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B4.1 The Blood

GCSE Combined Science · AQA · Chapter B4: Organising Animals and Plants

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: blood as a tissue: plasma, red blood cells, white blood cells and platelets. 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 blood as a tissue made of plasma with red blood cells, white blood cells and platelets suspended in it
  • List what plasma transports and where each substance is carried
  • Explain how red blood cells are adapted to carry oxygen
  • Describe the roles of white blood cells and of platelets in clotting
  • Recognise blood cells in diagrams and use magnification calculations

Key words

  • Tissue: A group of cells with a similar structure and function. Blood is a liquid tissue.
  • Plasma: The straw-coloured liquid part of blood, mostly water, that carries cells and dissolved substances.
  • Haemoglobin: The red protein in red blood cells that joins with oxygen.
  • Oxyhaemoglobin: Haemoglobin joined to oxygen. It forms in the lungs and breaks down again in the tissues.
  • Phagocyte: A white blood cell that engulfs pathogens and digests them with enzymes.
  • Lymphocyte: A white blood cell that makes antibodies and antitoxins.
  • Platelets: Small fragments of cells, with no nucleus, that start the clotting process.

Blood is a tissue

Blood is made of a liquid, plasma, with three kinds of cell or cell part suspended in it: red blood cells, white blood cells and platelets. They work together to transport substances and to defend the body, so blood counts as a tissue. An adult has roughly 5 litres of it.

If a blood sample is spun fast in a centrifuge, the heavier cells sink to the bottom and the parts separate into layers.

Worked example: percentages from a spun sample

A 5.0 cm³ sample of blood is spun. The plasma layer is 2.8 cm³. What percentage of the sample is plasma?
percentage = (part ÷ whole) × 100 = (2.8 ÷ 5.0) × 100 = 56%

Answer: 56% plasma, so 44% is cells and platelets

Plasma: the transport liquid

Plasma is more than 90% water. It carries the blood cells around the body, and many substances travel dissolved in it.

  • Carbon dioxide (waste from respiration); Carried from all respiring organs; Carried to the lungs, to be breathed out
  • Urea (waste made when excess amino acids are broken down); Carried from the liver; Carried to the kidneys, to be removed in urine
  • Soluble products of digestion (glucose, amino acids); Carried from the small intestine; Carried to all cells of the body
  • Hormones; Carried from the glands that make them; Carried to their target organs
  • Antibodies and antitoxins; Carried from white blood cells; Carried to wherever there is an infection

Exam tip: Oxygen is not mainly carried in the plasma. Almost all of it is carried by the haemoglobin inside red blood cells.

Red blood cells carry oxygen

Red blood cells pick up oxygen in the lungs and carry it to every cell for respiration. They are by far the most common blood cell: a healthy adult has roughly 4 to 6 million in every mm³ of blood.

  • Biconcave disc shape (thinner in the middle); How it helps large surface area to volume ratio, so oxygen diffuses in and out quickly
  • No nucleus; How it helps more space for haemoglobin, so each cell carries more oxygen
  • Packed with haemoglobin; How it helps haemoglobin + oxygen ⇌ oxyhaemoglobin: it loads oxygen in the lungs and unloads it in the tissues
  • Small and flexible; How it helps squeezes through narrow capillaries, close to the body cells

Worked example: actual size from a micrograph

In a light micrograph a red blood cell measures 24 mm across. The magnification is × 3000. How big is the real cell in µm?
actual size = image size ÷ magnification. Change mm to µm first: 24 mm = 24 000 µm.
actual size = 24 000 ÷ 3000 = 8.0 µm

Answer: 8.0 µm

White blood cells and platelets

White blood cells are much less common than red blood cells, but they are larger and each one has a nucleus. They are part of the immune system, which defends the body against pathogens (more in B5).

  • Phagocytes engulf pathogens and digest them with enzymes (phagocytosis).
  • Lymphocytes make antibodies, which lock on to markers called antigens on a pathogen and clump pathogens together, and antitoxins, which neutralise toxins.

Platelets are tiny fragments of cells with no nucleus. At a cut they gather and release clotting factors. These turn the soluble protein fibrinogen in the plasma into insoluble threads of fibrin, which form a mesh.

Key fact: A low platelet count means blood clots slowly, so a person bruises and bleeds easily. A high white blood cell count often means the body is fighting an infection.

Common mistakes

  • Saying red blood cells have a nucleus. Mature red blood cells have no nucleus; white blood cells do.
  • Writing that plasma carries oxygen. Oxygen is carried by haemoglobin in red blood cells.
  • Calling platelets cells. They are fragments of cells.
  • Saying the biconcave shape gives "more space". It gives a larger surface area for diffusion; having no nucleus gives more space for haemoglobin.
  • Forgetting to change mm to µm (× 1000) in magnification calculations.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows things seen in a blood smear. Which letter shows a structure that has no nucleus and helps the blood to clot? (Choose from: A, B, C, D.)

Answer: C

Working: C shows platelets: small fragments of cells with no nucleus that start the clotting process. A and D are red blood cells; B is a white blood cell.

Question 2. True or false? Platelets have a nucleus.

Answer: False

Working: False. Platelets are fragments of cells, so they have no nucleus.

Question 3. A white blood cell is 12 µm across. In a photograph it measures 54 mm across. What is the magnification of the photograph?

Answer: 4500

Working: 54 mm = 54 000 µm. Magnification = 54 000 ÷ 12 = × 4500.

Related lessons

Continue in the full course

This lesson is section B4.1 (AGCSC0401) 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.

 

 

 

 

 

 

 

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