Blood Vessels

Blood vessels form the network of tubes that carry blood around the body. In this lesson you will learn about the three main types of blood vessel — arteries, veins and capillaries — their structural differences, their functions, and how they are adapted for their specific roles. This is a key part of the AQA GCSE PE specification (3.1.1.2) and is frequently tested through comparison and application questions.

---

Overview of Blood Vessels

The circulatory system contains approximately 100,000 kilometres of blood vessels. These vessels form a closed loop that carries blood from the heart to the body's tissues and back again. The three types of blood vessel are:

1. Arteries — carry blood away from the heart
2. Veins — carry blood back to the heart
3. Capillaries — tiny vessels where exchange of gases, nutrients and waste products occurs between the blood and body tissues

Exam Tip: A useful mnemonic to remember that arteries carry blood away from the heart: Arteries = Away.

---

Arteries

Structure

Arteries carry blood away from the heart at high pressure. Their structure is specifically adapted for this role:

• Thick, muscular walls — the walls of arteries contain a thick layer of smooth muscle and elastic tissue. This allows them to withstand the high pressure of blood being pumped from the ventricles, and to stretch and recoil with each heartbeat.
• Thick outer wall — provides strength and support.
• Small lumen (internal diameter) — the relatively narrow internal space maintains high blood pressure as blood is pushed through.
• No valves (except in the aorta and pulmonary artery) — the high pressure of blood flow is sufficient to keep blood moving in one direction without valves.

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.
• The elastic tissue in artery walls allows them to stretch when the heart contracts (systole) and recoil when the heart relaxes (diastole), which helps to smooth out the pulses of blood flow into a more continuous stream.

Pulse

You can feel the pulse at certain points on the body where an artery passes close to the skin surface (e.g., the radial artery at the wrist, the carotid artery at the neck). The pulse is caused by the artery walls stretching and recoiling with each heartbeat.

---

Arterioles

As arteries move further from the heart, they branch into smaller vessels called arterioles. Arterioles play a crucial role in controlling blood flow to different parts of the body:

• They have a layer of smooth muscle that can contract (vasoconstriction) or relax (vasodilation).
• This allows the body to redirect blood flow during exercise — increasing flow to working muscles and decreasing flow to less active organs. This process is called blood shunting or vascular shunting and is covered in detail in the next lesson.

---

Veins

Structure

Veins carry blood back to the heart at low pressure. Their structure reflects this:

• Thin walls — veins have thinner walls than arteries with less smooth muscle and elastic tissue, because the blood inside them is at much lower pressure.
• Large lumen (internal diameter) — the wide internal space reduces resistance to blood flow, making it easier for blood to flow back to the heart at low pressure.
• Valves — veins contain one-way valves at regular intervals. These valves prevent the backflow of blood, which is essential because blood in veins is flowing at low pressure and, in the legs, must flow against gravity.

Function

• Transport deoxygenated blood from the body's tissues back to the heart (except the pulmonary veins, which carry oxygenated blood from the lungs to the heart).
• The valves and the squeezing action of surrounding skeletal muscles (the skeletal muscle pump) help to push blood through the veins back towards the heart.

The Skeletal Muscle Pump

When skeletal muscles contract during movement, they squeeze the veins that run through and alongside them. This compresses the veins and pushes blood towards the heart. The valves in the veins prevent blood from flowing backwards. This mechanism is called the skeletal muscle pump and is one reason why a cool-down after exercise is important — continued gentle movement keeps the muscle pump active, preventing blood from pooling in the extremities.

---

Capillaries

Structure

Capillaries are the smallest blood vessels in the body — so small that red blood cells must pass through them in single file. Their structure is uniquely adapted for exchange:

• Walls only one cell thick — this creates a very short diffusion distance, allowing rapid exchange of oxygen, carbon dioxide, nutrients and waste between the blood and surrounding tissues.
• Very narrow diameter — capillaries are only about 8–10 micrometres wide. This forces blood to flow slowly, giving maximum time for exchange to occur.
• Huge network — there are billions of capillaries forming a dense network (capillary bed) around every tissue in the body, providing a massive surface area for exchange.
• No smooth muscle or elastic tissue — the walls consist of a single layer of thin, flat endothelial cells.

Function

• Allow the exchange of substances between the blood and body tissues.
• Oxygen and nutrients pass from the blood through the thin capillary walls into the tissue cells.
• Carbon dioxide and other waste products pass from the tissue cells through the capillary walls into the blood.
• This exchange occurs by diffusion, driven by concentration gradients.

---

Comparison Table: Arteries, Veins and Capillaries

This is one of the most commonly tested tables in AQA GCSE PE. You should be able to reproduce this from memory:

Feature	Arteries	Veins	Capillaries
Direction of blood flow	Away from the heart	Towards the heart	Connect arteries to veins
Blood pressure	High	Low	Very low
Wall thickness	Thick (muscular and elastic)	Thin (less muscle and elastic tissue)	One cell thick
Lumen size	Small (narrow)	Large (wide)	Very small (microscopic)
Valves	No (except at the aorta/pulmonary artery)	Yes (to prevent backflow)	No
Blood type (usually)	Oxygenated (except pulmonary artery)	Deoxygenated (except pulmonary veins)	Both (transition from oxygenated to deoxygenated)
Pulse	Yes (can be felt)	No	No
Speed of blood flow	Fast	Slow	Very slow
Main function	Transport blood at high pressure	Return blood to heart at low pressure	Exchange of gases, nutrients and waste

Exam Tip: AQA loves comparison questions. Be ready for questions like: "Compare the structure of an artery and a vein" or "Explain how the structure of a capillary is related to its function." Always link structure to function in your answers — do not just describe the structure.

---

How Blood Vessel Structure Relates to Function

This is a crucial skill for the exam — linking structural features to their function:

Arteries

Structural Feature	How It Relates to Function
Thick, muscular, elastic walls	Withstand high blood pressure from the heart; stretch and recoil to maintain pressure
Small lumen	Maintains high blood pressure
No valves needed	High pressure keeps blood flowing in one direction

Veins

Structural Feature	How It Relates to Function
Thin walls	Blood is at low pressure, so thick walls are not needed
Large lumen	Reduces resistance, allows blood to flow easily at low pressure
Valves present	Prevent backflow of blood, which is essential because pressure is too low to prevent it naturally

Capillaries

Structural Feature	How It Relates to Function
Walls one cell thick	Short diffusion distance allows rapid gas exchange
Very narrow diameter	Blood flows slowly, allowing maximum time for exchange
Dense network (capillary bed)	Huge total surface area for exchange

---

The Pathway of Blood Through the Vessels

Blood flows through the vessels in the following order:

graph LR
    A[Heart] --> B[Arteries]
    B --> C[Arterioles]
    C --> D[Capillaries]
    D --> E[Venules]
    E --> F[Veins]
    F --> A

    style A fill:#e74c3c,color:#fff
    style B fill:#c0392b,color:#fff
    style C fill:#d35400,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#2980b9,color:#fff
    style F fill:#4a90d9,color:#fff

1. The heart pumps blood into large arteries at high pressure.
2. Arteries branch into smaller arterioles, which control blood distribution.
3. Arterioles lead into capillary beds, where exchange occurs.
4. Capillaries merge into small venules.
5. Venules merge into larger veins, which carry blood back to the heart.

---

Blood Vessel Changes During Exercise

During exercise, blood vessels undergo significant changes to meet the increased demands of working muscles: