Blood and Its Components

Objectives

• Blood

• Components of Blood

• Functions of Blood

Blood

Blood is a vital fluid that circulates throughout the body, delivering oxygen and nutrients while removing waste products.

Basic Characteristics of Blood

• Average volume: 4.5–6 liters in an adult (about 7–8% of body weight)

• pH: ~7.35–7.45 (slightly alkaline)

• Temperature: ~38°C (slightly warmer than normal body temperature)

Blood consists of several components, each performing specific functions necessary for the proper functioning of the body.

Components of Blood

Blood consists of two main components:

1. Plasma (≈55%)

2. Formed Elements (≈45%)

Formed elements include:

• Erythrocytes (Red Blood Cells – RBCs)

• Leukocytes (White Blood Cells – WBCs)

• Thrombocytes (Platelets)

Plasma (≈55% of Blood Volume)

Plasma is the liquid portion of blood, appearing as a pale-yellow fluid composed mainly of water, proteins, and dissolved substances.

Composition of Plasma

1. Water (≈90–92%)

• Acts as a solvent for substances

• Helps in transport of materials

• Regulates body temperature

2. Plasma Proteins (≈7–8%)

• Albumin: Maintains osmotic pressure

• Globulins: Provide immunity

• Fibrinogen: Helps in blood clotting

3. Other Solutes

• Ions: Sodium, calcium, chloride, potassium, etc.

• Nutrients: Glucose, amino acids, lipids

• Waste products: Urea, uric acid

• Hormones and enzymes

Note:
Plasma without clotting factors is known as Serum.

Formed Elements (≈45% of Blood Volume)

These are the cellular components of blood.

They include:

• Red Blood Cells (RBCs) – Erythrocytes

• White Blood Cells (WBCs) – Leukocytes

• Platelets – Thrombocytes

Red Blood Cells (RBCs) – Erythrocytes

• Percentage of formed elements: ~99%

• Count:

  • Males: 4.7–6.1 million cells/μL

  • Females: 4.2–5.4 million cells/μL

• Lifespan: Approximately 120 days

• Formation: Bone marrow

• Destruction: Mainly in the spleen

Structure

• Shape: Biconcave, flexible disc

• Provides increased surface area for gas exchange

Main Component

• Hemoglobin (about 33% of cell weight)

Function

• Transport of oxygen (O₂) from lungs to tissues

• Transport of carbon dioxide (CO₂) from tissues to lungs

White Blood Cells (WBCs) – Leukocytes

• Percentage of formed elements: <1%

• Count: 4,000–11,000 cells/μL

Main Function

• Defense and immunity

• Protection against infections and foreign substances

WBCs are divided into two major groups:

1. Granulocytes

These cells contain granules in their cytoplasm.

Types:

• Neutrophils (55–70%)

  • Perform phagocytosis of bacteria

  • Act as first responders during infection

• Eosinophils (1–4%)

  • Involved in allergic reactions

  • Defense against parasitic infections

• Basophils (<1%)

  • Release histamine

  • Mediate allergic and inflammatory responses

2. Agranulocytes

These cells do not contain cytoplasmic granules.

Types:

• Monocytes (2–8%)

  • Perform phagocytosis

  • Differentiate into macrophages

• Lymphocytes

  • Responsible for specific immune responses

  • Include B cells, T cells, and Natural Killer (NK) cells

Platelets (Thrombocytes)

• Count: 150,000–400,000/μL

• Shape: Small, disc-shaped cell fragments

• Origin: Fragments of megakaryocytes in bone marrow

Functions

• Blood clotting

• Repair of damaged blood vessels

• Formation of platelet plug during injury

Functions of Blood

Blood performs several essential functions in the body.

1. Transport

• Oxygen via hemoglobin

• Nutrients from digestive tract

• Waste products to kidneys and lungs

• Hormones to target organs

2. Protection

• Protection against infections (WBCs, antibodies)

• Blood clotting to prevent excessive blood loss

3. Regulation

• Body temperature regulation through heat distribution

• pH buffering via bicarbonate and proteins

• Fluid balance maintained through osmotic pressure