Blood Vessels

This lesson covers the three types of blood vessel — arteries, veins, and capillaries — as required by the OCR GCSE PE specification (J587). You need to know the structure and function of each type and be able to explain how their structural differences relate to their functions.

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Overview of Blood Vessels

Blood vessels are the tubes that carry blood around the body. There are three types:

graph TD
    A["Blood Vessels"] --> B["Arteries"]
    A --> C["Veins"]
    A --> D["Capillaries"]
    B --> E["Carry blood AWAY<br>from the heart"]
    C --> F["Carry blood TOWARDS<br>the heart"]
    D --> G["Connect arteries to veins;<br>site of exchange"]

    style A fill:#4a90d9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#9b59b6,color:#fff

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Arteries

Arteries carry blood away from the heart to the body's tissues. They carry blood at high pressure because it has just been pumped by the powerful ventricles.

Structure

• Thick, muscular walls — to withstand the high pressure of blood being pumped from the heart.
• Thick layer of elastic tissue — allows the artery to stretch and recoil with each heartbeat (this is what you feel as your pulse).
• Small internal diameter (lumen) — relative to the wall thickness, which maintains high pressure.
• No valves — the high pressure keeps blood flowing in the correct direction.

Function

• Transport oxygenated blood from the heart to the body (except the pulmonary artery, which carries deoxygenated blood to the lungs).
• Maintain high blood pressure to ensure blood reaches all tissues.
• The muscular walls can contract (vasoconstriction) or relax (vasodilation) to control blood flow.

Exam Tip: Remember: Arteries carry blood Away from the heart. The key structural feature is their thick, muscular, elastic walls.

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Veins

Veins carry blood towards the heart from the body's tissues. They carry blood at low pressure because the blood has passed through the capillary network and lost most of its pressure.

Structure

• Thinner walls — veins do not need thick walls because blood pressure is low.
• Less muscle and elastic tissue — compared to arteries.
• Larger internal diameter (lumen) — to allow blood to flow easily at low pressure.
• Valves present — to prevent the backflow of blood and ensure it flows in one direction towards the heart.

Function

• Transport deoxygenated blood from the body back to the heart (except the pulmonary veins, which carry oxygenated blood from the lungs).
• The valves ensure blood flows in the correct direction despite the low pressure.
• The movement of surrounding skeletal muscles (the muscle pump) helps squeeze blood through the veins and back to the heart — this is one reason why moving during a cool-down is important.

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Capillaries

Capillaries are the smallest blood vessels. They connect arteries to veins and are the site of exchange between blood and the body's tissues.

Structure

• Extremely thin walls — only one cell thick, which allows substances to pass through easily by diffusion.
• Very small diameter — so small that red blood cells must pass through in single file.
• Huge network — capillaries form a vast network throughout every tissue in the body, ensuring every cell is close to a capillary.
• No muscle or elastic tissue — the walls are just a single layer of cells.

Function

• Enable the exchange of gases, nutrients, and waste between the blood and the body's cells.
• Oxygen and nutrients diffuse from the blood through the thin capillary wall into the surrounding tissue cells.
• Carbon dioxide and waste products diffuse from the tissue cells through the capillary wall into the blood.
• At the lungs, capillaries surround the alveoli to enable gaseous exchange (O₂ in, CO₂ out).
• At the muscles, capillaries deliver oxygen for aerobic respiration and remove carbon dioxide.