4.1 The Blood

A free GCSE Biology lesson for AQA (8461) 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
• State the roles of white blood cells and platelets
• Recognise the different blood cells in a diagram or photograph and calculate their size or magnification

Key words

• Tissue: A group of cells with a similar structure and function. Blood is a liquid tissue.
• Plasma: The pale yellow liquid part of blood. It carries the cells and many dissolved substances.
• Haemoglobin: The red protein in red blood cells that joins with oxygen.
• Oxyhaemoglobin: Haemoglobin that has joined with oxygen. It forms in the lungs and splits up in the tissues.
• Phagocyte: A white blood cell that engulfs and digests pathogens.
• Lymphocyte: A white blood cell that makes antibodies and antitoxins.
• Platelets: Small fragments of cells, with no nucleus, that help the blood to clot.

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. Because it is a group of cells working together to transport substances and defend the body, blood counts as a tissue.

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

Worked example: percentages from a spun sample

A 6.0 cm³ sample of blood is spun. The plasma layer is 3.3 cm³. What percentage of the sample is plasma?
percentage = (part ÷ whole) × 100 = (3.3 ÷ 6.0) × 100 = 55%

Answer: 55% plasma, so 45% is cells

Plasma: the transport liquid

Plasma is mostly water. It carries the blood cells around the body and also carries many substances dissolved in it.

• Carbon dioxide (waste from respiration); Carried from all respiring organs; Carried to the lungs, to be breathed out
• Urea (waste from breaking down amino acids); 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 the most common blood cell: a healthy adult has about 4 to 6 million in every mm³ of blood.

• Biconcave disc shape (thinner in the middle); How it helps gives a larger surface area to volume ratio, so oxygen diffuses in and out faster
• No nucleus; How it helps more space inside 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 can squeeze through narrow capillaries, close to the body cells

Worked example: actual size from a micrograph

In a photograph taken with a light microscope, a red blood cell is 30 mm across. The magnification is × 4000. How big is the real cell in µm?
actual size = image size ÷ magnification. Change mm to µm first: 30 mm = 30 000 µm.
actual size = 30 000 ÷ 4000 = 7.5 µm

Answer: 7.5 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 body's defence against pathogens (you will meet them again in Chapter 5).

• Phagocytes engulf pathogens and digest them (phagocytosis).
• Lymphocytes make antibodies, which stick to pathogens, and antitoxins, which neutralise the toxins that bacteria release.

Platelets are tiny fragments of cells with no nucleus. At a cut they help the blood to clot: they trigger a chain of reactions that makes a mesh of protein fibres (fibrin) which traps red blood cells. The clot stops blood loss and keeps pathogens out while the skin heals.

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; the missing 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. Which part of the blood carries urea from the liver to the kidneys? (Choose from: Red blood cells, White blood cells, Platelets, Plasma.)

Answer: Plasma

Working: Urea is dissolved in the plasma, which carries it from the liver to the kidneys.

Question 2. True or false? Mature red blood cells have a nucleus.

Answer: False

Working: False. Red blood cells have no nucleus, which leaves more room for haemoglobin.

Question 3. A red blood cell is 7.5 µm across. In a photograph it measures 45 mm across. What is the magnification of the photograph?

Answer: 6000

Working: 45 mm = 45 000 µm. Magnification = 45 000 ÷ 7.5 = × 6000.

Related lessons

• Previous lesson: 3.8 Required Practicals: Organisation and the Digestive System
• Next lesson: 4.2 The Blood Vessels
• In this chapter: 4.3 The Heart
• In this chapter: 4.4 Helping the Heart
• In this chapter: 4.5 Breathing and Gas Exchange

Continue in the full course

This lesson is section 4.1 (AGCBH0401) 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.