In 1628 the English physician William Harvey published a short book usually called De Motu Cordis (On the Motion of the Heart and Blood). For centuries, doctors had followed the ancient Greek physician Galen, who taught that blood was made in the liver and used up by the body. Harvey argued instead that the heart pumps the same blood round and round in a circuit. One of his demonstrations needed nothing more than a band tied round an arm: the veins swelled below it, and pressing along a vein showed that its valves let blood move only towards the heart.
Harvey could not see how blood got from the arteries into the veins. That missing link was found after his death: in 1661 the Italian anatomist Marcello Malpighi, using a microscope, described tiny vessels in the lungs of a frog. We call them capillaries.
This lesson covers the heart and blood vessels in AQA GCSE Biology Topic 2 (Organisation). It's also useful for Combined Science.
A double circulatory system
The heart is an organ that pumps blood round the body in a double circulatory system. The right ventricle pumps blood to the lungs, where gas exchange takes place. The left ventricle pumps blood round the rest of the body. So on one full trip round the body, blood passes through the heart twice.
Inside the heart
Follow one drop of blood:
- Deoxygenated blood from the body arrives in the right atrium through the vena cava.
- It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs.
- Oxygenated blood returns through the pulmonary vein to the left atrium.
- It passes into the left ventricle, which pumps it out through the aorta to the body.
Valves stop blood flowing backwards. AQA doesn't require their names. The heart muscle gets its own oxygen from the coronary arteries, which branch off the aorta.
The left ventricle has the thickest wall because it pumps blood at higher pressure, all the way round the body. The resting heart rate is set by a group of cells in the right atrium that act as a natural pacemaker. An artificial pacemaker is an electrical device that corrects an irregular heart rate.
Arteries, veins and capillaries
| Vessel | Structure | Why |
|---|---|---|
| Artery | Thick muscular, elastic walls | Carries blood away from the heart at high pressure |
| Vein | Thinner walls, wide lumen, valves | Blood returns at low pressure; valves stop backflow |
| Capillary | Walls one cell thick | Short diffusion distance to and from cells |
You may be asked to calculate a rate of blood flow: rate = volume ÷ time. If 2000 cm³ flows in 4 minutes, the rate is 500 cm³ per minute.
The lungs
AQA limits the lungs to the trachea, the bronchi, the alveoli and the capillary network round the alveoli. Alveoli are adapted for gas exchange: a very large surface area, thin walls, a good blood supply, and breathing keeps the concentration gradients steep.
When the heart goes wrong
In coronary heart disease, layers of fatty material build up inside the coronary arteries and narrow them. Less blood reaches the heart muscle, so it lacks oxygen.
- Stents hold a narrowed coronary artery open.
- Statins lower blood cholesterol, slowing the build-up of fatty material. They must be taken long term and can have side effects.
- A faulty valve may not open fully or may leak. It can be replaced with a biological or mechanical valve.
- In heart failure, a donor heart, or heart and lungs, can be transplanted. Artificial hearts are occasionally used to keep a patient alive while waiting, or to let the heart rest and recover.
You need to weigh up the benefits and risks of drugs, devices and transplants.
Common exam mistakes
- "Arteries carry oxygenated blood." The pulmonary artery carries deoxygenated blood. Arteries carry blood away from the heart.
- Mixing up left and right. Heart diagrams are drawn as if the person faces you, so the left side is on the right of the page.
- "The left ventricle pumps more blood." Both ventricles pump about the same volume. The left pumps at higher pressure.
- "Capillaries have thin walls so blood flows faster." The thin wall is for a short diffusion distance.
- Saying statins unblock arteries. They slow further build-up. A stent opens the artery.
Frequently asked questions
What is the difference between arteries and veins at GCSE?
Arteries carry blood away from the heart at high pressure and have thick muscular, elastic walls. Veins carry blood back at low pressure, with thinner walls and valves.
Why is the left side of the heart thicker?
The left ventricle pumps blood all round the body, which needs higher pressure than pumping to the nearby lungs.
Which blood vessel carries deoxygenated blood away from the heart?
The pulmonary artery, from the right ventricle to the lungs.
How is coronary heart disease treated?
With stents to hold the coronary arteries open and statins to lower cholesterol. Transplants are used in heart failure.
Try it
Say each answer before you flip the card. Then put yourself in Harvey's place: you're sure blood goes round in a circle, but you can't see how it gets from the arteries to the veins. What would you have tested next?
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