, arterioles contribute significantly to:

Regulation of tissue blood flow
Peripheral vascular resistance
Blood pressure regulation

1.8 Capillaries

Capillaries are the smallest blood vessels.
They connect arterioles with venules.
Their walls are extremely thin, allowing exchange between blood and tissues.
Substances exchanged include:

Oxygen
Carbon dioxide
Glucose
Amino acids
Water
Electrolytes
Hormones
Metabolic waste

Thus, the capillary circulation is particularly important for tissue perfusion and exchange.

1.9 Venules

Venules collect blood from capillary beds.
Small venules join together to form larger veins.
They also participate in inflammatory processes because leukocytes can migrate through venular walls into tissues.

1.10 Veins

Veins carry blood toward the heart.
Most systemic veins contain deoxygenated blood.

Exception

Pulmonary veins carry oxygenated blood from the lungs to the heart.
Compared with arteries, veins generally have:

Thinner walls
Larger lumens
Lower pressure

Many veins contain valves that help prevent backflow.

1.11 Blood Volume

The average adult contains approximately: 5–6 liters of blood
Blood represents approximately: 7–8% of body weight
Blood volume varies according to:

Age
Body size
Sex
Pregnancy
Hydration
Disease conditions

A newborn infant has a much smaller absolute blood volume, approximately several hundred milliliters depending on body size.

1.12 Composition of Blood

Blood is a specialized connective tissue (connect, support, protect, separate, and transport different parts of the body).
It consists of two major components:

1. Plasma

The fluid component.
Approximately 55% of blood volume.

2. Formed elements

The cellular and cell-derived components, primarily:

Erythrocytes
Leukocytes
Platelets

1.13 Plasma

Plasma is the pale yellow fluid portion of blood.
It provides the medium in which blood cells circulate.
Plasma contains approximately:

90–95% water
Proteins
Electrolytes
Nutrients
Hormones
Enzymes
Dissolved gases
Metabolic products
Waste products

1.13.1 Water

Water is the major constituent of plasma.
Functions include:

Acting as a solvent
Transporting dissolved substances
Distributing heat
Supporting chemical reactions

1.14 Plasma Proteins

The major plasma proteins include:

Albumin

Albumin is the most abundant plasma protein.
Major functions:

Maintenance of colloid osmotic pressure
Transport of various substances
Binding of hormones, fatty acids, drugs and other molecules

Globulins

Globulins include several groups of proteins involved in:

Immunity
Transport
Inflammatory responses

Fibrinogen

Fibrinogen is an important protein involved in blood coagulation.
During coagulation:
Fibrinogen → Fibrin
Fibrin forms the structural framework of a blood clot.

Detailed coagulation mechanisms will be discussed later in the course.

1.15 Other Components of Plasma

Electrolytes

Important plasma electrolytes include:

Sodium
Potassium
Calcium
Chloride
Bicarbonate
Phosphate

They contribute to:

Osmotic balance
Acid–base balance
Nerve function
Muscle contraction
Cellular function

Nutrients

Blood transports nutrients absorbed from the gastrointestinal tract.

Examples include:

Glucose
Amino acids
Fatty acids
Glycerol
Vitamins
Minerals

Hormones

Blood transports hormones from endocrine glands to target organs.

Examples:

Insulin
Thyroxine
Cortisol
Growth hormone

Waste Products

Blood carries metabolic waste products from tissues to organs responsible for their elimination.

Examples:

Waste	Major route
Carbon dioxide	Lungs
Urea	Kidneys
Creatinine	Kidneys
Uric acid	Mainly kidneys

1.16 Formed Elements of Blood

The formed elements consist of:

1. Erythrocytes

Also called red blood cells (RBCs).

2. Leukocytes

Also called white blood cells (WBCs).

3. Platelets

Also called thrombocytes.

1.17 Erythrocytes

Erythrocytes are the most numerous formed elements of blood.
Their primary function is the transport of respiratory gases.

Major functions

Transport oxygen
Participate in carbon dioxide transport
Contribute to acid–base buffering

Mature human RBCs are:

Biconcave
Anucleate
Flexible

They contain large quantities of hemoglobin.

1.18 Leukocytes

Leukocytes are white blood cells involved primarily in immune defense and inflammatory responses.

The five major types are:

Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils

They can be broadly divided into:

Granulocytes

Neutrophils
Eosinophils
Basophils

Agranulocytes

Lymphocytes
Monocytes

1.19 Platelets

Platelets are small, membrane-bound cytoplasmic fragments derived from megakaryocytes.
Their primary role is in hemostasis.
Major functions include:

Adhesion
Activation
Aggregation
Formation of the primary platelet plug
Participation in coagulation
Support of clot formation

1.20 Functions of Blood

The major functions of blood can be classified into:

1. Transportation
2. Regulation
3. Protection

1.20.1 Transportation

Oxygen

Hemoglobin within RBCs transports oxygen from the lungs to tissues.

Lungs → Blood → Tissues

Nutrients

Blood transports absorbed nutrients to cells.

Examples:

Glucose
Amino acids
Lipids
Vitamins

Metabolic Waste

Blood transports waste products away from tissues.

Examples:

CO₂ → lungs
Urea → kidneys
Creatinine → kidneys

Hormones

Blood transports hormones from endocrine glands to their target tissues.

1.21 Regulatory Functions

Blood contributes to maintenance of homeostasis, meaning the relatively stable internal environment required for normal cellular function.

1.21.1 Regulation of pH

Normal blood pH is approximately:

7.35–7.45

Blood contains several buffering systems, particularly:

Bicarbonate
Hemoglobin
Plasma proteins
Phosphate

The lungs and kidneys also contribute substantially to acid–base regulation.

1.21.2 Regulation of Osmotic Pressure

Plasma proteins, especially albumin, contribute to maintaining plasma colloid osmotic pressure.

This helps regulate the movement of water between:

Blood
Interstitial fluid

1.21.3 Temperature Regulation

Blood absorbs heat produced by metabolically active tissues and distributes it throughout the body.
Increased blood flow to the skin facilitates heat loss.

1.22 Protective Functions

1.22.1 Protection Against Blood Loss

When a blood vessel is damaged, blood components participate in hemostasis.
At a basic level:

Vascular injury → Platelet response → Coagulation → Fibrin clot

This prevents excessive blood loss.

1.22.2 Immune Protection

Leukocytes provide protection against:

Bacteria
Viruses
Fungi
Parasites
Foreign substances

Different WBC populations have specialized functions.

1.23 Relationship Between Blood Constituents and Their Functions

Component	Main function
Plasma water	Solvent and transport medium
Albumin	Osmotic pressure and transport
Globulins	Immunity and transport
Fibrinogen	Coagulation
Electrolytes	Fluid, acid–base and cellular regulation
RBCs	Respiratory gas transport
Hemoglobin	Primarily oxygen transport
WBCs	Immune defense
Platelets	Hemostasis

1.24 Introduction to Blood Cell Disorders

Blood disorders may affect:

Number of blood cells
Size of blood cells
Shape of blood cells
Cellular morphology
Cellular function
Blood-cell production
Blood-cell destruction
Hemoglobin
Bone marrow
Coagulation mechanisms

Major categories

RBC disorders

Examples:

Anemia
Erythrocytosis/polycythemia
Hemoglobin disorders

WBC disorders

Examples:

Leukocytosis
Leukopenia
Leukemias

Platelet disorders

Examples:

Thrombocytopenia
Thrombocytosis
Platelet function disorders

Coagulation disorders

Examples:

Disorders associated with abnormal coagulation factor function or deficiency

1.25 Hematology Laboratory: Basic Concept

A hematology laboratory converts a clinical specimen into laboratory information that can assist clinical decision-making.

The basic process is:

Patient → Specimen → Laboratory test → Result → Interpretation → Clinical decision

For example:

Patient → Blood specimen → CBC → Hemoglobin/RBC/WBC/platelet results → Interpretation

Red-cell investigations

Hemoglobin estimation
RBC count
PCV/HCT
RBC indices
Reticulocyte count
ESR
Osmotic fragility

White-cell investigations

Total leukocyte count
Differential leukocyte count
Absolute cell counts where applicable

Platelet investigations

Platelet count
Other platelet-related investigations

Morphological investigations

Peripheral blood film
RBC morphology
WBC morphology
Platelet morphology
Detection of abnormal/immature cells

Parasitological examination

Identification of malarial parasites in peripheral blood films