Blood and Its Components

Blood is a tissue made up of cells and cell fragments suspended in a liquid. Each component has a specific function. Understanding the composition of blood and the roles of each part is essential for Edexcel GCSE Biology.

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The Four Components of Blood

Blood consists of four main components:

1. Plasma (~55% of blood volume)
2. Red blood cells (erythrocytes)
3. White blood cells (leucocytes)
4. Platelets (thrombocytes)

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1. Plasma

Plasma is the straw-coloured liquid that makes up about 55% of blood volume. It is mostly water and acts as the transport medium for the blood.

What Plasma Carries

Substance	From	To
Glucose	Small intestine (after digestion)	All body cells (for respiration)
Amino acids	Small intestine (after protein digestion)	Cells (for protein synthesis)
Carbon dioxide	Body cells (waste from respiration)	Lungs (to be exhaled) — dissolved in plasma
Urea	Liver (from breakdown of excess amino acids)	Kidneys (to be filtered out and excreted in urine)
Hormones	Endocrine glands	Target organs
Antibodies	Lymphocytes (white blood cells)	Throughout the body
Heat energy	Active organs (e.g. muscles, liver)	Throughout the body (helps regulate temperature)
Mineral ions	Small intestine	Where needed
Proteins	Liver	Throughout the body

Exam tip: A common exam question asks you to name three substances transported by plasma. Prepare at least five examples so you can confidently choose three.

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2. Red Blood Cells (Erythrocytes)

Red blood cells are the most numerous cells in the blood. Their primary function is to transport oxygen from the lungs to every cell in the body.

Adaptations of Red Blood Cells

Adaptation	How it helps
Biconcave disc shape	Increases the surface area to volume ratio, allowing faster diffusion of oxygen in and out of the cell
No nucleus	More space inside the cell for haemoglobin, the oxygen-carrying molecule
Contain haemoglobin	Haemoglobin binds to oxygen to form oxyhaemoglobin — this is reversible, allowing oxygen to be picked up in the lungs and released at the tissues
Small and flexible	Can squeeze through narrow capillaries, getting close to body cells
Produced in bone marrow	Continuous production replaces old cells (lifespan ~120 days)

The Oxygen-Haemoglobin Reaction

In the lungs (high oxygen concentration):

haemoglobin + oxygen → oxyhaemoglobin

In the tissues (low oxygen concentration):

oxyhaemoglobin → haemoglobin + oxygen

This is a reversible reaction — oxygen is picked up where concentration is high and released where concentration is low.

Exam tip: When describing red blood cell adaptations, always state the adaptation AND explain how it helps the cell carry out its function. For example: "Red blood cells have no nucleus, which provides more space for haemoglobin, allowing more oxygen to be carried."

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3. White Blood Cells (Leucocytes)

White blood cells are part of the immune system. They defend the body against pathogens (disease-causing microorganisms).

There are two main types you need to know:

Phagocytes

• Engulf and digest pathogens (a process called phagocytosis)
• They surround the pathogen, engulf it into the cell, and destroy it using enzymes
• They are non-specific — they attack any pathogen they encounter

Lymphocytes

• Produce antibodies — specific proteins that bind to antigens (unique molecules on the surface of pathogens)
• Each type of lymphocyte produces antibodies specific to one type of antigen
• Antibodies cause pathogens to clump together (agglutinate), making them easier for phagocytes to destroy
• Some lymphocytes produce antitoxins that neutralise toxins released by pathogens
• Memory lymphocytes remain in the blood after an infection, allowing a faster immune response if the same pathogen enters the body again — this is the basis of immunity

Key Differences from Red Blood Cells

Feature	Red blood cells	White blood cells
Nucleus	No nucleus	Have a nucleus
Number	Very abundant (~5 million per mm³)	Far fewer (~7,000 per mm³)
Function	Transport oxygen	Defend against pathogens
Shape	Biconcave disc	Irregular/variable (phagocytes can change shape)

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4. Platelets (Thrombocytes)

Platelets are small cell fragments (not complete cells). They have no nucleus. Their main role is in blood clotting.

The Clotting Process

When a blood vessel is damaged:

1. Platelets arrive at the wound site and clump together to form a platelet plug
2. Platelets release chemicals that trigger a cascade of reactions
3. The enzyme thrombin converts the soluble protein fibrinogen (in plasma) into insoluble fibrin
4. Fibrin forms a mesh of threads across the wound
5. Red blood cells become trapped in the fibrin mesh, forming a blood clot
6. The clot dries and hardens to form a scab

Why Blood Clotting is Important

• Prevents excessive blood loss (haemorrhage) from wounds
• Prevents entry of pathogens — the clot seals the wound, acting as a barrier against microorganisms
• Provides a framework for tissue repair underneath

Exam tip: The clotting cascade is a sequence — make sure you know the correct order: platelets → thrombin (enzyme) → fibrinogen converted to fibrin → fibrin mesh → clot → scab.

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Blood Types and Transfusions (Supplementary)

Although not heavily examined, it is useful to know that blood types (A, B, AB, O) are determined by antigens on the surface of red blood cells. Mismatched transfusions cause the immune system to attack the donated blood, leading to dangerous clumping (agglutination).

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Comparing All Blood Components

Component	Structure	Function	Key Feature
Plasma	Liquid (mostly water)	Transports dissolved substances	Carries glucose, urea, CO₂, hormones
Red blood cells	Biconcave disc, no nucleus	Transport oxygen	Contain haemoglobin
White blood cells	Irregular shape, have nucleus	Immune defence	Phagocytes engulf; lymphocytes make antibodies
Platelets	Cell fragments, no nucleus	Blood clotting	Trigger fibrin mesh formation

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Summary

• Plasma is the liquid part of blood that transports dissolved substances including glucose, urea, CO₂, hormones and amino acids
• Red blood cells are adapted to carry oxygen with their biconcave shape, lack of nucleus and haemoglobin content
• White blood cells fight infection — phagocytes engulf pathogens, lymphocytes produce antibodies
• Platelets are cell fragments that initiate the clotting process to seal wounds and prevent blood loss

Exam tip: In multiple-choice questions, remember that red blood cells and platelets do NOT have a nucleus, but white blood cells DO. This is one of the most commonly tested distinctions.

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Worked Example: Oxygen Transport Numbers