Blood and Its Components

Blood is a tissue made up of several different types of cell suspended in a liquid called plasma. Each component of blood has a specific role in maintaining life. This lesson covers the composition of blood and the functions of each component, as required by the AQA GCSE Combined Science Trilogy specification (8464).

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Composition of Blood

Blood is made up of four main components:

graph TD
    B["Blood"] --> P["Plasma (~55%)"]
    B --> RBC["Red blood cells (~45%)"]
    B --> WBC["White blood cells (<1%)"]
    B --> PLT["Platelets (<1%)"]

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Plasma

Plasma is the straw-coloured liquid part of blood. It makes up about 55% of blood volume and acts as the transport medium, carrying dissolved substances around the body.

Substance Transported	From → To
Red blood cells, white blood cells and platelets	Throughout the body
Glucose	Small intestine → body cells (for respiration)
Amino acids	Small intestine → body cells (for protein synthesis)
Carbon dioxide	Body cells → lungs (as dissolved CO₂ or as hydrogen carbonate ions)
Urea	Liver → kidneys (for excretion)
Hormones	Endocrine glands → target organs
Antibodies and antitoxins	White blood cells → sites of infection
Heat energy	Core organs → extremities (helps regulate body temperature)

Exam Tip: A common exam question is "Name THREE substances transported by plasma." Practise answering this quickly — good examples are glucose, urea and carbon dioxide.

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Red Blood Cells (Erythrocytes)

Red blood cells are the most abundant 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 internal space is available for haemoglobin, so each cell can carry more oxygen
Contains haemoglobin	Haemoglobin binds reversibly with oxygen to form oxyhaemoglobin
Small and flexible	Can squeeze through narrow capillaries, ensuring oxygen reaches every tissue

Oxygen Transport

The key chemical reaction for oxygen transport:

$$\text{Haemoglobin} + \text{Oxygen} \rightleftharpoons \text{Oxyhaemoglobin}$$Haemoglobin+Oxygen⇌Oxyhaemoglobin

• In the lungs (high oxygen concentration): haemoglobin binds with oxygen to form oxyhaemoglobin
• In body tissues (low oxygen concentration): oxyhaemoglobin releases oxygen for cells to use in aerobic respiration

Exam Tip: The double arrow (⇌) shows this is a reversible reaction. Oxygen binds to haemoglobin in the lungs and is released in the tissues. Make sure you use the term "oxyhaemoglobin" in exam answers.

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White Blood Cells (Leucocytes)

White blood cells are part of the immune system and defend the body against disease. They are larger than red blood cells, have a nucleus, and are far less numerous.

Types of White Blood Cell

Type	Action	How It Works
Phagocytes	Phagocytosis (engulfing)	Detect pathogens, engulf and digest them inside the cell
Lymphocytes	Produce antibodies	Recognise specific antigens on pathogens and produce antibodies that lock onto and destroy them
Lymphocytes	Produce antitoxins	Neutralise toxins produced by bacteria

graph TD
    WBC["White Blood Cells"] --> PH["Phagocytes"]
    WBC --> LY["Lymphocytes"]
    PH --> PH1["Engulf and digest pathogens (phagocytosis)"]
    LY --> LY1["Produce specific antibodies"]
    LY --> LY2["Produce antitoxins to neutralise toxins"]

Exam Tip: Know the difference between phagocytes and lymphocytes. Phagocytes are non-specific — they engulf any pathogen. Lymphocytes are specific — they produce antibodies that match specific antigens on a particular pathogen.

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Platelets

Platelets are small cell fragments (they do not have a nucleus). They play a crucial role in blood clotting.

The Clotting Process

When a blood vessel is damaged:

1. Platelets arrive at the wound site
2. They clump together and trigger the production of a mesh of protein fibres called fibrin
3. The fibrin mesh traps red blood cells and forms a blood clot (a scab on the surface)

Functions of blood clotting:

• Seals the wound — prevents further blood loss
• Prevents entry of pathogens — the clot acts as a physical barrier against microorganisms
• Provides a framework for repair of the damaged tissue

Exam Tip: Platelets are NOT cells — they are cell fragments. They have no nucleus. Don't call them "platelet cells" in an exam.

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Comparison Table: Blood Components

Component	Structure	Function	Has a Nucleus?
Plasma	Straw-coloured liquid	Transports dissolved substances (glucose, urea, CO₂, hormones, etc.)	N/A (not a cell)
Red blood cells	Biconcave disc, no nucleus, contains haemoglobin	Transport oxygen as oxyhaemoglobin	No
White blood cells	Larger, irregular shape, with nucleus	Defend against disease (phagocytosis, antibody production, antitoxins)	Yes
Platelets	Small cell fragments, no nucleus	Cause blood clotting at wound sites	No

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Worked Example

Question: Explain how the structure of a red blood cell is related to its function. [4 marks]

Answer:

1. Red blood cells have a biconcave disc shape, which gives them a large surface area to volume ratio — this increases the rate of diffusion of oxygen into and out of the cell ✓
2. They have no nucleus, which leaves more space inside the cell for haemoglobin — this means each cell can carry more oxygen ✓
3. They contain haemoglobin, which binds with oxygen in the lungs to form oxyhaemoglobin and releases it in body tissues ✓
4. They are small and flexible, allowing them to squeeze through narrow capillaries to deliver oxygen close to every body cell ✓

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Common Mistakes