AQA GCSE Biology 8461: Blood – Revision Notes

These are condensed recall notes for 4.2.2.3 Blood of the AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards. Organisation is assessed on Paper 1, set at Foundation and Higher Tier. Nothing in 4.2.2.3 is marked (HT only) or (biology only), so every line applies to both tiers. There is no required practical here, but the specification suggests observing and drawing blood cells under a microscope.

The heart and vessels that carry the blood are in the Principles of organisation; the heart and blood vessels revision notes. How white blood cells destroy pathogens is covered in more depth in the Infection and response revision notes. The course hub is AQA GCSE Biology, and the printable checklist lists every point. To find your weak spots fast, try the free 10-minute diagnostics.

Blood is a tissue

Blood is a tissue: a group of cells (plus cell fragments) with a shared function, carried in a liquid.

blood = plasma (liquid) + red blood cells + white blood cells + platelets (suspended in the plasma)

It is not an organ: it is one tissue, not several tissues working together.

The four components

Component	What it is	Function
Plasma	Pale yellow liquid, mostly water	Carries the cells and dissolved substances around the body
Red blood cells	Small cells, no nucleus, packed with haemoglobin	Carry oxygen from the lungs to the body cells
White blood cells	Larger cells, each with a nucleus	Defend the body against pathogens (part of the immune system)
Platelets	Tiny fragments of cells, no nucleus	Help the blood clot at a wound

What plasma carries

Substance	From	To
Glucose and amino acids (soluble products of digestion)	Small intestine	Body cells
Carbon dioxide	Respiring body cells	Lungs
Urea	Liver	Kidneys
Hormones	Glands	Target organs
Antibodies and other proteins	Made in the body	Around the body

Plasma also carries the blood cells and platelets themselves, and transfers heat around the body.

Adaptations to function

Red blood cells

Feature	Effect
Biconcave disc shape (dented on both sides)	Large surface area for oxygen to diffuse in and out
No nucleus	More room inside for haemoglobin
Haemoglobin	Red pigment that binds oxygen in the lungs and releases it in the tissues
Small and flexible	Can squeeze through narrow capillaries

White blood cells

• They have a nucleus (often large or lobed), which makes them easy to tell apart from red cells.
• Some engulf and digest pathogens (phagocytosis).
• Some make antibodies, which target particular pathogens, and antitoxins, which neutralise toxins.
• Some can change shape to squeeze out of capillaries into infected tissue.

Platelets

• Small fragments of cells with no nucleus.
• At a wound they help form a clot, which dries to a scab.
• The clot stops blood loss and stops pathogens getting in through the wound.

Recognising blood cells in a photograph or diagram

You see	It is
Many round cells, pale in the centre, no nucleus	Red blood cells
Fewer, larger cells with a darkly stained nucleus	White blood cells
Tiny specks or fragments between the cells	Platelets
The background liquid (not visible as cells)	Plasma

Red blood cells far outnumber white blood cells in any normal blood smear, so a field of view is mostly red cells.

Method in steps: observing and drawing blood cells

1. Use a prepared, stained blood smear slide (the stain shows up nuclei).
2. Start with the lowest-power objective and focus with the coarse focus.
3. Switch to a higher power and sharpen with the fine focus.
4. Draw in pencil with clear, unshaded outlines, the right relative sizes, and ruled label lines.
5. Write the magnification (eyepiece x objective) beside the drawing.

Worked reminder: actual size

actual size = image size / magnification

A red blood cell measures 3.5 mm across in a photo at x500. Actual size = 3.5 / 500 = 0.007 mm. Convert: 0.007 x 1000 = 7 µm. Always convert to µm for cells.

Evaluating blood products

The specification asks you to weigh the risks of using blood products, such as transfusions of donated blood.

Benefit	Risk
Replaces blood lost in an accident, surgery or childbirth	A pathogen could be passed on in donated blood (so donations are screened)
Platelets or plasma can be given to people who need just that part	The donated blood must be matched to the recipient, or their immune system may react against it
Can save a life quickly	Supply depends on donors and blood must be stored correctly

In an evaluation, give a benefit and a risk, then a judgement (for example: the benefit of saving a life usually outweighs a small, controlled risk of infection).

Must-know distinctions

• Red vs white blood cells: carry oxygen, no nucleus, very many vs defend against pathogens, nucleus, fewer.
• Platelets vs cells: platelets are fragments of cells, not whole cells.
• Plasma vs blood: plasma is the liquid part only. Blood is plasma plus everything suspended in it.
• Haemoglobin vs red blood cell: haemoglobin is the protein pigment; the red blood cell is the cell that contains it.

Quick self-test

1. Name the four components of blood.
2. Is blood a tissue or an organ? Explain.
3. Give two ways a red blood cell is adapted to carry oxygen, and the effect of each.
4. Name the substance in red blood cells that binds oxygen.
5. How can you tell a white blood cell from a red blood cell in a micrograph?
6. Give two ways white blood cells defend the body.
7. What are platelets, and what do they do?
8. Name three substances carried in plasma, and where each is carried to.
9. A white blood cell is 6 mm wide in a photo at x500. Calculate its actual width in µm.
10. Give one benefit and one risk of a blood transfusion.

Answers

1. Plasma, red blood cells, white blood cells and platelets.
2. A tissue. It is one group of cells with a shared function, suspended in plasma, not several tissues working together.
3. Any two: biconcave shape gives a large surface area for diffusion; no nucleus leaves more room for haemoglobin; small and flexible to pass through capillaries.
4. Haemoglobin.
5. The white blood cell is larger and has a nucleus; the red blood cell has no nucleus.
6. Any two: engulfing and digesting pathogens; making antibodies; making antitoxins.
7. Fragments of cells with no nucleus. They help the blood clot, which stops blood loss and keeps pathogens out of wounds.
8. Any three: glucose and amino acids to body cells; carbon dioxide to the lungs; urea to the kidneys; hormones to target organs.
9. 6 / 500 = 0.012 mm = 12 µm.
10. Benefit: replaces lost blood and can save a life. Risk: a pathogen could be passed on, or the blood could be wrongly matched, causing an immune reaction.

Where marks are usually lost

• Saying red blood cells have “no nucleus so they can carry more oxygen” without mentioning haemoglobin.
• Calling platelets cells, or giving them a nucleus.
• Writing that white blood cells “fight disease” without saying how (engulf, antibodies, antitoxins).
• Forgetting that plasma carries carbon dioxide and urea, not just food.
• Calling blood an organ.
• Leaving an actual-size answer in mm when the question asks for µm.

Frequently asked questions

Is blood a tissue or an organ? Blood is a tissue: a group of cells and cell fragments with a similar function, in a liquid called plasma. It is not an organ.

What are the four components of blood? Plasma, red blood cells, white blood cells and platelets.

What do platelets do? Platelets are small fragments of cells with no nucleus. At a wound they help form a clot, which stops blood loss and stops pathogens getting in.

How do white blood cells defend the body? Some white blood cells engulf and digest pathogens by phagocytosis, and some make antibodies and antitoxins. Saying they “fight disease” without saying how loses marks.

What does plasma carry? Plasma carries the blood cells and platelets, and substances such as carbon dioxide, urea, glucose, hormones and proteins.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.2 Organisation: 4.2.2.3 Blood.