4.2 The Blood Vessels

A free GCSE Biology lesson for AQA (8461) from OBA School: arteries, veins and capillaries, the double circulatory system and rate of blood flow. 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

• Describe the structure of arteries, veins and capillaries
• Explain how the structure of each type of blood vessel is related to its function
• Describe the double circulatory system and explain why it is efficient
• Calculate the rate of blood flow and a pulse rate

Key words

• Artery: A blood vessel that carries blood away from the heart, under high pressure.
• Vein: A blood vessel that carries blood back to the heart, under low pressure.
• Capillary: A tiny blood vessel with a wall one cell thick, where substances are exchanged with body cells.
• Lumen: The space inside a blood vessel that the blood flows through.
• Valve: A flap of tissue that stops blood flowing backwards.
• Double circulatory system: A system in which blood passes through the heart twice for each complete journey around the body.

Three types of blood vessel

Blood is pumped by the heart through a closed system of tubes. Arteries carry blood away from the heart, divide into millions of capillaries inside every organ, and the capillaries join up into veins that bring the blood back.

• Artery; Structure thick walls of muscle and elastic fibres; small lumen; How the structure fits the function withstands the high pressure from each heartbeat; elastic fibres stretch and then recoil, which evens out the flow (this is the pulse you feel)
• Capillary; Structure wall one cell thick; very narrow; huge network; How the structure fits the function short diffusion distance for oxygen, glucose and carbon dioxide; reaches close to every cell
• Vein; Structure thinner walls; large lumen; valves; How the structure fits the function blood is at low pressure; the large lumen lets it flow easily; valves stop it flowing backwards

Exam tip: Arteries carry blood away from the heart. Describe the pressure in each vessel, not the oxygen: the pulmonary artery carries deoxygenated blood.

Exchange in the capillaries

Capillaries are where the real work of the circulation happens. Oxygen and soluble food molecules such as glucose diffuse out of the blood into the body cells, and carbon dioxide and other wastes diffuse in. Because body cells keep using oxygen, there is always a concentration gradient from the blood to the cells.

Key fact: No cell in the body is more than a few cells away from a capillary. The thin wall and the short distance make diffusion fast.

The double circulatory system

Humans have a double circulatory system: two separate circuits joined at the heart.

• The right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen and gives up carbon dioxide.
• The oxygenated blood returns to the left side of the heart, which pumps it to all the other organs of the body.
• Deoxygenated blood returns from the body to the right side of the heart, and the cycle repeats.

Blood loses pressure as it squeezes through the capillaries of the lungs. Sending it back to the heart to be pumped a second time means it reaches the body at high pressure, so oxygen is delivered quickly to active tissues. Fish have a single circulation and their blood travels much more slowly after the gills.

Exam tip: When tracing the route of blood, name every structure in order: vena cava → right side of heart → pulmonary artery → lungs → pulmonary vein → left side of heart → aorta → body.

Rate of blood flow and pulse rate

A rate tells you how much happens in each unit of time.

rate of blood flow = volume of blood ÷ time (for example ml/min or ml/s)

pulse rate (beats per minute) = number of beats ÷ time in minutes

Worked example: rate of blood flow

A doctor measures that 2.7 litres of blood pass through a large artery in 30 seconds. Calculate the rate of blood flow in ml/s.
2.7 litres = 2.7 × 1000 = 2700 ml
rate = 2700 ÷ 30 = 90 ml/s

Answer: 90 ml/s (this is 5400 ml/min)

Worked example: pulse rate

Jo counts 19 beats in 15 seconds at the artery in her wrist. What is her pulse rate in beats per minute?
15 seconds is a quarter of a minute, so multiply by 4: 19 × 4 = 76

Answer: 76 beats per minute

Common mistakes

• Saying arteries always carry oxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs.
• Saying veins have thick walls. Veins have thin walls and a large lumen; arteries have thick walls.
• Writing that capillary walls are "one cell wall thick". They are one cell thick (animal cells have no cell wall).
• Saying the heart pumps blood "twice as fast" in a double circulation. The point is that blood passes through the heart twice, so it is pumped to the body at high pressure.
• Forgetting to change litres to ml (× 1000) or minutes to seconds (× 60) in rate calculations.

Revision questions

Try these, then check your answers below each one.

Question 1. Which type of blood vessel has valves to stop blood flowing backwards? (Choose from: Arteries, Capillaries, Veins.)

Answer: Veins

Working: Veins carry blood at low pressure, so they have valves to stop it flowing backwards.

Question 2. True or false? The pulmonary artery carries oxygenated blood.

Answer: False

Working: False. It carries deoxygenated blood from the right ventricle to the lungs.

Question 3. 1.8 litres of blood flow through an artery in 30 seconds. Calculate the rate of blood flow in ml/s.

Answer: 60 ml/s

Working: 1.8 litres = 1800 ml. Rate = 1800 ÷ 30 = 60 ml/s.

Related lessons

• Previous lesson: 4.1 The Blood
• Next lesson: 4.3 The Heart
• In this chapter: 4.4 Helping the Heart
• 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 4.2 (AGCBH0402) 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.