Blood and Blood Vessels

This lesson covers the components of blood and the structure and function of the three types of blood vessel — arteries, veins and capillaries — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe each blood component, explain how blood vessels are adapted for their function and interpret data on blood composition.

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Components of Blood

Blood is a tissue made up of four main components:

Component	Description	Function
Plasma	Straw-coloured liquid that makes up about 55 % of blood volume	Transports dissolved substances: glucose, amino acids, CO₂, urea, hormones, antibodies, heat
Red blood cells (erythrocytes)	Biconcave disc shape; no nucleus; packed with haemoglobin	Transport oxygen from lungs to body tissues
White blood cells (leucocytes)	Larger than red blood cells; have a nucleus; several types	Defend the body against pathogens (part of the immune system)
Platelets	Small cell fragments; no nucleus	Help blood clot at wound sites to prevent blood loss and entry of pathogens

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Red Blood Cells

Red blood cells are highly specialised for carrying oxygen.

Adaptation	How it helps
Biconcave disc shape	Increases surface area to volume ratio for faster oxygen diffusion
No nucleus	More space for haemoglobin, so each cell carries more oxygen
Contains haemoglobin	Haemoglobin binds reversibly to oxygen to form oxyhaemoglobin
Small and flexible	Can squeeze through narrow capillaries to deliver oxygen close to every cell

The key reaction:

$\text{haemoglobin} + \text{oxygen} \rightleftharpoons \text{oxyhaemoglobin}$haemoglobin+oxygen⇌oxyhaemoglobin

In the lungs (high O₂ concentration), haemoglobin picks up oxygen. In the tissues (low O₂ concentration), oxyhaemoglobin releases oxygen for respiration.

Exam Tip: The reaction is reversible (shown by the ⇌ arrow). Haemoglobin loads oxygen in the lungs and unloads it in the tissues. This is a common question.

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White Blood Cells

White blood cells are part of the immune system. You need to know two main types:

Type	Function
Phagocytes	Engulf and digest pathogens by phagocytosis
Lymphocytes	Produce antibodies (proteins that bind to specific antigens on pathogens) and antitoxins that neutralise toxins

graph TD
    A["Pathogen enters body"] --> B["Phagocytes detect pathogen"]
    B --> C["Phagocyte engulfs pathogen (phagocytosis)"]
    A --> D["Lymphocytes recognise antigens"]
    D --> E["Produce specific antibodies"]
    E --> F["Antibodies bind to antigens → pathogen destroyed"]
    D --> G["Produce antitoxins"]
    G --> H["Antitoxins neutralise toxins"]

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Platelets

Platelets are small fragments of larger cells (megakaryocytes). They do not have a nucleus.

When a blood vessel is damaged:

1. Platelets gather at the wound site.
2. They trigger a series of chemical reactions that convert the soluble protein fibrinogen into insoluble fibrin threads.
3. The fibrin threads form a mesh that traps red blood cells, forming a blood clot (scab).

The clot:

• Prevents blood loss (haemorrhage)
• Prevents entry of pathogens through the wound

Exam Tip: Platelets are not whole cells — they are cell fragments. This is a common mistake in exams.

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Blood Plasma

Plasma is the liquid component of blood. It is mostly water and acts as the transport medium for many dissolved substances:

Substance carried in plasma	Where from → where to
Glucose	Small intestine → body cells
Amino acids	Small intestine → body cells
Carbon dioxide	Body cells → lungs (as dissolved CO₂ or hydrogencarbonate ions)
Urea	Liver → kidneys (for excretion)
Hormones	Endocrine glands → target organs
Antibodies	Lymphocytes → throughout the body
Heat	Core organs → extremities (helps regulate temperature)

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The Three Types of Blood Vessel

Arteries

Arteries carry blood away from the heart (under high pressure).

Feature	Purpose
Thick muscular walls	Withstand the high pressure of blood pumped from the heart
Thick layer of elastic tissue	Stretches and recoils with each heartbeat to maintain blood pressure (pulse)
Small lumen (relative to wall thickness)	Maintains high pressure
No valves (except at the base of the aorta and pulmonary artery)	Blood pressure is high enough to prevent backflow

Veins

Veins carry blood towards the heart (under low pressure).

Feature	Purpose
Thin walls with less muscle and elastic tissue	Blood is at lower pressure, so thick walls are not needed
Large lumen	Reduces resistance to blood flow; allows blood to flow easily at low pressure
Valves	Prevent backflow of blood; ensure one-way flow back to the heart

Capillaries

Capillaries are the smallest blood vessels. They connect arteries to veins and are where exchange of substances occurs.

Feature	Purpose
Very thin walls (one cell thick)	Very short diffusion distance for gases, nutrients and waste
Very narrow lumen	Red blood cells pass through in single file, close to every body cell
Large total surface area	Vast network maximises the area for exchange
Permeable walls	Allow substances to pass through easily

graph LR
    A["Heart"] -->|"Arteries (high pressure, thick walls)"| B["Arterioles"]
    B -->|"Capillaries (thin walls, exchange)"| C["Venules"]
    C -->|"Veins (low pressure, valves)"| D["Heart"]

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Comparing Blood Vessels

Feature	Artery	Vein	Capillary
Direction of blood	Away from heart	Towards heart	Between arteries and veins
Blood pressure	High	Low	Dropping
Wall thickness	Thick (muscle + elastic tissue)	Thinner	One cell thick
Lumen size	Small (relative)	Large	Very narrow
Valves	No (mostly)	Yes	No
Function	Carry blood at high pressure	Return blood to heart	Exchange of substances

Exam Tip: Remember: Arteries carry blood Away from the heart. Veins have Valves. Capillaries are where exchange happens because their walls are just one cell thick.

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Summary