Circulatory System ICSE Class 10: Blood, Heart, Blood Vessels and Lymph

What Circulatory System is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Circulatory System is in the 3. Human Anatomy and Physiology (i) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

Blood: plasma and red blood cells

Blood is a fluid connective tissue: plasma (about 55%) and formed elements (about 45%).

Plasma: mostly water (about 90%) with dissolved proteins (albumin, globulins, fibrinogen and prothrombin for clotting), glucose, amino acids, salts, hormones, urea and CO₂. It carries dissolved food, wastes and hormones.

Red blood cells (erythrocytes): biconcave, disc-shaped cells with no nucleus and no mitochondria in mammals; packed with haemoglobin, which carries oxygen (as oxyhaemoglobin). Made in the red bone marrow; live about 120 days; old ones are destroyed in the spleen and liver.

Why are mammalian RBCs efficient?

• No nucleus: more space for haemoglobin, and the biconcave shape gives a larger surface area for oxygen exchange.
• No mitochondria: they do not use up any of the oxygen they carry (they respire anaerobically).

Worked example

Why are RBCs efficient in carrying oxygen though they lack a nucleus and mitochondria?

Try it first, then show the solutionSolution

1. No nucleus: more room for haemoglobin; biconcave shape increases surface area.
2. No mitochondria: the cell does not consume the oxygen it is carrying.

Answer: More haemoglobin and surface area; no oxygen used by the cell itself.

White blood cells and platelets

White blood cells (leucocytes): colourless, nucleated, can change shape (amoeboid) and squeeze out through capillary walls into tissues, which is called diapedesis. They defend the body.

• Granulocytes (granular cytoplasm, lobed nucleus): neutrophils (most numerous; phagocytosis, engulfing germs), eosinophils (allergy, parasitic infections), basophils (release heparin and histamine).
• Agranulocytes: lymphocytes (produce antibodies), monocytes (phagocytosis; become macrophages in tissues).

Platelets (thrombocytes): tiny cell fragments without a nucleus; they help blood clotting.

Worked example

Name the white blood cells that (a) engulf bacteria, (b) produce antibodies.

Try it first, then show the solutionSolution

1. (a) Phagocytosis is mainly by neutrophils (and monocytes).
2. (b) Antibodies are made by lymphocytes.

Answer: (a) Neutrophils (b) Lymphocytes

Blood coagulation (brief)

When a blood vessel is injured:
1. Damaged tissues and platelets release thromboplastin (thrombokinase).
2. Thromboplastin, with calcium ions (Ca²⁺), converts inactive prothrombin into active thrombin.
3. Thrombin converts soluble fibrinogen into insoluble fibrin threads.
4. The fibrin threads form a mesh that traps blood cells, forming a clot that seals the wound.

Vitamin K is needed to make prothrombin in the liver. Heparin prevents clotting inside the vessels.

Worked example

Arrange in order of clotting: fibrin, thromboplastin, thrombin, prothrombin.

Try it first, then show the solutionSolution

1. Injury releases thromboplastin.
2. Thromboplastin acts on prothrombin to form thrombin.
3. Thrombin converts fibrinogen to fibrin.

Answer: Thromboplastin → prothrombin → thrombin → fibrin.

Tissue fluid, lymph and lymphatic organs

Tissue fluid: part of the plasma that leaks out of the capillaries into the spaces between cells. It bathes the cells, bringing food and oxygen and taking away wastes. It contains no RBCs and fewer proteins.

Lymph: tissue fluid that enters the thin lymph capillaries. It is a colourless fluid with many lymphocytes. Lymph vessels carry it, in one direction, through lymph nodes back into the blood (into veins near the neck). Lymph also absorbs fats from the intestine.

Lymphatic organs (brief):

• Spleen: the largest lymphatic organ, on the left side of the abdomen; destroys old RBCs, makes lymphocytes, and stores blood.
• Tonsils: in the throat; trap and destroy germs entering through the mouth and nose.

Worked example

Give two differences between blood and lymph.

Try it first, then show the solutionSolution

1. Blood has RBCs; lymph has no RBCs.
2. Blood flows in a circuit pumped by the heart; lymph flows one way from tissues back to the veins.

Answer: As above.

Structure of the heart

The heart lies in the thoracic cavity between the lungs, slightly to the left, enclosed in a double-layered pericardium containing pericardial fluid (reduces friction and protects the heart).

• Four chambers: two upper, thin-walled atria (receive blood) and two lower, thick-walled ventricles (pump blood). The left ventricle has the thickest wall, since it pumps blood to the whole body. A septum separates the left and right sides.
• Valves: tricuspid between the right atrium and right ventricle; bicuspid (mitral) between the left atrium and left ventricle; semilunar valves at the bases of the pulmonary artery and aorta. Valves keep blood flowing one way.
• Chordae tendineae: tendons attaching the cusps of the tricuspid and bicuspid valves to papillary muscles of the ventricle walls, preventing the valves from turning back into the atria.
• Coronary arteries supply the heart muscle itself.

Main vessels: superior and inferior venae cavae → right atrium (deoxygenated blood); pulmonary artery from the right ventricle → lungs (deoxygenated); pulmonary veins from the lungs → left atrium (oxygenated); aorta from the left ventricle → body (oxygenated).

Worked example

Give the exact location of (a) the bicuspid valve, (b) the pericardium.

Try it first, then show the solutionSolution

1. (a) Between the left atrium and the left ventricle.
2. (b) A double membrane surrounding the heart.

Answer: As above.

Working of the heart and double circulation

Systole is contraction; diastole is relaxation. In one heartbeat (about 0.8 s at 72 beats per minute):
1. Atria contract (atrial systole), pushing blood into the ventricles.
2. Ventricles contract (ventricular systole); the tricuspid and bicuspid valves close ('lubb' sound) and blood is forced into the pulmonary artery and aorta.
3. Ventricles relax (diastole); the semilunar valves close ('dubb' sound) and the atria refill.

Double circulation: blood passes twice through the heart in one complete round of the body.

• Pulmonary circulation: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.
• Systemic circulation: left ventricle → aorta → body organs → venae cavae → right atrium.

This keeps oxygenated and deoxygenated blood separate, so tissues get blood rich in oxygen at high pressure.

Worked example

Trace the path of a red blood cell from the right atrium back to the right atrium.

Try it first, then show the solutionSolution

1. Right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs.
2. Lungs → pulmonary vein → left atrium → bicuspid valve → left ventricle → aorta.
3. Aorta → body capillaries → venae cavae → right atrium.

Answer: As above.

Blood vessels, pulse and blood pressure

Feature	Artery	Vein	Capillary
Wall	thick, muscular, elastic	thin, less muscle	one cell thick (endothelium)
Lumen	narrow	wide	very narrow
Valves	absent (except at the base of the aorta and pulmonary artery)	present	absent
Blood flow	away from the heart, under high pressure, in spurts	towards the heart, low pressure, smooth	links arteries and veins; slow
Function	carry blood to organs	return blood to the heart	exchange of materials with cells

Arteries carry oxygenated blood except the pulmonary artery; veins carry deoxygenated blood except the pulmonary veins.

Pulse: the rhythmic expansion of an artery wall with each heartbeat, felt at the wrist (radial artery); normally about 72 per minute.
Blood pressure: the pressure of blood on the walls of arteries; normal about 120/80 mm Hg (systolic/diastolic), measured with a sphygmomanometer.

Worked example

How do the wall and the lumen of an artery differ from those of a vein?

Try it first, then show the solutionSolution

1. Artery: thick, muscular, elastic wall; vein: thin wall.
2. Artery: narrow lumen and no valves; vein: wide lumen with valves.

Answer: As above.

ABO blood groups, Rh factor and the hepatic portal system

ABO system: based on antigens (A, B) on RBCs and antibodies (anti-a, anti-b) in plasma.

Group	Antigen on RBC	Antibody in plasma
A	A	anti-b
B	B	anti-a
AB	A and B	none (universal recipient)
O	none	anti-a and anti-b (universal donor)

If incompatible blood is transfused, the antibodies cause the donor's RBCs to clump (agglutinate).

Rh factor: an antigen on the RBCs of Rh-positive people; absent in Rh-negative people. If an Rh-negative mother carries an Rh-positive foetus, she may form anti-Rh antibodies; in a later Rh-positive pregnancy these can destroy the foetus's RBCs (erythroblastosis foetalis).

Hepatic portal system: blood from the stomach and intestines is carried by the hepatic portal vein to the liver (not straight to the heart). The liver regulates the amount of digested food in blood (stores excess glucose as glycogen) and removes toxins before the blood goes on via the hepatic vein to the inferior vena cava. The liver also receives oxygenated blood by the hepatic artery. The kidney receives blood by the renal artery and returns it by the renal vein.

Worked example

Why is a person with blood group O called a universal donor?

Try it first, then show the solutionSolution

1. Group O RBCs carry no A or B antigens.
2. So the recipient's antibodies have nothing to react with.

Answer: O RBCs have no A or B antigens.

Circulatory System in real life

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

Topics in this chapter

• Structure and Working of the Human Heart
• Composition of Blood and Blood Groups
• Artery, Vein and Capillary, and the Lymphatic System

Circulatory System: key facts

Pulmonary circulation

$RV\to \text{pulmonary artery}\to \text{lungs}\to \text{pulmonary veins}\to LA$

Blood is oxygenated.

Systemic circulation

$LV\to \text{aorta}\to \text{body}\to \text{venae cavae}\to RA$

Blood gives up oxygen.

Clotting cascade

$\text{prothrombin}\stackrel{{\text{thromboplastin, Ca}}^{2+}}{\to }\text{thrombin};\text{fibrinogen}\stackrel{\text{thrombin}}{\to }\text{fibrin}$

Fibrin traps cells to form the clot.

Normal values

$\text{pulse}\approx 72/\text{min};BP\approx 120/80\text{mm Hg}$

Systolic/diastolic.

Hepatic portal route

$\text{stomach, intestine}\to \text{hepatic portal vein}\to \text{liver}\to \text{hepatic vein}\to \text{inferior vena cava}$

Liver processes absorbed food first.

Circulatory System: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Circulatory System

The question patterns to expect, and the method that works for each.

Tricks to remember Circulatory System

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

• Try before you Bi

  TRIcuspid on the RIght; BIcuspid (mitral) on the left.

  Use it when: Valve positions.

• A for Away

  Arteries carry blood Away from the heart; veins return it.

  Use it when: Defining artery and vein (pulmonary ones are exceptions in oxygen content, not direction).

• Lubb = Lids of AV valves

  'Lubb' is the AV valves shutting; 'Dubb' is the semilunar valves.

  Use it when: Heart sounds.

• O gives, AB takes

  O is the universal donor (no antigens); AB the universal recipient (no antibodies).

  Use it when: Blood group MCQs.

Circulatory System: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 2 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Circulatory System

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

• ✗Mistake: Wrong location of valves and pericardium

  CISCE examiners' comments, 2024: candidates placed the bicuspid valve between the right atrium and right ventricle, or said 'on the left side of the heart', and said the pericardium is on the wall or in the heart.

  Wrong working

  Bicuspid valve: left side of the heart.

  Right working

  Between the left atrium and the left ventricle; pericardium surrounds the heart.

  ✓Do this: Name both chambers a valve lies between.

  Usually costs: 1 mark each.

• ✗Mistake: Thick and thin without saying what

  CISCE examiners' comments, 2024: for artery-vein differences, candidates wrote 'arteries are thick, veins thin' without wall or lumen, or gave functions instead of structure.

  Wrong working

  Arteries are thick; veins are thin.

  Right working

  Arteries have thick, muscular walls and a narrow lumen; veins have thin walls, a wide lumen and valves.

  ✓Do this: Name the part: wall, lumen, valves.

  Usually costs: 1 mark.

• ✗Mistake: Half an answer on RBC efficiency

  CISCE examiners' comments, 2024: candidates failed to explain both the absence of the nucleus and of mitochondria.

  Wrong working

  RBCs have no nucleus, so they carry more haemoglobin.

  Right working

  No nucleus: more haemoglobin; no mitochondria: they do not use the oxygen they carry.

  ✓Do this: Two features asked = two reasons.

  Usually costs: 1 mark.

• ✗Mistake: Short forms and confused vessels

  CISCE examiners' comments, 2025: on a heart diagram, candidates confused pulmonary artery and vein, used short forms like P.A., wrote vena cava for aorta, misspelt aorta, and wrote pleura for pericardium.

  Wrong working

  P.A. carries blood to the body.

  Right working

  Pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.

  ✓Do this: Full names, correct spellings, check the direction of flow.

  Usually costs: 1 mark each.

• ✗Mistake: Wrong WBC type

  CISCE examiners' comments, 2025: candidates wrote lymphocytes instead of neutrophils for phagocytosis, or wrote just 'WBCs'.

  Wrong working

  Phagocytosis: lymphocytes.

  Right working

  Phagocytosis: neutrophils (and monocytes); antibodies: lymphocytes.

  ✓Do this: Name the specific type.

  Usually costs: 1 mark.

• ✗Mistake: Capillaries vs blood vessels in diapedesis

  CISCE examiners' comments, 2024: defining diapedesis, candidates wrote 'blood vessels' instead of capillaries.

  Wrong working

  WBCs squeeze out of blood vessels.

  Right working

  WBCs squeeze out through the walls of capillaries.

  ✓Do this: Only capillary walls are thin enough.

  Usually costs: 1 mark.

Practice questions on Circulatory System

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

1. Q1 · 1 mark · MCQBoard level

   The valve between the left atrium and left ventricle is the:
   (a) tricuspid
   (b) bicuspid
   (c) semilunar
   (d) pulmonary

   Hint

   Try before you Bi.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   Which blood vessel carries oxygenated blood?
   (a) Pulmonary artery
   (b) Pulmonary vein
   (c) Vena cava
   (d) Hepatic portal vein

   Hint

   From the lungs.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerHarder board level

   Odd one out, with category: spleen, tonsils, lymph node, liver.

   Hint

   Lymphatic organs.

   Answer

   Liver; the others are lymphatic organs.

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Veins have valves.
   Reason (R): Blood in veins flows at low pressure and could flow backwards.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Why valves?

   Answer

   (a)

5. Q5 · 2 marks · Very short answerBoard level

   Name: (a) the membrane around the heart; (b) the pigment that carries oxygen; (c) the instrument for measuring blood pressure; (d) the vessel that brings blood to the kidney.

   Hint

   Half mark each.

   Answer

   (a) Pericardium (b) Haemoglobin (c) Sphygmomanometer (d) Renal artery

6. Q6 · 2 marks · Short answerHarder board level

   Give two differences between systemic and pulmonary circulation.

   Hint

   Start, end, destination.

   Answer

   Systemic starts at the left ventricle and supplies the body, ending in the right atrium; pulmonary starts at the right ventricle and goes to the lungs, ending in the left atrium.

7. Q7 · 2 marks · Short answerHarder board level

   What is the significance of the hepatic portal system?

   Hint

   Liver first.

   Answer

   Blood from the digestive organs reaches the liver first, which regulates the amount of glucose and other digested food entering the general circulation (storing excess as glycogen) and removes toxins.

8. Q8 · 3 marks · Short answerHarder board level

   A patient with group A blood is given group B blood by mistake. Explain what happens using antigens and antibodies.

   Hint

   Anti-b in A plasma.

   Answer

   Group A plasma contains anti-b antibodies. The donor's group B RBCs carry B antigens, so they are agglutinated (clumped), which can block vessels and be fatal.

9. Q9 · 2 marks · Short answerHOTS / competency

   A person's blood clots very slowly after a cut. Suggest two possible reasons based on the clotting process.

   Hint

   Platelets, vitamin K, calcium.

   Answer

   Too few platelets (less thromboplastin); deficiency of vitamin K (less prothrombin made). Also possible: low calcium ions.

10. Q10 · 3 marks · Short answerHOTS / competency

    Explain why the walls of capillaries are only one cell thick, and why blood flows slowly in them.

    Hint

    Exchange.

    Answer

    Capillaries are the site of exchange of oxygen, food and wastes with cells; a one-cell-thick wall gives a very short diffusion path. Slow flow (large total cross-section) gives more time for exchange.

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. 1.Lifespan of an RBC?Show answer

   About 120 days.

2. 2.Where are RBCs formed?Show answer

   Red bone marrow.

3. 3.Most numerous WBC?Show answer

   Neutrophils.

4. 4.Protein converted to fibrin in clotting?Show answer

   Fibrinogen.

5. 5.Valve between right atrium and right ventricle?Show answer

   Tricuspid valve.

6. 6.Artery carrying deoxygenated blood?Show answer

   Pulmonary artery.

7. 7.Normal blood pressure?Show answer

   About 120/80 mm Hg.

8. 8.Vessel bringing blood from the intestine to the liver?Show answer

   Hepatic portal vein.

9. 9.Universal donor blood group?Show answer

   O.

10. 10.Largest lymphatic organ?Show answer

    Spleen.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain why RBCs lack a nucleus and mitochondria?
• ☐Can you name the WBC types and their functions?
• ☐Can you list the four steps of clotting?
• ☐Can you draw and label the heart with all valves and vessels?
• ☐Can you trace pulmonary and systemic circulation?
• ☐Can you fill in the ABO antigen-antibody table?
• ☐Can you name the vessels entering and leaving the liver and kidney?

Circulatory System: frequently asked questions