The heart, Blood vessels and Blood — AQA GCSE Biology

SPEC 4.2.2.2–4.2.2.3 • ORGANISATION — CIRCULATION

The Heart, Blood Vessels & Blood

The double pump, three vessel types, and four blood components — using paired-comparison wording the mark scheme actually rewards.

The Two-Mark Trap

One of the most reliable two-mark questions on Paper 1 asks you to compare the structure of an artery and a vein. Sounds easy. Most students lose at least one mark.

AQA mark schemes reward paired wording — "artery has a thick er layer of muscle than vein" scores. "Artery has thick muscle" on its own does not. No comparator → no mark. — recurring pattern across AQA biology mark schemes

What You Get From This Lesson

The four heart chambers, the three vessel types, and the four blood components — explained using the paired-comparison wording the examiner gives marks for, plus the rate calculations the spec demands.

What You'll Learn

By the end of this lesson, you'll be able to:

Lesson Objectives

Identify the four chambers of the heart and the four major vessels — and explain why the left ventricle has a thicker wall.

Compare arteries, veins, and capillaries using paired-comparison statements that match mark-scheme wording.

Name the four components of blood and what each does, including the three named adaptations of red blood cells.

Spec ref: 4.2.2.2 (heart and blood vessels) and 4.2.2.3 (blood). Paper 1, Higher tier.

The Heart: A Double Pump

The heart has four chambers — two atria (top, receive blood) and two ventricles (bottom, pump blood out).

Memory Trick

Atria are Above, Ventricles are below (V points down). Right side handles deoxygenated blood; left side handles oxygenated.

The Four Major Vessels

AQA limits the named vessels to five: vena cava, pulmonary artery, pulmonary vein, aorta, and coronary arteries. Learn each one's job — and which blood it carries.

VesselBloodFrom → To

Vena cavaDeoxygenatedBody → right atrium

Pulmonary arteryDeoxygenated ⚠️Right ventricle → lungs

Pulmonary veinOxygenated ⚠️Lungs → left atrium

AortaOxygenatedLeft ventricle → body

Coronary arteriesOxygenatedSupply the heart muscle itself

Examiner Alert

The pulmonary vessels are the exception to the general rule that "arteries carry oxygenated blood, veins carry deoxygenated blood." Mark schemes test this every series.

Why The Left Ventricle Is Thicker

One of the most common short-answer questions: "Explain why the left ventricle wall is thicker than the right ventricle wall." The PEE answer:

Model Answer (3 marks)

Point: The left ventricle pumps blood around the whole body, while the right ventricle only pumps blood to the lungs.

Evidence: The whole-body circuit is much longer and has greater resistance to flow than the lung circuit.

Explain: The left ventricle therefore needs to generate a higher pressure, so it has a thicker, more muscular wall to contract more forcefully.

Double Circulation Definition

Blood passes through the heart twice per circuit: once through the right side to the lungs (pulmonary circuit), once through the left side to the body (systemic circuit).

The Pacemaker

The heart's natural resting rhythm is set by a group of cells acting as a pacemaker.

Natural Pacemaker

A group of cells in the right atrium .

Sends electrical signals that cause the heart muscle to contract.

Sets the natural resting heart rate.

Artificial Pacemaker

An electrical device fitted to the patient.

Used to correct irregularities in the heart rate (e.g. an irregular heart beat / arrhythmia).

Sends regular signals to the heart muscle.

Mark-Scheme Note

For "what does an artificial pacemaker treat?" — the mark scheme accepts "an irregular heart beat", arrhythmia, or fibrillation. It does not accept "cardiovascular disease" or "heart murmur".

Three Blood Vessel Types

The body contains three types of blood vessel — and you must be able to relate structure to function for each one.

Artery

Carries blood AWAY from the heart.

Thick muscular + elastic wall.

Narrow lumen.

No valves .

High pressure.

Capillary

Connects arteries to veins.

Walls are one cell thick .

Permeable — substances diffuse through.

Site of exchange with tissues.

Vein

Carries blood BACK to the heart.

Thin wall.

Wide lumen.

Has valves .

Low pressure.

Function Link

Every structural feature exists for a reason: thick artery walls withstand high pressure from the heart; capillary walls one cell thick give a short diffusion distance; vein valves prevent backflow at low pressure.

Cross-Section: Artery vs Vein

This is the kind of diagram exam questions are built around. By eye, you should spot three structural differences immediately.

Mark-Scheme Wording

Use the suffix -er: thicker, thinner, narrower, wider. AQA explicitly accepts these and rejects bare adjectives. And — do not write "cell wall" for a blood vessel; the mark scheme rejects this term.

Blood: A Tissue, Not A Liquid

Blood is a tissue consisting of plasma, in which red blood cells, white blood cells, and platelets are suspended. The spec lists exactly these four components.

1. Plasma

The liquid part of blood. Transports dissolved substances: carbon dioxide, urea, glucose, amino acids, hormones, antibodies — and heat.

2. Red Blood Cells

Carry oxygen from the lungs to body tissues. Contain haemoglobin, which binds oxygen.

3. White Blood Cells

Defence against pathogens. Phagocytes change shape to engulf and digest pathogens; lymphocytes produce antibodies and antitoxins. Unlike red blood cells, white blood cells have a nucleus.

4. Platelets

Small cell fragments that help blood clot at wound sites — preventing blood loss and pathogen entry.

Red Blood Cell Adaptations

For any question on "how is a red blood cell adapted to its function?", name all three — mark schemes typically award one mark per adaptation.

Biconcave disc

A flattened, dipped shape gives a large surface area for oxygen to diffuse in and out.

No nucleus

Frees up more space inside the cell for haemoglobin — so each red blood cell can carry more oxygen.

Contains haemoglobin

A red protein that binds oxygen in the lungs and releases it in respiring tissues.

The Double Circulation Loop

Trace blood from the body, through the heart, and out again — this is the path that any "describe the journey of a blood cell" question expects.

In Words

Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs (pick up O₂, drop off CO₂) → pulmonary vein → left atrium → left ventricle → aorta → body.

Blood Flow Rate Calculations

Spec 4.2.2.2 requires you to use rate calculations for blood flow (maths skills MS 1a, 1c). The formula is simple — but you must watch the units the question asks for.

Blood Flow Rate

rate = volume ÷ time

Worked Calculation

A person's heart pumps 70 cm³ of blood per beat at a resting rate of 75 beats per minute. Calculate the blood flow rate in cm³ per minute.

Blood flow rate = 70 cm³ × 75 beats/min

= 5,250 cm³/min

Exam Tip

Always check the requested units — "per minute" vs "per second" means × or ÷ 60. Show your working: examiners credit method even if the final number is wrong. And if a question gives you the rate during exercise vs at rest, the comparison itself often carries the mark.

Worked Example

A diagram shows the cross-sections of an artery and a vein. Describe two ways the structure of the artery differs from the structure of the vein.2 marks

POINT

Two paired structural differences between artery and vein — each statement must reference both vessels or use a comparator.

EVIDENCE

AQA mark schemes for this style of question typically accept: thicker muscle / thicker elastic tissue / narrower lumen / no valves (any two).

EXPLAIN

Model answer: "The artery has a thicker layer of muscle than the vein. The artery has a narrower lumen than the vein." ✓✓

Writing only "artery has thick muscle" scores zero on this part — no comparator. Mark-scheme convention: precise paired wording (thicker muscle, narrower lumen) wins full marks; a vague "thicker wall" usually earns a 1-mark fallback at most; bare adjectives score nothing.

Three Examiner Alerts

Alert 1 — Paired comparisons

For any compare question on arteries vs veins, every statement must be paired — "thicker than" or "narrower than" — not just "thick" or "narrow". AQA mark schemes consistently enforce this rule.

Alert 2 — The pulmonary exception

The pulmonary artery carries deoxygenated blood. The pulmonary vein carries oxygenated blood. These are the exceptions to the general rule. Mark schemes test this regularly.

Alert 3 — Name all three RBC adaptations

When explaining red blood cell adaptations, name all three: biconcave (large surface area), no nucleus (more space for haemoglobin), contains haemoglobin (binds oxygen). Mark schemes typically award 1 mark per adaptation.

Common Errors — Wrong vs Right

Wrong

"Artery has thick wall."

Right

"Artery has a thicker wall than vein." (paired comparison)

Wrong

"All arteries carry oxygenated blood."

Right

"Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood."

Wrong

"Veins have thick walls."

Right

"Veins have thinner walls than arteries, a wider lumen, and contain valves."

Wrong

"White blood cells carry oxygen."

Right

"Red blood cells carry oxygen via haemoglobin; white blood cells fight infection."

Three Things To Remember

1. The Heart Is A Double Pump

Right side pumps deoxygenated blood to the lungs (via pulmonary artery); left side pumps oxygenated blood to the body (via aorta). The left ventricle has a thicker wall because it pumps at higher pressure around the whole body.

2. Three Vessel Types — Paired Wording

Arteries (thicker wall, narrower lumen, no valves — away from heart); veins (thinner wall, wider lumen, valves — back to heart); capillaries (walls one cell thick — exchange site). Always use -er comparators.

3. Four Blood Components

Plasma (transports dissolved substances + heat), red blood cells (carry oxygen via haemoglobin — biconcave, no nucleus), white blood cells (phagocytes engulf, lymphocytes produce antibodies), platelets (cell fragments — clotting).

Try It Yourself

Explain two ways a red blood cell is adapted to carry oxygen efficiently. 2 marks

POINT

Two structural adaptations linked to oxygen transport.

EVIDENCE

The cell is biconcave — a flattened, dipped shape. It also has no nucleus.

EXPLAIN

The biconcave shape gives a large surface area, so oxygen can diffuse in and out faster. The absence of a nucleus leaves more space for haemoglobin, so the cell can carry more oxygen. ✓✓

Either two of: biconcave (large SA) / no nucleus (more haemoglobin) / contains haemoglobin (binds O₂) would score. Each adaptation needs a function linked to it — not just naming the feature.

Up Next

You've covered the structure of the circulatory system. Next lesson is what goes wrong with it.

Lesson 2.07 — Coronary Heart Disease, Health & Cancer

What happens when coronary arteries narrow, the three named treatments (stents, statins, faulty valve replacement), the definition of health, risk factors for non-communicable disease, and benign vs malignant tumours.

Before You Move On

Test yourself — can you list the four chambers and the four major vessels from memory? Can you give two paired-comparison statements for artery vs vein? If yes, you're ready for 2.07.