❤️ Circulatory System – Full Breakdown
ICSE Class 10
Biology
June 2026
The circulatory system is the body's transport superhighway. Every cell depends on it for oxygen, nutrients, and waste removal. ICSE loves this chapter because it connects anatomy (heart structure), physiology (cardiac cycle), and clinical applications (blood pressure, ECG). This chapter also has the most diagram-based questions in the board exam. Written by someone who scored 93.4% in ICSE 2026 — these are the exact notes I used.
1) What is Circulation?
Circulation: The continuous flow of blood through the heart and blood vessels, delivering oxygen and nutrients to tissues and removing waste products.
The human circulatory system is a closed, double circulatory system — blood always stays inside vessels (closed), and it passes through the heart twice per complete circuit (double). This is more efficient than open or single circulation because it maintains high pressure and keeps oxygenated & deoxygenated blood separate.
2) Types of Circulatory Systems
| Type | Feature | Examples |
|---|---|---|
| Open | Blood (haemolymph) flows into body cavities, bathes organs directly. Low pressure, slow. | Arthropods, molluscs (snails, insects) |
| Closed | Blood always confined to vessels. High pressure, fast, directional. | Vertebrates (humans), annelids (earthworm) |
| Type | Feature | Examples |
|---|---|---|
| Single | Blood passes through heart once per cycle. Gills → body → heart. | Fish (2-chambered heart) |
| Incomplete double | Heart has 3 chambers; some mixing of oxygenated & deoxygenated blood. | Amphibians, most reptiles |
| Complete double | 4-chambered heart; no mixing. Highly efficient. | Birds, mammals (humans) |
⚠️ Board question classic: "Why is double circulation more efficient than single circulation?"
Answer: (1) Oxygenated & deoxygenated blood remain separate → tissues get maximum oxygen. (2) Higher blood pressure is maintained for systemic circulation. (3) Faster delivery of oxygen to active tissues like muscles and brain.
Answer: (1) Oxygenated & deoxygenated blood remain separate → tissues get maximum oxygen. (2) Higher blood pressure is maintained for systemic circulation. (3) Faster delivery of oxygen to active tissues like muscles and brain.
3) The Heart — Structure
The heart is a hollow, muscular organ about the size of a fist, located in the thoracic cavity between the lungs (mediastinum). It pumps blood through the body 24/7 — about 100,000 beats per day.
External Features
- Pericardium: Double-walled protective sac enclosing the heart. Contains pericardial fluid (reduces friction during beats).
- Epicardium: Outer layer of the heart wall.
- Myocardium: Middle, thickest layer — cardiac muscle. Responsible for contraction.
- Endocardium: Inner lining — smooth endothelium that lines the chambers and valves.
Internal Structure — 4 Chambers
| Chamber | Position | Receives From | Pumps To | Wall Thickness |
|---|---|---|---|---|
| Right Atrium | Upper right | Venae cavae (superior & inferior) | Right ventricle (via tricuspid valve) | Thin |
| Right Ventricle | Lower right | Right atrium | Lungs (via pulmonary artery, through pulmonary valve) | Moderate |
| Left Atrium | Upper left | Pulmonary veins (from lungs) | Left ventricle (via bicuspid/mitral valve) | Thin |
| Left Ventricle | Lower left | Left atrium | Aorta → all body tissues (via aortic valve) | Thickest (needs to push blood to entire body) |
💡 My memory trick: The left ventricle has the thickest wall because it pumps blood to the whole body (high pressure needed). The right ventricle only pumps to the lungs (nearby, less pressure needed). The atria are thin because they just push blood into the ventricles (short distance, gravity-assisted).
Valves of the Heart
Valves prevent backflow of blood. They open and close passively due to pressure changes.
| Valve | Location | Number of Cusps | Prevents Backflow From → To |
|---|---|---|---|
| Tricuspid | Between RA & RV | 3 | RV → RA |
| Bicuspid (Mitral) | Between LA & LV | 2 | LV → LA |
| Pulmonary Semilunar | Between RV & Pulmonary artery | 3 (half-moon shaped) | Pulmonary artery → RV |
| Aortic Semilunar | Between LV & Aorta | 3 (half-moon shaped) | Aorta → LV |
Chordae tendineae ("heart strings") are fibrous cords connecting the cusps of AV valves to the papillary muscles in the ventricles. They prevent the valves from inverting (flipping inside out) during ventricular contraction.
📖 Mnemonic for valves: "Try Before You Buy" — Tricuspid (right AV) → Bicuspid (left AV) → pulmonarY → Body (aortic). Or simply remember: Right side = Tricuspid (3), Left side = Bicuspid (2). The semilunar valves are both 3-cusped and half-moon shaped.
4) Blood Vessels — Arteries, Veins & Capillaries
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction of flow | Away from heart | Toward heart | Arteries → Tissues → Veins |
| Blood carried | Usually oxygenated (except pulmonary artery) | Usually deoxygenated (except pulmonary vein) | Both (site of exchange) |
| Wall thickness | Thick (3 layers — tunica intima, media, adventitia) | Thin (less muscle, more collagen) | One cell thick (endothelium only) |
| Lumen | Narrow | Wide | Very narrow (one RBC wide) |
| Valves | Absent | Present (prevent backflow) | Absent |
| Pressure | High (pulsatile) | Low (non-pulsatile) | Very low |
| Function | Carry blood away under high pressure | Return blood to heart, valves prevent pooling in limbs | Exchange of gases, nutrients, wastes between blood & tissues |
⚠️ Common mistake: Students think ALL arteries carry oxygenated blood and ALL veins carry deoxygenated blood.
Exception to remember: Pulmonary artery carries deoxygenated blood (from heart to lungs). Pulmonary vein carries oxygenated blood (from lungs to heart). The naming is based on direction — arteries carry blood away from the heart, veins carry blood toward the heart.
Exception to remember: Pulmonary artery carries deoxygenated blood (from heart to lungs). Pulmonary vein carries oxygenated blood (from lungs to heart). The naming is based on direction — arteries carry blood away from the heart, veins carry blood toward the heart.
5) Double Circulation — The Complete Circuit
Blood passes through the heart twice in one complete circuit. This ensures oxygenated and deoxygenated blood never mix.
Pulmonary Circulation (Heart → Lungs → Heart)
- Deoxygenated blood from right ventricle → pulmonary artery → lungs
- In lung capillaries: CO₂ released, O₂ picked up
- Oxygenated blood → pulmonary veins → left atrium
- Purpose: Oxygenation of blood (replenish O₂, remove CO₂)
Systemic Circulation (Heart → Body → Heart)
- Oxygenated blood from left ventricle → aorta → all body tissues via branching arteries
- In tissue capillaries: O₂ & nutrients delivered, CO₂ & wastes collected
- Deoxygenated blood → venae cavae (superior from upper body, inferior from lower body) → right atrium
- Purpose: Supply oxygen & nutrients to tissues, remove wastes
💡 Flowchart for board diagrams:
This is the most important flowchart in ICSE Biology. Practice drawing it — arrows and labels included.
Body → Venae cavae → RA → Tricuspid → RV → Pulmonary artery → Lungs
Lungs → Pulmonary vein → LA → Bicuspid → LV → Aorta → Body
This is the most important flowchart in ICSE Biology. Practice drawing it — arrows and labels included.
6) The Cardiac Cycle — How the Heart Beats
The cardiac cycle is the sequence of events in one complete heartbeat, lasting about 0.8 seconds (at 72 beats/min). It has three phases:
| Phase | Duration | What Happens | AV Valves | Semilunar Valves | Heart Sound |
|---|---|---|---|---|---|
| Atrial Systole (contraction) | 0.1 s | Atria contract → blood pushed into ventricles (last 30% of filling) | Open | Closed | — |
| Ventricular Systole (contraction) | 0.3 s | Ventricles contract → pressure rises → AV valves close → SL valves open → blood ejected into aorta & pulmonary artery | Closed | Open | Lub (AV valves closing) = 1st heart sound |
| Diastole (relaxation) | 0.4 s | Ventricles relax → SL valves close (to prevent backflow) → AV valves open → passive filling of ventricles | Open | Closed | Dub (SL valves closing) = 2nd heart sound |
⚠️ Heart sounds — board favourite:
Lub: Closing of AV valves (tricuspid & bicuspid) at start of ventricular systole.
Dub: Closing of semilunar valves (pulmonary & aortic) at start of diastole.
If there's a "whooshing" sound after lub or dub, it indicates a heart murmur — usually due to a faulty valve that doesn't close properly, causing backflow.
Lub: Closing of AV valves (tricuspid & bicuspid) at start of ventricular systole.
Dub: Closing of semilunar valves (pulmonary & aortic) at start of diastole.
If there's a "whooshing" sound after lub or dub, it indicates a heart murmur — usually due to a faulty valve that doesn't close properly, causing backflow.
7) Blood Pressure & Pulse
Blood Pressure (BP): The force exerted by blood on the walls of arteries. Measured in mmHg using a sphygmomanometer.
| Term | Meaning | Normal Value |
|---|---|---|
| Systolic Pressure | Pressure during ventricular contraction (when blood is ejected) | ~120 mmHg |
| Diastolic Pressure | Pressure during ventricular relaxation (when heart fills) | ~80 mmHg |
| Pulse Pressure | Systolic − Diastolic | ~40 mmHg |
| Hypertension | Persistently high BP (>140/90) | Increases risk of heart attack, stroke |
| Hypotension | Abnormally low BP (<90/60) | Can cause dizziness, fainting |
Pulse: The rhythmic expansion and contraction of an artery as blood is ejected during ventricular systole. Can be felt at superficial arteries like radial (wrist), carotid (neck), brachial (elbow). Normal resting pulse: 60–100 beats/min.
8) Blood — Composition & Functions
Blood is a fluid connective tissue (~5 litres in an adult male). Slightly alkaline (pH 7.35–7.45), ~38°C.
Composition
| Component | Percentage | Key Facts |
|---|---|---|
| Plasma (liquid) | ~55% | 91% water + 7% proteins (albumin, globulin, fibrinogen) + 2% nutrients, hormones, wastes, gases |
| RBCs (Erythrocytes) | ~45% (majority of formed elements) | 5–5.5 million/mm³; 120-day lifespan; no nucleus; biconcave; contain haemoglobin |
| WBCs (Leukocytes) | ~1% (4,000–11,000/mm³) | Nucleated; fight infection; types: granulocytes (neutrophils, eosinophils, basophils) & agranulocytes (lymphocytes, monocytes) |
| Platelets (Thrombocytes) | ~1.5–4 lakh/mm³ | Cell fragments from megakaryocytes; essential for clotting (haemostasis) |
Functions of Blood
- Transport: O₂ (via haemoglobin in RBCs), CO₂, nutrients, hormones, enzymes, wastes
- Regulation: Body temperature (distributes heat), pH (buffer systems), water balance
- Protection: Immunity (WBCs, antibodies), clotting (platelets, fibrinogen prevent blood loss)
9) Blood Groups — ABO & Rh System
ABO System
Based on A and B antigens on RBC surface and corresponding antibodies in plasma.
| Blood Group | Antigen on RBC | Antibody in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A & B | None | AB only | A, B, AB, O (Universal Recipient) |
| O | None | Anti-A & Anti-B | A, B, AB, O (Universal Donor) | O only |
⚠️ Why is O the universal donor but AB the universal recipient?
Group O has NO A or B antigens → donor RBCs won't be attacked by recipient's antibodies (but plasma from O has anti-A & anti-B, so only packed RBCs are used).
Group AB has NO antibodies → can receive any RBC type without immune reaction.
Group O has NO A or B antigens → donor RBCs won't be attacked by recipient's antibodies (but plasma from O has anti-A & anti-B, so only packed RBCs are used).
Group AB has NO antibodies → can receive any RBC type without immune reaction.
Rh Factor
- Rh-positive (Rh+): D antigen present on RBC (~85% of Indians)
- Rh-negative (Rh-): D antigen absent (~15%)
- Clinical significance: If an Rh- mother carries an Rh+ fetus, her immune system may produce anti-D antibodies. In subsequent Rh+ pregnancies, these antibodies can cross the placenta and destroy fetal RBCs → Erythroblastosis fetalis (haemolytic disease of newborn). Prevented by RhoGAM (anti-D immunoglobulin) injection at 28 weeks and within 72 hours of delivery.
10) Blood Clotting (Coagulation)
A protective mechanism that prevents blood loss when a blood vessel is damaged. Involves a cascade of clotting factors (I to XIII).
Steps of Clotting:
- Vascular spasm: Damaged vessel constricts to reduce blood flow
- Platelet plug formation: Platelets adhere to exposed collagen and aggregate at the site (forms a temporary plug)
- Coagulation cascade:
- Damaged tissue + platelets release clotting factors
- Prothrombin activator (from cascade) converts prothrombin → thrombin
- Thrombin converts fibrinogen (soluble) → fibrin (insoluble)
- Fibrin forms a mesh that traps RBCs & platelets → clot
- Clot retraction & repair: Clot shrinks, edges of wound pulled together, tissue repair follows
Clotting factors: Most are produced in the liver and require Vitamin K for synthesis. This is why liver disease and Vitamin K deficiency cause bleeding disorders.
💡 Board favourite: The prothrombin → thrombin (with Ca²⁺) and fibrinogen → fibrin steps are the most commonly asked in reasoning questions. Memorise the sequence: Injury → Clotting factors released → Prothrombin → Thrombin → Fibrinogen → Fibrin (mesh) → Clot.
11) Lymphatic System (Overview for ICSE)
The lymphatic system is a one-way transport system that returns interstitial fluid (called lymph) from tissues to the blood.
- Lymph: Similar to plasma but with less protein. Contains WBCs (mainly lymphocytes).
- Lymph capillaries: Begin as blind-ended tubes in tissue spaces
- Lymph vessels: Have valves (like veins); lymph flows due to muscle contraction
- Lymph nodes: Swellings along lymph vessels that filter lymph and produce lymphocytes. Swollen nodes = sign of infection.
- Lymphatic ducts: Thoracic duct & right lymphatic duct → drain into subclavian veins
Functions of lymphatic system:
- Returns excess tissue fluid to blood (prevents oedema/swelling)
- Absorbs & transports digested fats (lacteals in small intestine absorb fatty acids → chyle)
- Defence — lymph nodes filter pathogens & produce lymphocytes & antibodies
12) Common ICSE Board Questions
1-Mark Definitions:
- Define: Systole, Diastole, Pulse, Blood pressure, Heart murmur, Lymph, Phagocytosis
- What is double circulation? Name the two circuits.
- Which chamber of the heart has the thickest wall? Why?
- Name the valve between left atrium and left ventricle.
- What is the difference between pulmonary artery and pulmonary vein (blood carried)?
- Define universal donor and universal recipient.
3-Mark Reasoning Questions:
- "The left ventricle has a thicker wall than the right ventricle." Explain why.
- Differentiate between arteries and veins (any 4 points).
- Explain the flow of blood through the heart with the help of a flowchart.
- What is the difference between "lub" and "dub" heart sounds? What causes each?
- Why is group O called the universal donor but AB the universal recipient?
- Explain Rh incompatibility in pregnancy. How is it managed?
- Describe the steps of blood clotting in sequence.
- What is the role of the lymphatic system? How does lymph differ from blood?
- Why does a person with hypertension have a higher risk of heart disease?
5-Mark Questions / Diagrams:
- Draw a neat labelled diagram of the internal structure of the human heart. Label: all 4 chambers, valves, chordae tendineae, and major blood vessels.
- Draw a flowchart showing the complete double circulation (pulmonary + systemic). Use arrows.
- Describe the cardiac cycle with a table showing what happens during atrial systole, ventricular systole, and diastole.
- Draw a table comparing arteries, veins, and capillaries under 6+ headings.
- Explain the ABO blood group system with a compatibility table. What happens if incompatible blood is transfused?
- Describe the components of blood and the function of each in a table format.
🧠 From my exam (93.4% scorer): The heart diagram is the most predicted diagram in ICSE Biology. Practice drawing it at least 5 times from memory before the exam. Pay special attention to:
→ Position of valves (tricuspid on right, bicuspid on left)
→ Chordae tendineae connecting valves to papillary muscles
→ Which blood vessels carry oxygenated vs deoxygenated blood
→ The thickness difference between left and right ventricle walls
The examiner looks for NEAT labels and correct arrows. A messy diagram loses marks even if content is correct.
→ Position of valves (tricuspid on right, bicuspid on left)
→ Chordae tendineae connecting valves to papillary muscles
→ Which blood vessels carry oxygenated vs deoxygenated blood
→ The thickness difference between left and right ventricle walls
The examiner looks for NEAT labels and correct arrows. A messy diagram loses marks even if content is correct.
13) Quick Reference Tables (Print This)
Heart Chambers — One Look
| Chamber | Receives | Sends To | Valve |
|---|---|---|---|
| RA | Venae cavae | RV | Tricuspid |
| RV | RA | Lungs (Pulmonary art.) | Pulmonary SL |
| LA | Pulmonary veins | LV | Bicuspid |
| LV | LA | Aorta (Body) | Aortic SL |
Key Differences — Quick Summary
Artery vs Vein:
- Artery: thick wall, narrow lumen, no valves, high pressure, away from heart
- Vein: thin wall, wide lumen, valves present, low pressure, toward heart
- Exception: Pulmonary artery has deoxygenated blood; pulmonary vein has oxygenated blood
Systemic vs Pulmonary Circulation:
- Systemic: LV → body → RA (oxygenates tissues)
- Pulmonary: RV → lungs → LA (oxygenates blood)
Systole vs Diastole:
- Systole = contraction (higher pressure, valves closed/open accordingly)
- Diastole = relaxation (filling of chambers)
⚠️ Final advice: This chapter is the most diagram-heavy in the ICSE Biology syllabus. The heart diagram, blood flow chart, and cardiac cycle table are guaranteed to appear in the board exam. Don't just read — practice drawing. Set a timer for 5 minutes and draw the heart from memory every day in the week before the exam. I did this and it was THE reason I scored full marks in this chapter.