Circulatory System
1. Components of the Circulatory System
| Component | Function |
|---|---|
| HEART | PUMPS blood throughout the body |
| BLOOD VESSELS | TUBES that carry blood |
| BLOOD | TRANSPORTS oxygen, nutrients, CO₂, and wastes |
| LYMPH | Returns tissue fluid to the bloodstream |
2. The Human Heart
A MUSCULAR organ located in the CHEST CAVITY (slightly left of centre).
Four Chambers
| Chamber | Position | Function |
|---|---|---|
| RIGHT ATRIUM | Upper RIGHT | Receives DEOXYGENATED blood from the body |
| RIGHT VENTRICLE | Lower RIGHT | Pumps blood to the LUNGS |
| LEFT ATRIUM | Upper LEFT | Receives OXYGENATED blood from the lungs |
| LEFT VENTRICLE | Lower LEFT | Pumps blood to the ENTIRE body |
Valves of the Heart
| Valve | Location | Function |
|---|---|---|
| Tricuspid valve | Between RIGHT atrium and ventricle | Prevents BACKFLOW of blood |
| Bicuspid (Mitral) valve | Between LEFT atrium and ventricle | Prevents BACKFLOW of blood |
| Semilunar valves | At the BASE of pulmonary artery and aorta | Prevent BACKFLOW from arteries |
3. Blood Vessels
| Feature | ARTERIES | VEINS | CAPILLARIES |
|---|---|---|---|
| Direction | Carry blood AWAY from heart | Carry blood TOWARD heart | Connect arteries and veins |
| Wall thickness | THICK and muscular | THINNER | VERY THIN (one cell thick) |
| Valve | NO valves | PRESENT (prevent backflow) | NO valves |
| Blood pressure | HIGH | LOW | LOW |
| Blood colour (usually) | OXYGENATED (bright red) except pulmonary | DEOXYGENATED (dark red) except pulmonary | — |
| Location | DEEPER in body | CLOSER to skin surface | In all TISSUES |
4. Blood Components
| Component | Percentage | Function |
|---|---|---|
| PLASMA | ~55% | Liquid matrix. Transports nutrients, hormones, wastes, salts |
| RED BLOOD CELLS (ERYTHROCYTES) | ~44% | Contain HAEMOGLOBIN (carries OXYGEN). No NUCLEUS (in mammals). BICONCAVE shape |
| WHITE BLOOD CELLS (LEUKOCYTES) | ~1% | FIGHT infections. Have a NUCLEUS. Various types (phagocytes, lymphocytes) |
| PLATELETS (THROMBOCYTES) | <1% | BLOOD CLOTTING. Fragments of cells |
Worked Example: Functions of WBC types:
- NEUTROPHILS: ENGULF bacteria (phagocytosis)
- LYMPHOCYTES: Produce ANTIBODIES
- MONOCYTES: ENGULF dead cells and debris
5. Double Circulation
The blood passes through the heart TWICE in one complete circuit.
Pulmonary Circulation (Heart → Lungs → Heart)
RIGHT ventricle → Pulmonary artery → Lungs (CO₂ removed, O₂ added) → Pulmonary veins → LEFT atrium
Systemic Circulation (Heart → Body → Heart)
LEFT ventricle → Aorta → Body tissues (O₂ delivered, CO₂ picked up) → Vena cava → RIGHT atrium
'Double circulation means OXYGENATED and DEOXYGENATED blood do NOT mix (except in some congenital heart defects). This makes BIRD and MAMMAL circulation highly EFFICIENT.'
6. Pulse and Blood Pressure
Pulse: The EXPANSION and CONTRACTION of an artery with each heartbeat.
- Normal adult: 72–80 beats per minute (at rest)
Blood Pressure: The FORCE exerted by blood against the walls of arteries.
- Systolic (ventricles CONTRACT): ~120 mm Hg
- Diastolic (ventricles RELAX): ~80 mm Hg
7. Lymphatic System
A NETWORK of tissues and organs that helps maintain FLUID balance and defend against infection.
| Component | Function |
|---|---|
| LYMPH | CLEAR fluid (similar to plasma but LESS protein). Contains WBCs |
| Lymph nodes | FILTER lymph. Trap bacteria and cancer cells |
| Lymph vessels | Collect EXCESS tissue fluid and return it to the BLOOD |
| Spleen | Filters BLOOD. Stores platelets. Produces lymphocytes |
| Tonsils | TRAP pathogens entering through the mouth and nose |
Difference: Blood vs Lymph
- Blood circulates in a CLOSED system. Lymph flows in ONE direction (toward the heart).
- Blood carries oxygen. Lymph does NOT.
- Blood has RBCs. Lymph has ONLY WBCs.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'All arteries carry oxygenated blood' | The PULMONARY artery carries DEOXYGENATED blood to the lungs |
| 'All veins carry deoxygenated blood' | PULMONARY veins carry OXYGENATED blood from the lungs to the heart |
| 'The left ventricle pumps blood to the lungs' | The LEFT ventricle pumps blood to the BODY (systemic). The RIGHT ventricle pumps to the LUNGS (pulmonary) |
| 'Plasma is just water' | Plasma is 90% water + 10% DISSOLVED substances (proteins, nutrients, hormones, gases, wastes) |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Name chambers of heart, components of blood
- 3-mark questions: Differentiate between arteries and veins
- 4-mark questions: Double circulation — labelled diagram
- 6-mark questions: Functions of blood components or lymphatic system
Self-Test
Q1. Name the four chambers of the human heart. A1. Right Atrium, Right Ventricle, Left Atrium, Left Ventricle.
Q2. What is the function of red blood cells? A2. RBCs contain HAEMOGLOBIN which BINDS to oxygen in the lungs and CARRIES it to all body tissues. Their biconcave shape INCREASES surface area for gas exchange.
Q3. Differentiate between arteries and veins. A3. Arteries carry blood AWAY from the heart (thick walls, high pressure, NO valves). Veins carry blood TOWARD the heart (thinner walls, low pressure, HAVE valves).
Q4. What is double circulation? A4. Blood passes through the heart TWICE: PULMONARY circulation (heart → lungs → heart) and SYSTEMIC circulation (heart → body → heart). This keeps oxygenated and deoxygenated blood SEPARATE.
Q5. What is the role of platelets in the blood? A5. Platelets are INVOLVED in blood CLOTTING. At a wound site, they release chemicals that CONVERT fibrinogen into fibrin, forming a NET that traps RBCs to create a CLOT.
Q6. What is the function of the lymphatic system? A6. The lymphatic system RETURNS excess tissue fluid to the blood, FILTERS lymph (traps pathogens in lymph nodes), and helps the IMMUNE system fight infections.
