Edexcel iGCSE Biology-3.9 The Blood- Study Notes- New Syllabus

Composition of Blood

📝 Introduction

• Blood is a specialised connective tissue that circulates throughout the body.
• It transports essential substances to cells and removes waste products.
• Blood also helps protect the body from infection and prevents excessive blood loss after injury.
• Human blood is made up of four main components: plasma, red blood cells, white blood cells and platelets.
• Each component has a specialised structure that allows it to perform a specific function efficiently.

🩸 Components of Blood

Blood consists of:

• Red blood cells (Erythrocytes)
• White blood cells (Leucocytes)
• Platelets (Thrombocytes)
• Plasma

Each component works together to keep the body functioning normally.

🔴 Red Blood Cells (RBCs / Erythrocytes)

Definition

• Red blood cells are specialised blood cells responsible for transporting oxygen around the body.

Structure of Red Blood Cells

Biconcave Disc Shape

• Red blood cells have a biconcave disc shape.
• They are thinner in the centre and thicker around the edges.

Importance

• Provides a large surface area for rapid diffusion of oxygen.
• Shortens the diffusion distance between oxygen and haemoglobin.

No Nucleus

• Mature human red blood cells do not contain a nucleus.

Importance

• Creates more space for haemoglobin.
• Allows each cell to carry a larger amount of oxygen.

Contain Haemoglobin

• Red blood cells are packed with haemoglobin, a red pigment containing iron.

Importance

• Haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin.
• Oxygen is released when the blood reaches body tissues.

Flexible Cell Membrane

• The membrane is flexible and elastic.

Importance

• Allows red blood cells to squeeze through very narrow capillaries without breaking.

Function of Red Blood Cells

• Transport oxygen from the lungs to body cells.
• Carry a small amount of carbon dioxide from tissues back to the lungs.
• Ensure cells receive enough oxygen for aerobic respiration.

⚪ White Blood Cells (WBCs / Leucocytes)

Definition

• White blood cells are specialised cells that protect the body against disease-causing microorganisms (pathogens).
• They are much fewer in number than red blood cells.
• Unlike red blood cells, they contain a nucleus.

Types of White Blood Cells

There are many types of white blood cells, but the two main types studied are:

• Phagocytes
• Lymphocytes

🦠 Phagocytes

Structure

• Large, irregularly shaped cells.
• Can change shape to surround pathogens.
• Contain many digestive enzymes.

Function

Phagocytes protect the body by:

• Surrounding invading pathogens.
• Engulfing them.
• Digesting them using enzymes.

This process is called phagocytosis.

🛡️ Lymphocytes

Structure

• Small cells with a large nucleus.

Function

Lymphocytes defend the body by:

• Producing antibodies.
• Antibodies attach specifically to antigens on pathogens.
• This helps destroy the pathogens.

Some lymphocytes also form memory cells.

These allow the body to respond much more quickly if the same pathogen enters again.

🟡 Platelets (Thrombocytes)

Definition

• Platelets are small fragments of cells found in the blood.
• They are not complete cells because they do not contain a nucleus.

Structure

• Very small.
• Irregular in shape.
• Produced from larger cells in the bone marrow.

Function

Platelets help stop bleeding by:

• Gathering at the site of a damaged blood vessel.
• Releasing chemicals that start the clotting process.
• Helping form a fibrin mesh.
• Trapping blood cells to produce a blood clot.

Importance of Blood Clotting

• Prevents excessive blood loss.
• Seals the wound.
• Reduces the risk of pathogens entering the body.

🟠 Plasma

Definition

• Plasma is the straw-coloured liquid part of blood.
• It forms about 55% of the total blood volume.

Composition of Plasma

Plasma consists mainly of:

• Water
• Dissolved proteins
• Mineral ions
• Nutrients
• Hormones
• Waste products

Functions of Plasma

Plasma transports many substances throughout the body.

Nutrients

Such as:

• Glucose
• Amino acids
• Lipids
• Vitamins

Waste Products

Including:

• Carbon dioxide
• Urea

These are transported to organs where they can be removed.

Hormones

• Carries hormones from endocrine glands to target organs.

Antibodies

• Transports antibodies produced by lymphocytes.

Heat

• Distributes heat around the body.
• Helps maintain a stable body temperature.

🌟 How the Blood Components Work Together

• Red blood cells transport oxygen needed for respiration.
• White blood cells protect the body from infection.
• Platelets prevent excessive blood loss after injury.
• Plasma transports blood cells and dissolved substances throughout the body.

Together, these components maintain normal body function.

📊 Summary Table

⚡ Quick Recap
Blood has four main components:
Red blood cells – Carry oxygen using haemoglobin.
White blood cells – Protect against pathogens.
    • Phagocytes → Engulf and digest pathogens.
    • Lymphocytes → Produce antibodies and memory cells.
Platelets – Help blood clot and prevent blood loss.
Plasma – Liquid that transports blood cells, nutrients, hormones, wastes, antibodies and heat.