Describe the components and functions of blood and the structure of arteries, veins and capillaries, including vascular shunting in exercise

What this dot point is asking

SEAB wants you to describe what blood is made of and what it does, to compare the three types of blood vessel, and to explain vascular shunting during exercise. The central idea is that blood is the body's transport system, and the vessels are tuned both to carry it efficiently and to redirect it to whatever tissue needs it most.

The answer

The components of blood

Blood has four main components, each with a clear job.

• Red blood cells carry oxygen. They contain haemoglobin, which binds oxygen in the lungs and releases it in the tissues. They also help carry carbon dioxide back to the lungs.
• White blood cells defend the body, fighting infection and destroying disease-causing organisms.
• Platelets help blood to clot, sealing wounds so blood is not lost.
• Plasma is the straw-coloured liquid that carries the cells and transports dissolved nutrients, carbon dioxide, hormones and waste products.

The three types of blood vessel

The vessels are built to match the pressure and job at each point in the circulation.

• Arteries carry blood away from the heart. They have thick, muscular, elastic walls to cope with high pressure and a narrow lumen. They usually carry oxygenated blood (the pulmonary artery is the exception).
• Veins carry blood back to the heart at low pressure. They have thinner walls, a wide lumen and valves to stop blood flowing backwards. They usually carry deoxygenated blood (the pulmonary vein is the exception).
• Capillaries are tiny, with walls just one cell thick. This lets oxygen, carbon dioxide and nutrients pass quickly between the blood and the tissues. They form dense networks around active tissue.

Vascular shunting

During exercise the body redirects blood to where it is needed most, a process called vascular shunting.

• Arteries supplying the working muscles widen (vasodilation) so more oxygenated blood flows to them.
• Arteries supplying less active areas, such as the gut, narrow (vasoconstriction) so less blood goes there.

The result is that a much larger share of cardiac output reaches the muscles during exercise, while non-essential organs receive less.

Examples in context

Example 1. A distance runner near the end of a race. Sustained shunting keeps blood flowing to the leg muscles, with vasodilation in the muscle arteries and reduced flow to the gut. This is partly why eating just before a race feels uncomfortable: digestion and hard exercise compete for blood.

Example 2. A diver checking a graze. When the skin is cut, platelets gather at the wound and trigger clotting to seal it, while white blood cells move in to fight any infection. The same blood that transports oxygen also protects and repairs the body.

Try this

• Cue. List the four components of blood and one function of each. (Red cells: carry oxygen; white cells: fight infection; platelets: clot blood; plasma: transport liquid.)

• Cue. State two structural differences between an artery and a capillary. (An artery has a thick muscular wall and narrow lumen for high pressure; a capillary has a wall one cell thick for rapid exchange.)

• Cue. Explain why vascular shunting is useful during exercise. (It redirects a larger share of blood to the working muscles, delivering more oxygen and removing more carbon dioxide while reducing flow to inactive tissues.)