4.3 The Heart

A free GCSE Biology lesson for AQA (8461) from OBA School: structure of the heart, the route of blood, the natural pacemaker and coronary heart disease. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Label the chambers, valves and main blood vessels of the heart
• Describe the route blood takes through the heart and explain why the left ventricle has the thickest wall
• Explain how the natural pacemaker controls the resting heart rate
• Describe coronary heart disease and how stents and statins are used to treat it

Key words

• Atrium (plural atria): One of the two upper chambers of the heart. The atria receive blood from veins.
• Ventricle: One of the two lower chambers. The ventricles pump blood out into arteries.
• Aorta: The main artery leaving the left ventricle. It carries oxygenated blood to the body.
• Vena cava: The main vein bringing deoxygenated blood from the body to the right atrium.
• Coronary arteries: Arteries on the surface of the heart that supply the heart muscle with oxygenated blood.
• Pacemaker: A group of cells in the right atrium that sets the natural resting heart rate.
• Coronary heart disease (CHD): A disease in which fatty material builds up inside the coronary arteries and narrows them.

Structure of the heart

The heart is an organ made mostly of muscle tissue. It pumps blood around the body in a double circulatory system. It has four chambers: two atria on top and two ventricles below. The right and left sides are separated by a wall of muscle called the septum, so deoxygenated and oxygenated blood never mix.

Exam tip: Diagrams of the heart show it as if you are facing the person, so the right side of the heart appears on the left of the page.

The route of blood through the heart

Both sides of the heart work at the same time. The atria contract first and push blood into the ventricles; then the ventricles contract and force blood out into the arteries. Valves stop the blood flowing backwards, so it always moves in one direction.

• vena cava → right atrium; Left side (oxygenated blood) pulmonary vein → left atrium
• → valve → right ventricle; Left side (oxygenated blood) → valve → left ventricle
• → valve → pulmonary artery → lungs; Left side (oxygenated blood) → valve → aorta → rest of the body

The left ventricle has a thicker, more muscular wall than the right ventricle. It must pump blood at high pressure all the way round the body, while the right ventricle only pumps blood to the nearby lungs (at lower pressure, which also protects the delicate lung capillaries).

Heart muscle cells respire all the time, so they need their own blood supply. This comes from the coronary arteries, which branch off the aorta and spread over the surface of the heart.

Key fact: Atria receive, ventricles pump out. Arteries leave the ventricles; veins enter the atria.

The natural pacemaker and heart rate

The resting heart rate is controlled by a group of cells in the wall of the right atrium that act as a natural pacemaker. They send out small electrical impulses that spread through the heart muscle and make it contract.

Worked example: heart rate

A heart monitor shows that a patient's heart beats 26 times in 20 seconds. Calculate the heart rate in beats per minute.
1 minute = 60 s, and 60 ÷ 20 = 3, so multiply by 3: 26 × 3 = 78

Answer: 78 beats per minute

Coronary heart disease

In coronary heart disease, layers of fatty material build up inside the coronary arteries and narrow them. Less blood flows through, so the heart muscle gets less oxygen. This can cause chest pain; if an artery becomes blocked, part of the heart muscle is starved of oxygen and may die – a heart attack.

• Stent; How it works a small mesh tube is pushed into the narrowed artery and opened up, holding the artery open so blood flows freely; Points to weigh up works straight away and lasts a long time; needs a small operation with some risk of infection or blood clots
• Statins; How it works drugs that reduce the amount of cholesterol in the blood, which slows down the build-up of fatty material; Points to weigh up cheap and no surgery; must be taken every day for life; possible side effects such as muscle pain; take time to work

Worked example: how much narrower? (Higher tier)

A coronary artery's lumen is 3.0 mm wide. Fatty material reduces it to 1.5 mm. By what fraction is the cross-sectional area reduced?
area of a circle = πr². Halving the radius multiplies the area by (1/2)² = 1/4.
So the new area is 1/4 of the old area: a reduction of 3/4 (75%).

Answer: the area is reduced by 75%, even though the width only halved

Common mistakes

• Mixing up the sides: the heart's right side is on the left of a diagram.
• Saying the left ventricle has a thick wall "because it holds more blood". Both ventricles hold the same volume; the left one pumps at higher pressure to the whole body.
• Saying CHD blocks the arteries inside the chambers. CHD affects the coronary arteries that supply the heart muscle itself.
• Writing that the heart muscle is short of "blood cells". The problem is a lack of oxygen (and glucose) for respiration.
• Thinking statins unblock arteries. Statins lower blood cholesterol and slow further build-up; stents physically hold the artery open.

Revision questions

Try these, then check your answers below each one.

Question 1. Which chamber of the heart has the thickest muscular wall? (Choose from: Right atrium, Left atrium, Right ventricle, Left ventricle.)

Answer: Left ventricle

Working: The left ventricle pumps blood at high pressure round the whole body, so it has the thickest wall.

Question 2. True or false? The natural pacemaker is a group of cells in the right atrium.

Answer: True

Working: True. A group of cells in the right atrium acts as the natural pacemaker.

Question 3. Name the arteries that supply the heart muscle with oxygenated blood.

Answer: coronary arteries

Working: The coronary arteries supply the heart muscle.

Related lessons

• Previous lesson: 4.2 The Blood Vessels
• Next lesson: 4.4 Helping the Heart
• In this chapter: 4.1 The Blood
• In this chapter: 4.5 Breathing and Gas Exchange
• In this chapter: 4.6 Tissues and Organs in Plants

Continue in the full course

This lesson is section 4.3 (AGCBH0403) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.