BiologyICSE Class 10 · Circulatory System

Composition of Blood: Plasma, RBCs, WBCs, Platelets, Clotting and Blood Groups (ICSE Class 10 Biology)

Updated 16 October 2026 · Fact-checked

Blood is a fluid connective tissue of plasma (about 55%) and formed elements (about 45%). Red blood cells carry oxygen as oxyhaemoglobin and, in mammals, lack a nucleus and mitochondria. White blood cells defend the body by phagocytosis and antibodies. Platelets start clotting, in which fibrinogen becomes a mesh of fibrin.

Understand Composition of Blood and Blood Groups

Blood as a tissue. Blood is a fluid connective tissue: cells floating in a liquid matrix, the plasma. An adult has about 5 litres. Spun in a tube, it separates into pale yellow plasma (about 55%) above the packed formed elements (about 45%).

Plasma. About 90% water, with dissolved proteins (albumin; globulins including antibodies; fibrinogen and prothrombin for clotting), digested food (glucose, amino acids), salts, hormones, and wastes (urea, CO₂). Functions: transport of food, hormones, wastes and heat; clotting; defence.

Red blood cells (erythrocytes).

Biconcave discs, about 5 million per mm³ of blood.
No nucleus and no mitochondria in mammals (no endoplasmic reticulum either).
Packed with haemoglobin, an iron-containing pigment that joins loosely with O₂ to form oxyhaemoglobin in the lungs and releases it in the tissues.
Made in the red bone marrow; live about 120 days; old ones are broken down in the spleen and liver.

Why mammalian RBCs are efficient (both points needed).
1. No nucleus: more room for haemoglobin, and the biconcave shape gives a larger surface area for absorbing and carrying oxygen.
2. No mitochondria: they respire anaerobically, so they do not use up the oxygen they carry.

White blood cells (leucocytes). Colourless, nucleated, fewer than RBCs, can change shape and squeeze through the walls of capillaries into tissues: diapedesis.

Group	Type	Function
Granulocytes (granular cytoplasm, lobed nucleus)	Neutrophils	most numerous; phagocytosis (engulf germs)
	Eosinophils	allergies and parasitic infections
	Basophils	release heparin and histamine
Agranulocytes	Lymphocytes	produce antibodies
	Monocytes	phagocytosis; become macrophages

Platelets (thrombocytes). Tiny cell fragments with no nucleus; they start blood clotting.

Blood clotting (brief).
1. Injured tissue and platelets release thromboplastin.
2. Thromboplastin with calcium ions converts prothrombin into thrombin.
3. Thrombin converts soluble fibrinogen into insoluble fibrin threads.
4. Fibrin forms a mesh that traps blood cells: the clot seals the wound.
Vitamin K is needed to make prothrombin; heparin prevents clotting inside vessels.

ABO blood groups. Based on antigens on RBCs and antibodies in plasma.

Group	Antigen	Antibody	Can receive from
A	A	anti-b	A, O
B	B	anti-a	B, O
AB	A and B	none	all (universal recipient)
O	none	anti-a and anti-b	O only (universal donor)

Wrong transfusion makes the donor's RBCs clump (agglutinate).

Rh factor. An antigen on the RBCs of Rh-positive people. An Rh-negative mother carrying an Rh-positive baby may make anti-Rh antibodies, which in a later Rh-positive pregnancy can destroy the baby's RBCs (erythroblastosis foetalis).

Flow chart: clotting. Injury → platelets and tissues → thromboplastin → (+ Ca²⁺) prothrombin → thrombin → fibrinogen → fibrin mesh → clot.

Draw-and-label checklist: blood cells. RBC: a disc, face view round with a pale centre, side view dumb-bell shaped; label: biconcave, no nucleus. Neutrophil: round cell with a 3-5 lobed nucleus and granular cytoplasm. Lymphocyte: round cell with a large round nucleus filling most of it. Platelets: small irregular fragments.

Composition of Blood and Blood Groups: key facts

Blood cells: RBC biconcave, no nucleus, haemoglobin, carries O₂, 120 days
WBC nucleated, defence, diapedesis
Platelets fragments, clotting; Plasma 55%, formed elements 45%.
WBC types: $N e u t r o p hi l s : p ha g ocy t os i s ∣ L y m p h ocy t es : an t ib o d i es ∣ M o n ocy t es : p ha g ocy t os i s ∣ E os in o p hi l s : a l l er g y ∣ B a so p hi l s : h e p a r in, hi s t amin e$; Name the type, never just 'WBCs'.
Clotting and blood groups: Thromboplastin + Ca²⁺ prothrombin → thrombin
Thrombin fibrinogen → fibrin
O: universal donor
AB universal recipient; Vitamin K needed for prothrombin.

RBC	biconcave, no nucleus, haemoglobin, carries O₂, 120 days
WBC	nucleated, defence, diapedesis
Platelets	fragments, clotting

Thromboplastin + Ca²⁺	prothrombin → thrombin
Thrombin	fibrinogen → fibrin
	O: universal donor
AB	universal recipient

How to answer Composition of Blood and Blood Groups questions

ICSE asks you to name a WBC by its function, define diapedesis, explain RBC efficiency, order the steps of clotting, pick an odd one out among blood terms, or work out a safe transfusion.

1For RBC efficiency, give two reasons: no nucleus (more haemoglobin, more surface) and no mitochondria (no oxygen used).
2For WBCs, name the exact type: neutrophils engulf; lymphocytes make antibodies.
3For diapedesis, write 'through the walls of capillaries'.
4For clotting, keep the order thromboplastin → thrombin → fibrin.
5For transfusions, check whether the recipient's antibodies match any donor antigen.

Composition of Blood and Blood Groups in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Pus in a wound

Suppose a cut on your knee becomes swollen and fills with a yellowish fluid after two days.

Which blood cells are responsible, and how did they reach the wound?

Try it first, then show the solutionSolution

Germs entered through the cut.
Neutrophils squeezed out of nearby capillaries by diapedesis.
They engulfed and digested germs by phagocytosis, and many died in the process.
Pus is a mixture of dead neutrophils, germs and tissue fluid.

Answer: Neutrophils, which left the capillaries by diapedesis and engulfed the germs; pus is their remains.

Memory hook: Pus = fallen neutrophil soldiers.

Emergency blood

Suppose an accident victim arrives at a hospital and there is no time to test her blood group. The doctor orders O-negative blood.

Explain the choice.

Try it first, then show the solutionSolution

Group O RBCs have neither A nor B antigen.
So the recipient's anti-a or anti-b antibodies have nothing to clump.
Rh-negative cells also lack the Rh antigen, avoiding an anti-Rh reaction.
O-negative is therefore the safest choice when the group is unknown.

Answer: O-negative RBCs carry no A, B or Rh antigen, so they are least likely to be clumped.

Memory hook: O = zero antigens.

Tricks to remember Composition of Blood and Blood Groups

Short memory aids. Say each one aloud once, then use it on a question the same day.

Never Let Monkeys Eat Bananas
Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils: most to least common.
Use it when: Listing WBC types.
Thrombo-Thrombin-Threads
THromboplastin makes THrombin makes fibrin THreads.
Use it when: Order of clotting steps.
No nucleus, no mito, more oxygen
Empty RBC = more haemoglobin; no mitochondria = no oxygen spent.
Use it when: Two-point answer on RBC efficiency.

Composition of Blood and Blood Groups: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Phagocytic cell · 1 mark · MCQBoard level

The most numerous white blood cells, which engulf bacteria, are:
(a) lymphocytes (b) basophils (c) neutrophils (d) eosinophils

Try it first, then show the solutionSolution

Neutrophils are the most numerous WBCs and are phagocytic.

Answer: (c) neutrophils.

Example 2: An RBC with mitochondria · 2 marks · Short answerBoard level

Suppose a scientist could give a human red blood cell its own mitochondria. Predict one disadvantage of this, and state one other feature of the mature RBC that helps it carry more oxygen. (RBC efficiency was asked in 2024.)

Try it first, then show the solutionSolution

With mitochondria the RBC would respire aerobically and use up some of the oxygen it carries, so less would reach the tissues (1 mark).
It has no nucleus, leaving more room for haemoglobin, and its biconcave shape gives a large surface area for oxygen uptake (1 mark).

Answer: It would use some of the oxygen it carries; lacking a nucleus, it holds more haemoglobin and has a large biconcave surface.

Example 3: A tube without calcium · 3 marks · Short answerHarder board level

A blood sample is collected in a tube containing a chemical that removes all calcium ions. Predict whether it will clot, and explain your answer step by step.

Try it first, then show the solutionSolution

It will not clot (1 mark).
Calcium ions are needed, with thromboplastin, to convert prothrombin into thrombin; without them no thrombin forms (1 mark).
Without thrombin, soluble fibrinogen is not changed into fibrin threads, so no mesh forms to trap the cells (1 mark).

Answer: No clot: without Ca²⁺, prothrombin is not converted to thrombin, so fibrinogen does not become fibrin.

Example 4: Two cells in a smear · 3 marks · Short answerHarder board level

In a stained blood smear, cell X has a lobed nucleus and granular cytoplasm; cell Y has a large round nucleus filling most of the cell and clear cytoplasm. (a) Identify X and Y. (b) State one function of each. (c) Say whether each is a granulocyte or an agranulocyte.

Try it first, then show the solutionSolution

(a) X: neutrophil; Y: lymphocyte (1 mark).
(b) X engulfs germs by phagocytosis; Y produces antibodies (1 mark).
(c) X: granulocyte; Y: agranulocyte (1 mark).

Answer: (a) X neutrophil, Y lymphocyte. (b) Phagocytosis; antibody production. (c) Granulocyte; agranulocyte.

Example 5: Blood volume · 3 marks · Case-basedHOTS / competency

Suppose a person has 5 litres of blood, of which formed elements make up 45%. (a) Calculate the volume of plasma. (b) If RBCs live 120 days, roughly what percentage of RBCs is replaced each day? (c) Name the organ where most RBCs are made.

Try it first, then show the solutionSolution

(a) Plasma = 55% of 5 L = 2.75 L (1 mark).
(b) 1/120 × 100 ≈ 0.83% per day (1 mark).
(c) Red bone marrow (1 mark).

Answer: (a) 2.75 L. (b) About 0.83%. (c) Red bone marrow.

Example 6: Only group A in stock · 3 marks · Short answerHOTS / competency

Suppose four patients of groups A, B, AB and O need blood, and only group A blood is available. Which patients can safely receive it? Explain, in terms of antigens and antibodies, why one of the others cannot.

Try it first, then show the solutionSolution

Group A blood carries antigen A on its RBCs (1 mark).
Patients of groups A and AB can receive it, as their plasma has no anti-a antibodies (1 mark).
Group B and group O patients cannot: their plasma contains anti-a antibodies, which would clump (agglutinate) the donor RBCs (1 mark).

Answer: Groups A and AB can receive it; B and O cannot, because their anti-a antibodies agglutinate the A-cells.

Where students lose marks in Composition of Blood and Blood Groups

The slips examiners see most often, and what to write instead.

Mistake: Only one reason for RBC efficiency
CISCE examiners' comments (2024): candidates failed to explain both the absence of the nucleus and the absence of mitochondria.
Do this: Two features, two reasons.
Usually costs: 1 mark.
Mistake: Writing 'WBCs' for a specific job
CISCE examiners' comments (2025): candidates wrote lymphocytes instead of neutrophils for phagocytosis, or just 'WBCs' or 'leucocytes' without the type.
Do this: Neutrophils: phagocytosis; lymphocytes: antibodies.
Usually costs: 1 mark each.
Mistake: 'Blood vessels' in diapedesis
CISCE examiners' comments (2024): candidates wrote blood vessels instead of capillaries.
Do this: Only capillary walls, one cell thick, allow diapedesis.
Usually costs: 1 mark.
Mistake: Wrong category in odd-one-out
CISCE examiners' comments (2024): candidates gave the category as blood platelets instead of clotting of blood, and named parts of WBCs instead of granular WBCs.
Do this: Name the process or group the three terms share.
Usually costs: 1 mark.

Exam tips

Plasma 55%; RBC, WBC, platelets 45%.
RBC efficiency: no nucleus + no mitochondria, two points.
Diapedesis is through capillary walls.
O = universal donor; AB = universal recipient.

Practice questions on Composition of Blood and Blood Groups

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The protein in plasma that forms the threads of a clot is formed from:
(a) albumin (b) fibrinogen (c) haemoglobin (d) globulin
Hint
Fibrin's source.
Answer
(b) fibrinogen.
Q2 · 1 mark · Very short answerBoard level
Define diapedesis.
Hint
Which vessels?
Answer
The squeezing of white blood cells out through the walls of capillaries into the tissues.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A person of group O can receive blood only from group O.
Reason (R): Group O red blood cells carry no A or B antigens.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
What does the recipient's plasma contain?
Answer
(d). Both are true, but A is explained by the anti-a and anti-b antibodies in group O plasma, not by the antigens on group O cells.
Q4 · 2 marks · Short answerHarder board level
Odd one out, with the category of the rest: antibodies, fibrinogen, urea, haemoglobin.
Hint
Where is each found?
Answer
Haemoglobin; it is inside RBCs, while the rest are dissolved in plasma.
Q5 · 2 marks · Short answerHarder board level
Give two functions of plasma.
Hint
Transport and more.
Answer
Transports digested food, hormones and wastes such as urea; carries clotting proteins (fibrinogen, prothrombin) and antibodies (also distributes heat).
Q6 · 2 marks · Short answerHarder board level
Why is vitamin K given to some patients before surgery?
Hint
Prothrombin.
Answer
Vitamin K is needed by the liver to make prothrombin; enough prothrombin ensures normal clotting and prevents excess bleeding.
Q7 · 2 marks · Short answerHarder board level
Why is an Rh-negative mother given an anti-Rh injection soon after the birth of her first Rh-positive baby?
Hint
Act before her own antibodies form.
Answer
Some of the baby's Rh-positive RBCs enter her blood at birth. The injection destroys them before her own immune system makes anti-Rh antibodies, so a later Rh-positive baby is protected from erythroblastosis foetalis.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a blood report shows: RBC count low, haemoglobin low, WBC count normal, platelet count very low.
(a) Which symptom would the low haemoglobin cause? (1)
(b) What problem would the low platelet count cause? (1)
(c) Which organ makes all three kinds of cells? (1)
Hint
Oxygen and clotting.
Answer
(a) Tiredness and breathlessness (anaemia), as less oxygen is carried. (b) Slow clotting, so bleeding from small cuts continues longer. (c) Red bone marrow.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the lifespan of an RBC?Show answer
About 120 days.
2.Which WBCs produce antibodies?Show answer
Lymphocytes.
3.Which ion is needed for clotting?Show answer
Calcium (Ca²⁺).
4.Where are old RBCs destroyed?Show answer
Mainly in the spleen (and liver).
5.Which blood group has no antibodies in plasma?Show answer
AB.

Composition of Blood and Blood Groups: frequently asked questions

Are platelets cells?

They are cell fragments without a nucleus, counted among the formed elements.

Why does blood not clot inside vessels?

Clotting starts only when thromboplastin is released at an injury, and heparin in blood prevents clotting inside vessels.

Do I need the counts of each cell?

No exact counts are demanded; know that RBCs are the most numerous and neutrophils the most common WBC.

Is the Rh factor separate from the ABO group?

Yes. Every person has an ABO group and is also Rh-positive or Rh-negative, as in O-negative.