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.

---

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

---

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.

---

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.

---

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.

---

Comparison Table

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 (relative to wall)	Large	Very small (single file for RBCs)
Valves	No	Yes	No
Blood type (usually)	Oxygenated	Deoxygenated	Both (transitional)
Main function	Deliver blood from heart	Return blood to heart	Exchange of substances

Exam Tip: A common exam question asks you to compare the structure of arteries, veins, and capillaries. Use a table format in your answer — it is the clearest way to compare multiple features and examiners find it easy to mark.

---

Blood Vessels in a Sporting Context

Scenario	Vessel Type	Role
Blood being pumped from the left ventricle to the muscles	Aorta (artery)	Carries oxygenated blood at high pressure
Oxygen being delivered to muscle fibres during sprinting	Capillaries	Thin walls allow O₂ to diffuse into muscle cells
Deoxygenated blood returning from working legs	Veins (aided by muscle pump)	Return blood to the heart at low pressure
Checking a pulse during exercise	Artery (e.g., radial or carotid)	The pulse is the artery stretching and recoiling with each heartbeat
Blood pooling in the legs if an athlete stops suddenly	Veins	Without the muscle pump, blood collects in veins — this is why cool-downs include walking
Capillarisation after long-term training	Capillaries	New capillaries grow around muscles, improving oxygen delivery

---

The Muscle Pump and Venous Return

Because veins carry blood at low pressure, they need help to push blood back to the heart. The muscle pump is the mechanism by which contracting skeletal muscles squeeze veins and push blood towards the heart.

flowchart TD
    A["Skeletal muscles contract<br>during movement"] --> B["Muscles squeeze<br>surrounding veins"]
    B --> C["Blood is pushed<br>towards the heart"]
    C --> D["Valves prevent<br>backflow"]
    D --> E["Blood returns to<br>the right atrium"]

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

This is why:

• A warm-up increases venous return and prepares the cardiovascular system for exercise.
• A cool-down with light movement keeps the muscle pump active, preventing blood pooling.
• Standing completely still for long periods can cause dizziness because venous return decreases.

---

Common Exam Mistakes

• Saying all arteries carry oxygenated blood — the pulmonary artery carries deoxygenated blood. Similarly, not all veins carry deoxygenated blood — the pulmonary veins carry oxygenated blood.
• Confusing wall thickness — arteries have THICK walls; veins have THINNER walls; capillaries are ONE CELL thick.
• Forgetting that capillaries are the site of exchange — arteries and veins transport blood; capillaries are where actual exchange of substances occurs.
• Not mentioning valves in veins — valves are a key feature of veins that prevent backflow.
• Confusing the lumen — arteries have a smaller lumen (relative to their wall thickness); veins have a larger lumen.

---

Summary