B4.2 The Blood Vessels

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: arteries, veins and capillaries, tissue fluid, the double circulatory system and rates. 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 how substances are exchanged between capillaries and body cells
• 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.
• Tissue fluid: Liquid that leaks out of capillaries and bathes the body cells.
• 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

The heart pumps blood through a closed system of tubes. Arteries carry blood away from the heart and branch into millions of capillaries inside every organ. 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 (the pulse you feel at the wrist or neck)
• 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 (for example, down the legs)

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 circulation does its real work. Some plasma is squeezed out through the thin capillary walls and becomes tissue fluid, which bathes the cells. Oxygen and glucose diffuse from it into the cells; carbon dioxide and urea diffuse the other way and return to the blood.

Key fact: Because body cells keep using oxygen in respiration, there is always a concentration gradient from the blood to the cells. The thin wall makes the diffusion distance short.

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.
• 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 lung capillaries. Returning it to the heart to be pumped again means it reaches the body organs at high pressure, so oxygen and glucose are delivered quickly to active tissues.

Exam tip: Trace the route 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

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

During a scan, 3.3 litres of blood pass through a large artery in 40 seconds. Calculate the rate of blood flow in ml/s.
3.3 litres = 3.3 × 1000 = 3300 ml
rate = 3300 ÷ 40 = 82.5 ml/s

Answer: 82.5 ml/s (this is 4950 ml/min)

Worked example: pulse rate

Kim counts 13 beats in 10 seconds at the artery in her neck. What is her pulse rate in beats per minute?
10 seconds is one-sixth of a minute, so multiply by 6: 13 × 6 = 78

Answer: 78 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 a double circulation pumps blood "twice as fast". Blood passes through the heart twice, so it reaches 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. The diagram shows cross-sections of three blood vessels. Which letter shows a vein? (Choose from: X, Y, Z.)

Answer: X

Working: X is a vein: thin wall and large lumen. Y is an artery (thick wall, small lumen) and Z is a capillary (wall one cell thick).

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

Answer: True

Working: True. It carries blood that has just picked up oxygen in the lungs back to the left atrium.

Question 3. 2.4 litres of blood flow through an artery in 40 seconds. Calculate the rate of blood flow in ml/s.

Answer: 60 ml/s

Working: 2.4 litres = 2400 ml. Rate = 2400 ÷ 40 = 60 ml/s.

Related lessons

• Previous lesson: B4.1 The Blood
• Next lesson: B4.3 The Heart
• In this chapter: B4.4 Helping the Heart
• In this chapter: B4.5 Breathing and Gas Exchange
• In this chapter: B4.6 Tissues and Organs in Plants

Continue in the full course

This lesson is section B4.2 (AGCSC0402) of our free GCSE Combined Science 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 Combined Science course.