BiologyISC Class 11 · Body Fluids and Circulation

Blood: Composition, ABO and Rh Groups, Coagulation and Lymph

Updated 17 October 2026 · Fact-checked

Blood is a fluid connective tissue of plasma (55%) and formed elements (45%): erythrocytes, leucocytes and platelets. The ABO groups depend on antigens A and B on red cells and the Rh group on antigen D. Clotting converts fibrinogen into fibrin threads through thrombin. Lymph is a colourless fluid that returns tissue fluid to blood.

Notice that plasma makes up just over half of blood, and the formed elements are RBCs, WBCs and platelets.

Illustration drawn for QuestionBuddy.

Understand Blood Composition, Blood Groups and Coagulation

Plasma. About 55% of blood; 90 to 92% water, 6 to 8% proteins (fibrinogen for clotting, globulins for defence, albumins for osmotic balance), with nutrients, hormones, gases, and minerals such as Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻. Plasma without the clotting factors is serum.

Formed elements. *Erythrocytes (RBC):* biconcave, enucleate in mammals, about 5 to 5.5 million per mm³ of blood in adult men, containing haemoglobin (12 to 16 g per 100 mL of blood); formed in the red bone marrow; lifespan about 120 days; destroyed in the spleen ("graveyard of RBCs"). *Leucocytes (WBC):* colourless, nucleated, about 6000 to 8000 per mm³: neutrophils (the most abundant, 60 to 65%, phagocytic), lymphocytes (20 to 25%, B and T cells, immune responses), monocytes (6 to 8%, phagocytic), eosinophils (2 to 3%, allergic reactions) and basophils (0.5 to 1%, release histamine). *Platelets (thrombocytes):* cell fragments, about 1.5 to 3.5 lakh per mm³, involved in clotting.

ABO grouping. Antigens on the surface of RBCs and antibodies in plasma: group A has antigen A and anti-B; group B has antigen B and anti-A; group AB has both antigens and no antibodies (universal recipient); group O has no antigens and both antibodies (universal donor). A donor and recipient must be matched to avoid agglutination.

Rh grouping. About 80% of humans have the Rh antigen (Rh⁺), the rest are Rh⁻. If an Rh⁻ mother carries an Rh⁺ foetus, the mother's blood becomes sensitised at the first delivery; antibodies cross the placenta in a later pregnancy and destroy the foetal RBCs: erythroblastosis foetalis. It is prevented by giving the mother anti-Rh antibodies soon after the first delivery.

Coagulation. On injury, platelets release substances (thromboplastin) that, with Ca²⁺, convert inactive prothrombin into thrombin; thrombin converts soluble fibrinogen into insoluble fibrin threads, which trap blood cells to form the clot.

Lymph. The fluid in lymphatic vessels: tissue fluid that has entered lymph capillaries; colourless, with lymphocytes; it returns proteins and fluids to blood, carries absorbed fats from the intestine (lacteals) and helps in immunity.

Blood Composition, Blood Groups and Coagulation: key facts

Blood composition: Plasma 55% | Formed elements 45%; Plasma: 90 to 92% water, 6 to 8% proteins.
ABO groups: A: antigen A, anti-B
B: antigen B, anti-A
AB: A and B, none
O: none, anti-A and anti-B; AB universal recipient; O universal donor.
Cell counts: $R B C : 5 t o 5.5 mi l l i o n / m m^{3} ∣ W B C : 6000 t o 8000/ m m^{3} ∣ P l a t e l e t s : 1.5 t o 3.5 l ak h / m m^{3}$; Adult counts.
Clotting: $P r o t h r o mbin \to t h r o mbin (C a^{2 +}) ∣ F ib r in o g e n \to f ib r in$; Platelets release thromboplastin.

Blood Composition, Blood Groups and Coagulation 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.

A transfusion decision

Suppose a patient of blood group B urgently needs a transfusion, and donors of groups A, B, AB and O are available.

Say which donors are compatible and why.

Try it first, then show the solutionSolution

Group B has antigen B and anti-A antibodies in plasma.
Group B and group O donors are compatible: O red cells have no A or B antigen; B red cells have antigen B, which the patient's blood accepts.
Group A and AB donors carry antigen A, which would be agglutinated by the patient's anti-A.

Answer: Donors of group B and group O are compatible.

Memory hook: Check that the donor's antigens do not meet matching antibodies.

Tricks to remember Blood Composition, Blood Groups and Coagulation

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

O gives, AB receives
Group O is the universal donor; group AB is the universal recipient.
Use it when: Transfusion questions.
Rh trouble in the second baby
Rh⁻ mother, Rh⁺ foetus: the first baby is usually safe.
Use it when: Erythroblastosis foetalis.

Blood Composition, Blood Groups and Coagulation: solved examples

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

Example 1: Universal recipient · 1 mark · MCQBoard level

The universal recipient blood group is:
(a) A
(b) B
(c) AB
(d) O

Try it first, then show the solutionSolution

AB has no anti-A or anti-B antibodies in plasma.

Answer: (c) AB

Example 2: Components of blood · 3 marks · Short answerBoard level

Name the formed elements of blood and give one function and one count for each.

Try it first, then show the solutionSolution

Erythrocytes: carry O₂ and some CO₂; 5 to 5.5 million per mm³. (1 mark)
Leucocytes: defence against infection; 6000 to 8000 per mm³. (1 mark)
Platelets: blood clotting; 1.5 to 3.5 lakh per mm³. (1 mark)

Answer: RBC, WBC and platelets with counts and functions.

Example 3: ABO matching · 4 marks · Short answerHarder board level

Complete the table for a transfusion: donors A, B, AB and O to recipients A, B, AB and O (state which are compatible). Explain why AB can receive from all and O can donate to all.

Try it first, then show the solutionSolution

Recipient A: from A and O. Recipient B: from B and O. (1 mark)
Recipient AB: from A, B, AB and O. Recipient O: only from O. (1 mark)
AB has no antibodies, so no donor cells are agglutinated. (1 mark)
O red cells have no antigens, so no recipient antibodies react with them. (1 mark)

Answer: A and O, B and O, all, O only; AB lacks antibodies and O lacks antigens.

Example 4: Rh incompatibility · 4 marks · Short answerHarder board level

Explain erythroblastosis foetalis: why does it affect the second child and not usually the first, and how is it prevented?

Try it first, then show the solutionSolution

It occurs when an Rh⁻ mother carries an Rh⁺ foetus. (1 mark)
At the first delivery the mother's blood mixes with foetal blood and she forms anti-Rh antibodies. (1 mark)
In a later pregnancy these antibodies cross the placenta and destroy the RBCs of an Rh⁺ foetus. (1 mark)
It is prevented by giving the mother anti-Rh antibodies within 72 hours after the first delivery. (1 mark)

Answer: Sensitisation at first birth; antibodies attack the second foetus; anti-Rh prevention.

Example 5: Clotting and counts · 5 marks · Long answerHOTS / competency

Outline the mechanism of blood clotting. A blood sample of 0.5 mm³ contains 2.6 million RBC; find the count per mm³ and the RBC count in a litre of blood with a count of 5 million per mm³ (1 L = 10⁶ mm³).

Try it first, then show the solutionSolution

Platelets release thromboplastin at the injury; with Ca²⁺ it converts prothrombin to thrombin. (1.5 marks)
Thrombin converts fibrinogen to insoluble fibrin threads, forming the clot with blood cells. (1.5 marks)
Count per mm³ = 2.6 million/0.5 = 5.2 million. (1 mark)
In a litre: 5 × 10⁶ × 10⁶ = 5 × 10¹² RBCs. (1 mark)

Answer: Prothrombin → thrombin; fibrinogen → fibrin; 5.2 million per mm³; 5 × 10¹² per litre.

Where students lose marks in Blood Composition, Blood Groups and Coagulation

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

Mistake: Swapping universal donor and recipient
Calling AB the universal donor.
Do this: O is the universal donor; AB is the universal recipient.
Usually costs: 1 to 2 marks.
Mistake: Writing that mature RBCs have a nucleus
Wrong statement for mammals.
Do this: Mammalian RBCs are enucleate.
Usually costs: 1 mark.
Mistake: Wrong role of thrombin
Saying thrombin converts fibrin to fibrinogen.
Do this: Thrombin converts fibrinogen into fibrin.
Usually costs: 1 mark.

Exam tips

Present the ABO groups in a table.
Give the numerical counts of cells.
Write the clotting cascade in three steps.
Explain Rh incompatibility with a flow chart.

Practice questions on Blood Composition, Blood Groups and Coagulation

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 blood cells which are the most numerous leucocytes are:
(a) eosinophils
(b) basophils
(c) neutrophils
(d) monocytes
Hint
About 60 to 65%.
Answer
(c) neutrophils
Q2 · 2 marks · Short answerBoard level
What is the function of platelets?
Hint
They help to form clots.
Answer
They release substances that initiate blood clotting at the site of injury.
Q3 · 3 marks · Short answerHarder board level
Differentiate between plasma and serum.
Hint
One lacks clotting factors.
Answer
Plasma is the liquid part of blood with all proteins including fibrinogen. Serum is plasma from which clotting factors (fibrinogen) have been removed after clotting.
Q4 · 3 marks · Short answerHarder board level
A person of blood group AB⁺ receives blood of group O⁻. Is it safe? Explain.
Hint
AB has no antibodies against O.
Answer
Yes. O red cells have no A, B or Rh antigens, and AB⁺ has no anti-A, anti-B or anti-Rh antibodies, so there is no agglutination.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): A person with group O blood is called a universal donor. Reason (R): O red cells carry no A or B antigens. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think about agglutination.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a pregnant woman is Rh⁻ and her husband is Rh⁺, and this is her second pregnancy.
(i) Name the condition that may arise. (1 mark)
(ii) Name the antibodies involved. (1 mark)
(iii) State when preventive treatment should have been given. (1 mark)
(iv) State the genotype/phenotype of the foetus at risk. (1 mark)
Hint
Think about sensitisation.
Answer
(i) Erythroblastosis foetalis. (ii) Anti-Rh (anti-D) antibodies. (iii) After the first delivery. (iv) An Rh⁺ foetus.

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.Most abundant WBC?Show answer
Neutrophils.
2.Antibodies in group A?Show answer
Anti-B.
3.Clot protein?Show answer
Fibrin.
4.Graveyard of RBCs?Show answer
Spleen.

Blood Composition, Blood Groups and Coagulation: frequently asked questions

Why is blood group O the universal donor?

Its red cells carry no A or B antigens, so no antibodies in the recipient will react with them.

What is lymph?

Lymph is the tissue fluid that has entered the lymphatic capillaries. It is colourless, has no RBCs, contains fewer proteins than plasma, and contains lymphocytes.

Why does the first Rh⁺ baby usually escape the effects?

The mother has not yet formed antibodies at the time of the first pregnancy; sensitisation occurs at delivery.