Blood Vessels

This lesson covers the structure and function of the three types of blood vessel — arteries, veins, and capillaries — as required by the Edexcel GCSE PE specification (1PE0 — Topic 1). You need to know the structural differences between the three vessel types, how their structure relates to their function, and how they work together to deliver blood around the body.

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Overview of Blood Vessels

The body has three types of blood vessel, and blood always flows in the same order: arteries → capillaries → veins.

graph LR
    A["Heart"] --> B["Arteries"]
    B --> C["Arterioles<br>(small arteries)"]
    C --> D["Capillaries<br>(gas exchange)"]
    D --> E["Venules<br>(small veins)"]
    E --> F["Veins"]
    F --> A

    style A fill:#c0392b,color:#fff
    style D fill:#27ae60,color:#fff
    style F fill:#4a90d9,color:#fff

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Arteries

Arteries carry blood away from the heart to the tissues of the body. They carry blood under high pressure.

Structure

• Thick muscular walls — the walls contain a thick layer of smooth muscle and elastic tissue to withstand the high pressure of blood being pumped from the heart.
• Elastic tissue — allows the artery walls to stretch and recoil (spring back) with each heartbeat, helping to maintain blood pressure.
• Small lumen — the central channel (lumen) is relatively narrow compared to veins, which helps maintain high blood pressure.
• No valves — the high pressure from the heart is sufficient to keep blood moving forward; valves are not needed.

Function

• Transport oxygenated blood from the heart to the body's tissues (except the pulmonary artery, which carries deoxygenated blood to the lungs).
• Maintain high blood pressure to ensure blood reaches all parts of the body.

Key Arteries

Artery	Function
Aorta	Carries oxygenated blood from the left ventricle to the body
Pulmonary artery	Carries deoxygenated blood from the right ventricle to the lungs
Coronary arteries	Supply oxygenated blood to the heart muscle

Sporting relevance: During exercise, the smooth muscle in artery walls can vasodilate (widen) to increase blood flow to working muscles, or vasoconstrict (narrow) to redirect blood away from less active areas.

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Veins

Veins carry blood back to the heart from the tissues. They carry blood under low pressure.

Structure

• Thinner walls — veins have thinner walls than arteries because blood is under much lower pressure.
• Less smooth muscle and elastic tissue — the walls do not need to withstand high pressure.
• Larger lumen — the central channel is wider than in arteries, which helps blood flow despite the low pressure.
• Valves — veins contain one-way pocket valves that prevent blood from flowing backward due to gravity, particularly in the legs.

Function

• Transport deoxygenated blood from the body's tissues back to the heart (except the pulmonary veins, which carry oxygenated blood from the lungs).
• Work with the muscle pump and respiratory pump to return blood to the heart.

Key Veins

Vein	Function
Vena cava (superior and inferior)	Returns deoxygenated blood from the body to the right atrium
Pulmonary veins	Return oxygenated blood from the lungs to the left atrium

The Muscle Pump and Respiratory Pump

Because blood in the veins is under low pressure, the body uses two mechanisms to help return blood to the heart (known as venous return):

• Muscle pump — when skeletal muscles contract during physical activity, they squeeze the veins running through them, pushing blood towards the heart. The valves prevent it from flowing backward.
• Respiratory pump — during breathing, changes in pressure in the chest cavity help draw blood back towards the heart.

Sporting relevance: This is why a cool-down is important after exercise. Continuing gentle movement keeps the muscle pump working, preventing blood from pooling in the legs (which can cause dizziness or fainting).

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Capillaries

Capillaries are the smallest blood vessels, forming a vast network that connects arteries to veins. They are the site of gas exchange — where oxygen and nutrients are delivered to cells, and carbon dioxide and waste products are collected.

Structure

• Walls one cell thick — this allows for rapid diffusion of gases (oxygen and carbon dioxide) and nutrients between the blood and the surrounding tissues.
• Very narrow lumen — only wide enough for red blood cells to pass through in single file, bringing them close to the capillary walls.
• Huge surface area — the extensive network of capillaries provides a massive surface area for exchange.
• No valves and no smooth muscle — the walls are too thin.

Function

• Gas exchange — oxygen diffuses from the blood through the capillary wall into the tissue cells. Carbon dioxide diffuses from the tissue cells into the blood.
• Nutrient delivery — glucose and other nutrients pass from the blood to the cells.
• Waste removal — carbon dioxide and other metabolic waste products pass from the cells to the blood.

Sporting relevance: During exercise, the muscles need more oxygen. The extensive capillary network around muscle fibres allows for rapid gas exchange to meet this increased demand. Long-term aerobic training leads to capillarisation — an increase in the number of capillaries around the muscles, improving oxygen delivery.

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Comparing Blood Vessels

Feature	Arteries	Veins	Capillaries
Direction of flow	Away from the heart	Towards the heart	Between arteries and veins
Blood pressure	High	Low	Very low
Wall thickness	Thick (muscular, elastic)	Thinner	One cell thick
Lumen size	Small (narrow)	Large (wide)	Very small (single red blood cell)
Valves	No	Yes (prevent backflow)	No
Blood type	Mostly oxygenated*	Mostly deoxygenated*	Both (exchange occurs here)
Function	Transport blood at high pressure	Return blood at low pressure	Gas and nutrient exchange

*Exceptions: the pulmonary artery (deoxygenated) and the pulmonary veins (oxygenated).

graph TD
    A["Arteries"] --> B["Thick walls<br>Small lumen<br>High pressure<br>No valves"]
    C["Veins"] --> D["Thin walls<br>Large lumen<br>Low pressure<br>Valves present"]
    E["Capillaries"] --> F["One cell thick<br>Very small lumen<br>Very low pressure<br>Gas exchange"]

    style A fill:#c0392b,color:#fff
    style C fill:#4a90d9,color:#fff
    style E fill:#27ae60,color:#fff

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