What are the main components of blood and their functions in the human body?

Blood is composed of four main components: red blood cells, white blood cells, platelets, and plasma. Red blood cells transport oxygen from the lungs to the rest of the body and carry carbon dioxide back to the lungs for exhalation. White blood cells are part of the immune system and help fight infections. Platelets are involved in blood clotting, which prevents excessive bleeding. Plasma, the liquid portion, carries nutrients, hormones, and waste products throughout the body.

How does blood transport oxygen and carbon dioxide in the body?

Blood transports oxygen and carbon dioxide through red blood cells, which contain hemoglobin. Hemoglobin binds to oxygen in the lungs, forming oxyhemoglobin, and releases it to tissues where it is needed. Carbon dioxide, a waste product of cellular respiration, is transported back to the lungs in three forms: dissolved in plasma, chemically bound to hemoglobin, or as bicarbonate ions. This process is crucial for maintaining cellular respiration and energy production.

Why is blood considered a connective tissue in biology?

Blood is classified as a connective tissue because it originates from mesenchymal cells in the bone marrow and connects various systems in the body by transporting nutrients, gases, and waste products. Unlike other connective tissues, blood is unique in that it is a fluid, allowing it to circulate throughout the body and perform its transport functions efficiently.

What is the role of blood in the immune system?

Blood plays a critical role in the immune system through its white blood cells, which are responsible for defending the body against infections and foreign invaders. Different types of white blood cells, such as lymphocytes, neutrophils, and monocytes, work together to identify, attack, and destroy pathogens. Additionally, blood transports antibodies and other immune proteins that help neutralize harmful substances.

How does the circulatory system regulate body temperature using blood?

The circulatory system regulates body temperature through the process of vasodilation and vasoconstriction. When the body is too hot, blood vessels near the skin surface dilate, allowing more blood to flow through and release heat. Conversely, when the body is cold, blood vessels constrict to reduce blood flow to the skin, conserving heat. This thermoregulation process helps maintain a stable internal environment, essential for proper physiological functioning.

Why is "Saying red blood cells have a nucleus." a common mistake in Blood?

Why it happens: Most cells have a nucleus. How to avoid it: Mature human red blood cells LOSE their nucleus during development. This makes more space for haemoglobin (more O₂-carrying capacity).

Why is "Saying platelets ARE the clot." a common mistake in Blood?

Why it happens: They start the process. How to avoid it: Platelets TRIGGER the clot. The clot itself is FIBRIN (mesh) + trapped red blood cells. Platelets are the signal, not the structure.

Why is "Including red/white cells as part of 'plasma'." a common mistake in Blood?

Why it happens: Both are in blood. How to avoid it: PLASMA is the LIQUID part only — water + dissolved substances. Cells (red, white, platelets) are SUSPENDED in plasma but are NOT plasma.

Transport in AnimalsCambridge IGCSE Biology (0610)

Blood

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Blood — Cambridge IGCSE 0610 Biology Extended (2026)

A liquid tissue with four jobs: oxygen transport, defence, clotting, and carrying everything else dissolved.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

9.4 — Identify the components of blood (red cells, white cells, platelets, plasma).
9.4 — Describe the function of each.
9.4 — Describe the structure of red blood cells and link to function.
9.4 — Describe blood clotting.

Red blood cells — the oxygen carriers

Tiny biconcave discs packed with haemoglobin. No nucleus. Made in bone marrow.

Structure.

BICONCAVE DISC (~7 μm wide).
NO NUCLEUS, no most other organelles — leaves room for haemoglobin.
Packed with HAEMOGLOBIN — iron-containing protein.
Flexible membrane; squeezes through capillaries.

Adaptations:

Feature	Why
Biconcave disc	Large surface area for O₂ uptake; more flexible.
No nucleus	More space for haemoglobin = more O₂.
Packed with haemoglobin	Each Hb molecule binds 4 O₂.
Small + flexible	Can squeeze single-file through capillaries.

How haemoglobin works.

LUNGS (high O₂): Hb + 4 O₂ → OXYHAEMOGLOBIN (bright red).
TISSUES (low O₂): oxyhaemoglobin → Hb + 4 O₂ (deep red).
Reaction is REVERSIBLE — Hb is a carrier, not a permanent partner.

Lifespan.

~120 days.
Old red cells broken down in the LIVER and SPLEEN.
Iron from haemoglobin RECYCLED to make new red cells.
New cells made in BONE MARROW.

Worked qualitative. Why are people with sickle-cell anaemia anaemic?

A genetic mutation makes haemoglobin abnormal.
Cells become sickle-shaped under low O₂ → less effective + get stuck in capillaries.
Sickled cells break down faster (~10-20 days, not 120) → fewer red cells overall → anaemia.
Less O₂ to tissues → tiredness, breathlessness.

Cambridge tip. Always include 'no nucleus → more space for haemoglobin' in adaptation answers. Cambridge marks the explanation, not just the feature.

Biconcave disc.
No nucleus.
Packed with haemoglobin.
Lifespan ~120 days.
Made in bone marrow.

White blood cells — defence

Phagocytes engulf pathogens. Lymphocytes make antibodies. Both bigger than red cells, with nuclei.

Two main types of white blood cell, both bigger than red cells, both with nuclei.

1. Phagocytes.

Lobed nucleus, granular cytoplasm.
ENGULF and digest pathogens (bacteria, dead cells).
Process: pathogen → engulfed in vacuole → enzymes break it down → harmless products.
'Phago' = eating.

2. Lymphocytes.

Large round nucleus that fills most of the cell.
Make ANTIBODIES — Y-shaped proteins that BIND specifically to one type of pathogen.
Antibodies clump pathogens together (agglutination), mark them for destruction, or neutralise toxins.
Each lymphocyte makes one specific antibody.

Where they're made.

Bone marrow (most types).
Lymphocytes mature in the THYMUS or stay in bone marrow.
Stored in LYMPH NODES, SPLEEN, TONSILS.

Worked qualitative. Why does an infection often make lymph nodes SWELL?

Lymphocytes proliferate (divide rapidly) when responding to a pathogen.
More cells in nodes = swelling.
Once infection is cleared, swelling goes down.

Cambridge tip. Distinguish phagocytes (eat) and lymphocytes (make antibodies). Don't lump them as 'white blood cells fight infection'.

Phagocytes engulf pathogens.
Lymphocytes make antibodies.
Both have nuclei (red cells don't).
Made in bone marrow.

Clotting — sealing wounds

Platelets trigger; fibrinogen → fibrin mesh; red cells trapped → clot/scab.

Step 1. Blood vessel is damaged → platelets gather at the wound.

Step 2. Platelets release CHEMICAL FACTORS that trigger a chain of reactions.

Step 3. Soluble FIBRINOGEN (in plasma) is converted to insoluble FIBRIN.

$\text{fibrinogen (soluble)} \to \text{fibrin (insoluble)}$

Step 4. Fibrin forms a MESH of fine strands across the wound.

Step 5. Red blood cells get TRAPPED in the mesh → forms a CLOT (jelly-like plug).

Step 6. Clot dries → SCAB. Skin underneath repairs.

Why it matters:

Stops blood loss (limits bleeding).
Seals out pathogens — wound surface barrier.
Allows time for tissue repair.

Things that go wrong.

HAEMOPHILIA: genetic disorder where one of the clotting factors is missing → blood doesn't clot properly. Even small cuts bleed dangerously. Treated with replacement factor injections.
THROMBOSIS: clot forms inside an UNDAMAGED vessel → blocks flow. In a coronary artery → heart attack. In a brain artery → stroke.

Worked qualitative. Why is haemophilia much more common in MALES than females?

The faulty gene is on the X chromosome (sex-linked).
Males have only ONE X — if it carries the faulty allele, they're affected.
Females have TWO Xs — usually one is healthy → masks the faulty one.
Females can be CARRIERS (unaffected) but pass it to sons.

Cambridge tip. Memorise the chain: PLATELETS → FIBRINOGEN → FIBRIN → CLOT. Don't say 'blood thickens' — that's vague.

Platelets trigger.
Fibrinogen → fibrin (insoluble).
Fibrin mesh + trapped red cells = clot.
Dries to scab.
Haemophilia: faulty clotting (X-linked).

Plasma — the liquid medium

Pale yellow, ~90% water. Carries everything dissolved.

Composition.

~90% WATER.
Dissolved nutrients: GLUCOSE, AMINO ACIDS, mineral ions, vitamins.
Hormones: insulin, glucagon, adrenaline, etc.
Waste: UREA (from liver, going to kidneys), CO₂ (as bicarbonate).
ANTIBODIES (made by lymphocytes).
Plasma proteins: fibrinogen (clotting), albumin (osmotic balance), globulins.

Functions.

TRANSPORT — everything dissolved travels in plasma.
TEMPERATURE REGULATION — water carries heat from active organs (liver, muscles) to skin where it's lost.
pH BUFFERING — plasma proteins resist big pH swings.

Plasma vs serum.

Plasma = blood liquid INCLUDING fibrinogen and clotting factors.
Serum = plasma with the clotting factors REMOVED (i.e. what's left after clotting).
Antibody tests use serum.

Worked qualitative. Why does dehydration make blood 'thicker' (harder to flow)?

Less water in plasma → blood is more viscous.
Heart works harder to pump it.
Dehydration risks: heart strain, headaches, kidney stress.

Cambridge tip. Cambridge often asks 'what does plasma transport?' List THREE: glucose, urea, hormones (or antibodies). Three keeps the marks safe.

~90% water.
Carries glucose, amino acids, urea, CO₂, hormones, antibodies.
Helps regulate temperature.
Dehydration thickens blood.

How it’s examined

Blood appears every Paper 4 (5-10 marks). Common formats: name + function of components, RBC adaptations, clotting steps. Examiner reports flag students saying RBCs have a nucleus, calling platelets 'cells' (they're fragments), and confusing plasma with whole blood.

Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (9.4); 0610/42 Oct/Nov 2024 — Q8 (blood); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Blood

Step-by-step solutions to past-paper-style questions on blood, written exactly the way a tutor would explain them at the board.

1Components of blood
Core• blood components
Question
Name the FOUR main components of blood and state the function of each.
Step-by-step solution
1. Step 1
  Red blood cells (erythrocytes): carry OXYGEN (and some CO₂) using HAEMOGLOBIN.
2. Step 2
  White blood cells (leucocytes): defend against pathogens. Phagocytes engulf; lymphocytes make antibodies.
3. Step 3
  Platelets: small fragments that help BLOOD CLOTTING at wounds.
4. Step 4
  Plasma: pale yellow liquid (~90% water). Carries dissolved nutrients, hormones, urea, CO₂, antibodies.
Answer
Red cells (O₂), white cells (defence), platelets (clotting), plasma (transport medium).
2Adaptations of red blood cells
Extended• Adapted from 0610/42 Oct/Nov 2024 Q8• red blood cells
Question
Describe FOUR ways red blood cells are adapted for OXYGEN TRANSPORT.
Step-by-step solution
1. Step 1
  BICONCAVE DISC shape → large surface area for O₂ uptake; flexible to squeeze through capillaries.
2. Step 2
  No nucleus → more space for haemoglobin (more O₂ carried per cell).
3. Step 3
  Packed with HAEMOGLOBIN → iron-containing protein that binds O₂ reversibly.
4. Step 4
  Small and flexible → can squeeze through narrow capillaries one at a time.
Answer
Biconcave (surface area + flexibility), no nucleus (more haemoglobin), full of haemoglobin (binds O₂), small + flexible (capillary fit).
Examiner tip
Cambridge wants 'haemoglobin' explicitly + 'no nucleus to make space for more haemoglobin'. Memorise.
3How does haemoglobin transport oxygen?
Extended• haemoglobin
Question
Describe how HAEMOGLOBIN picks up oxygen at the lungs and releases it at body cells.
Step-by-step solution
1. Step 1
  At the LUNGS: O₂ concentration is HIGH. Haemoglobin BINDS O₂ → forms OXYHAEMOGLOBIN (red).
2. Step 2
  Reaction: $\text{Hb} + 4\text{O}_2 \to \text{HbO}_8$ (haemoglobin + 4 oxygen → oxyhaemoglobin).
3. Step 3
  At the BODY CELLS: O₂ concentration is LOW (cells are using it for respiration). Oxyhaemoglobin SPLITS to release O₂.
4. Step 4
  Reaction reverses: $\text{HbO}_8 \to \text{Hb} + 4\text{O}_2$ .
5. Step 5
  Reversibility is key — haemoglobin is a CARRIER, not a permanent partner. It loads where O₂ is plentiful, unloads where it's scarce.
Answer
Lungs (high O₂): Hb + O₂ → oxyhaemoglobin. Tissues (low O₂): oxyhaemoglobin → Hb + O₂. Reversible.
4How does blood clot?
Extended• clotting
Question
Outline the role of platelets and fibrinogen in blood clotting.
Step-by-step solution
1. Step 1
  When a blood vessel is cut, PLATELETS gather at the wound and release chemicals.
2. Step 2
  These chemicals trigger a series of reactions. The key step: SOLUBLE FIBRINOGEN (in plasma) is converted to INSOLUBLE FIBRIN.
3. Step 3
  FIBRIN forms a MESH of strands across the wound.
4. Step 4
  Red blood cells get TRAPPED in the mesh → forms a CLOT (plug) which seals the cut.
5. Step 5
  Clot dries to form a SCAB → barrier against pathogens; gives time for new tissue to grow.
Answer
Platelets trigger fibrinogen → fibrin (insoluble mesh) → traps red cells → clot/scab.

Key Definitions and Keywords — Blood

Definitions to memorise and the exact keywords mark schemes credit for blood answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Red blood cell (erythrocyte)
Examiner keyword
Biconcave disc-shaped cell containing haemoglobin. Carries oxygen from lungs to tissues.
Haemoglobin
Examiner keyword
Iron-containing protein in red blood cells that binds oxygen reversibly. Forms oxyhaemoglobin in lungs; releases O₂ at tissues.
White blood cell (leucocyte)
Examiner keyword
Cell of the immune system. Two main types: phagocytes (engulf pathogens) and lymphocytes (make antibodies).
Phagocyte
Type of white blood cell that engulfs and digests pathogens. Has a lobed nucleus.
Lymphocyte
Type of white blood cell that produces ANTIBODIES against pathogens.
Platelet
Small fragment of cell that helps blood clot at wounds. Releases factors that convert fibrinogen → fibrin.
Plasma
Examiner keyword
Pale yellow liquid component of blood (~90% water). Carries dissolved substances: glucose, amino acids, urea, hormones, antibodies.
Fibrin
Insoluble protein formed during clotting. Creates a mesh that traps red blood cells to form a clot.

Common Mistakes and Misconceptions — Blood

The traps other students keep falling into on blood questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.

✕Saying red blood cells have a nucleus.
Why it happens
Most cells have a nucleus.
How to avoid it
Mature human red blood cells LOSE their nucleus during development. This makes more space for haemoglobin (more O₂-carrying capacity).
✕Saying platelets ARE the clot.
Why it happens
They start the process.
How to avoid it
Platelets TRIGGER the clot. The clot itself is FIBRIN (mesh) + trapped red blood cells. Platelets are the signal, not the structure.
✕Including red/white cells as part of 'plasma'.
Why it happens
Both are in blood.
How to avoid it
PLASMA is the LIQUID part only — water + dissolved substances. Cells (red, white, platelets) are SUSPENDED in plasma but are NOT plasma.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Video lesson

Short walkthrough of the concepts students most often get stuck on.

Blood — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Blood

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Components of blood

2Adaptations of red blood cells

3How does haemoglobin transport oxygen?

4How does blood clot?

Red blood cell (erythrocyte)

Haemoglobin

White blood cell (leucocyte)

Phagocyte

Lymphocyte

Platelet

Plasma

Fibrin

✕Saying red blood cells have a nucleus.

✕Saying platelets ARE the clot.

✕Including red/white cells as part of 'plasma'.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Blood — frequently asked questions