The Circulatory System and the Heart: KS3 Biology Guide

The human circulatory system is a closed loop of blood vessels driven by the heart that delivers oxygen and nutrients to every cell in the body and carries away carbon dioxide and other waste products. Humans have a double circulatory system: the heart pumps blood along two separate circuits at the same time. This is essential KS3 biology content covered in Year 8.

What is the circulatory system?

The circulatory system has three main components:

The heart — a muscular pump that keeps blood moving.

Blood vessels — the network of tubes (arteries, veins and capillaries) through which blood travels.

Blood — the fluid that carries oxygen, nutrients, hormones and waste products.

Together these parts ensure that every living cell receives the oxygen and glucose it needs for respiration, and that waste carbon dioxide is removed and taken to the lungs to be exhaled.

How does double circulation work?

Humans (and all other mammals) have a double circulatory system, meaning the heart pumps blood through two loops simultaneously:

Pulmonary circulation — the right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen and releases carbon dioxide. Oxygenated blood then returns to the left side of the heart.

Systemic circulation — the left side of the heart pumps oxygenated blood to the rest of the body. As blood passes through tissues, it delivers oxygen and picks up carbon dioxide, becoming deoxygenated before returning to the right side of the heart.

Because the blood visits the heart twice on each complete circuit, this is called double circulation. Fish, by contrast, have a single circulation — blood passes through the heart only once per circuit, which is less efficient.

Structure of the heart

The heart is divided into four chambers:

Chamber | Side | Role
Right atrium | Right | Receives deoxygenated blood from the body via the vena cava
Right ventricle | Right | Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium | Left | Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle | Left | Pumps oxygenated blood to the body via the aorta

Valves

Four valves prevent backflow of blood:

Atrioventricular (AV) valves — between the atria and ventricles (tricuspid on the right, bicuspid/mitral on the left). They open to allow blood to flow from atria into ventricles, then snap shut when ventricles contract.

Semilunar valves — at the base of the pulmonary artery and aorta. They prevent blood from flowing back into the ventricles after each contraction.

Why is the left ventricle wall thicker?

The left ventricle has a much thicker muscular wall than the right ventricle. This is because it must pump blood all the way around the body (systemic circuit), which requires much greater force than pumping blood only to the nearby lungs (pulmonary circuit).

Blood vessels: arteries, veins and capillaries

Vessel | Direction of flow | Wall | Lumen | Pressure
Artery | Away from heart | Thick, elastic, muscular | Narrow | High
Vein | Towards heart | Thin | Wide | Low
Capillary | Through tissues | One cell thick | Very narrow | Low

Key points:

Arteries carry blood away from the heart (mostly oxygenated, except the pulmonary artery).

Veins return blood to the heart (mostly deoxygenated, except the pulmonary vein). Veins contain valves to stop blood flowing backwards at low pressure.

Capillaries are so thin that oxygen, glucose and waste products can diffuse directly through their walls into and out of cells — this is where the real exchange happens.

A common misconception: arteries always carry oxygenated blood. In fact, the pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood back. The rule is about direction (away from / towards the heart), not oxygen content.

A worked example: tracing one red blood cell

Follow a single red blood cell on a complete circuit, starting in the body's tissues:

The cell is deoxygenated and returns via the vena cava to the right atrium.

The AV valve opens; blood flows into the right ventricle.

The right ventricle contracts; blood is pushed through the semilunar valve into the pulmonary artery → lungs.

In the lungs, the red blood cell picks up oxygen (oxygenated) and carbon dioxide diffuses out.

Oxygenated blood travels via the pulmonary vein to the left atrium.

Blood flows into the left ventricle.

The left ventricle contracts powerfully; blood is pushed through the semilunar valve into the aorta → body tissues.

Oxygen diffuses into cells; carbon dioxide diffuses in. The cell is deoxygenated again.

Return to step 1.

The entire circuit takes approximately one minute at rest.

What does blood contain?

Blood is made up of four components:

Red blood cells (erythrocytes) — contain haemoglobin, which binds to oxygen to form oxyhaemoglobin. They have no nucleus (maximising space for haemoglobin) and a biconcave disc shape (maximising surface area for oxygen diffusion).

White blood cells (leucocytes) — part of the immune system; fight infection by engulfing pathogens (phagocytosis) or producing antibodies.

Platelets (thrombocytes) — tiny cell fragments that trigger blood clotting to seal wounds.

Plasma — the liquid component (about 55% of blood volume); transports dissolved substances including glucose, carbon dioxide, hormones and urea.

Factors that affect heart rate

At rest, the average adult heart beats 60–100 times per minute. Heart rate increases during exercise because muscles need more oxygen and produce more carbon dioxide. The heart also responds to:

Adrenaline — released in stressful or exciting situations ("fight or flight"). Increases heart rate and stroke volume.

Temperature — a higher body temperature speeds up heart rate slightly.

Fitness level — trained athletes often have lower resting heart rates because their hearts pump a larger volume of blood per beat (greater stroke volume).

According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should be taught the structure and functions of the human heart and the structure of the blood vessels in relation to their functions.

The NHS cardiovascular disease pages describe how the heart and blood vessels work together and the importance of a healthy circulatory system — directly relevant to the KS3 biology content on the heart and blood vessels.

Frequently asked questions

What is the difference between oxygenated and deoxygenated blood?

Oxygenated blood contains red blood cells with haemoglobin that has picked up oxygen (forming oxyhaemoglobin) in the lungs. It is bright red. Deoxygenated blood has released most of its oxygen to body tissues and appears darker red. The terms refer to oxygen content, not to which vessel the blood is in — although arteries carry oxygenated blood and veins carry deoxygenated blood throughout most of the body, the pulmonary artery and pulmonary vein are the important exceptions.

Why do veins have valves but arteries do not?

Arteries carry blood at high pressure directly from the heart's powerful contractions, so blood flows in the correct direction without help. Veins return blood to the heart at low pressure — sometimes against gravity (e.g., from the legs). Without valves, blood would pool and flow backwards. The valves open when blood moves towards the heart and snap shut when blood tries to flow back.

What is the role of haemoglobin in red blood cells?

Haemoglobin is an iron-containing protein that binds reversibly to oxygen. In the lungs, where oxygen concentration is high, haemoglobin picks up four oxygen molecules to form oxyhaemoglobin. In body tissues, where oxygen concentration is low, oxyhaemoglobin releases its oxygen (back to haemoglobin). This reversible binding is what allows red blood cells to act as efficient oxygen carriers. Each red blood cell contains about 250 million haemoglobin molecules.

How is the heart adapted for its function?

The heart has several adaptations: (1) thick muscular walls (especially the left ventricle) to generate the pressure needed to pump blood around the body; (2) valves to prevent backflow and ensure one-way flow; (3) its own blood supply via the coronary arteries, which branch off the aorta — without its own oxygen supply, the cardiac muscle would not be able to contract continuously; (4) an electrical conducting system (the sinoatrial node or "pacemaker") that coordinates contractions of the four chambers in the correct sequence.

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