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The double circulatory system, how the heart pumps blood, and how arteries, veins and capillaries are adapted.
Specification references this topic covers: AQA 4.2.2; Edexcel B8.7, B8.8.
Your heart beats all day and all night. Each beat pushes blood around your body. Blood carries oxygen and nutrients to every cell, so the heart never gets a day off.
This matters because the heart and blood vessels deliver what every cell needs. If they fail, cells run short of oxygen. It also explains pulse, blood pressure and heart attacks.
By the end you will be able to describe the route blood takes through the heart. You will explain how arteries, veins and capillaries are suited to their jobs. You will also calculate a rate of blood flow.
Blood travels around the body in tubes called blood vessels. The heart is the pump. Humans have a double circulatory system, which means there are two loops that meet at the heart.
In one loop, the right side of the heart pumps blood to the lungs to pick up oxygen. In the other loop, the left side pumps oxygenated blood around the body. Blood passes through the heart twice on each full circuit.
Think of a figure-of-eight running track. The heart is where the two loops cross. Blood loses pressure in the lungs, so the second pump boosts it again.
The heart is a muscular organ with four chambers. The two atria at the top receive blood. The two ventricles at the bottom pump blood out. A tip: atria is for arrive.
Valves sit between the chambers and in the arteries. They stop blood flowing backwards.
The wall of the left ventricle is thicker than the right. It pumps blood all around the body at high pressure. The right ventricle only pumps blood to the lungs.
Follow one drop of blood. It comes from the body into the right atrium along the vena cava. It passes into the right ventricle. This pumps it along the pulmonary artery to the lungs.
In the lungs the blood picks up oxygen. It returns along the pulmonary vein to the left atrium, then into the left ventricle. The left ventricle pumps it out along the aorta to the body.
The pulmonary artery carries deoxygenated blood. The pulmonary vein carries oxygenated blood. This is the exception to the rule that arteries carry oxygenated blood.
Arteries carry blood away from the heart at high pressure. They have thick, elastic, muscular walls and a narrow lumen, which is the space inside. Veins carry blood back to the heart at low pressure. They have thinner walls, a wide lumen and valves to stop backflow.
Capillaries are tiny vessels that link them. Their walls are one cell thick, so substances diffuse quickly between the blood and the cells. There are so many that their total surface area is very large.
Think of motorways out of town, small streets to every house, and roads back.
Heart rate is the number of beats per minute. It rises during exercise, so more oxygen reaches the muscles.
A group of cells in the right atrium is the natural pacemaker. It controls the resting heart rate. If it does not work properly, an artificial pacemaker can be fitted.
The heart muscle gets its own blood supply from the coronary arteries. If fatty deposits block them, the muscle cells do not get enough oxygen. This can cause a heart attack.
The rate of blood flow tells you how much blood passes a point in a set time. Use rate of blood flow = volume of blood ÷ time.
Think of a tap filling a bucket. If 10 litres arrive in 2 minutes, the rate is 10 ÷ 2 = 5 litres per minute.
Now try blood. Say 300 cm³ passes through an artery in 5 minutes. The rate is 300 ÷ 5 = 60 cm³ per minute. Always give the unit.
The heart's output can be worked out too. The stroke volume is the volume of blood pumped out in one beat, and the heart rate is the number of beats each minute. Cardiac output = stroke volume × heart rate. A stroke volume of 70 cm³ at 72 beats per minute gives 5040 cm³ per minute.
The double circulatory system has two loops. The right side pumps to the lungs and the left side pumps to the body.
Route: vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta.
Arteries are thick and elastic. Veins have valves. Capillaries are one cell thick.
Rate of blood flow = volume ÷ time.
Name the blood vessel that carries blood from the heart to the lungs.
Answer: the pulmonary artery
A capillary has walls that are one cell thick. Explain how this helps it do its job.
Answer: A thin wall gives a short diffusion distance so substances exchange quickly
Explain why blood passes through the heart twice on each circuit of the body in a double circulatory system.
Answer: The pressure drops in the lungs so the heart pumps it again to increase the pressure and deliver oxygen faster
The heart and blood vessels practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
How many chambers are there in the human heart?
Answer: 4
Name the blood vessel that carries blood from the heart to the body.
Answer: aorta
Which type of blood vessel has valves?
Answer: veins
Name the smallest blood vessels, which are one cell thick.
Answer: capillaries
What name is given to the space inside a blood vessel?
Answer: lumen
What name is given to the two chambers at the top of the heart, which receive blood?
Answer: atria
Name the vessel that brings blood from the lungs to the heart.
Answer: pulmonary vein
Which chamber of the heart has the thickest wall?
Answer: left ventricle
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) Name the blood vessel that carries blood into the right atrium. [1 mark] (b) Describe how the structure of an artery is related to its function. [3 marks]
Answer: (a) vena cava (b) thick muscular and elastic walls to withstand high pressure, and a narrow lumen to keep the pressure high
Describe the route taken by a red blood cell from the right atrium back to the right atrium. [4 marks]
Answer: Right atrium to right ventricle, pulmonary artery to the lungs, pulmonary vein to the left atrium, left ventricle, aorta to the body, then the vena cava back to the right atrium.
Saying all arteries carry oxygenated blood
The pulmonary artery carries deoxygenated blood to the lungs.
Labelling the heart the wrong way round
The left side of the diagram is the right side of the heart, because you look at it from the front.
Saying veins have thick walls
Veins have thin walls. Arteries have thick, elastic walls.
Saying the atria pump blood around the body
The ventricles pump the blood out. The atria receive blood.
The heart and blood vessels is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
Saying all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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