Human Body — Circulatory System
1. Functions of the Circulatory System
The CIRCULATORY SYSTEM transports substances throughout the body.
Main Functions
- Transport of oxygen and nutrients to cells.
- Transport of carbon dioxide and wastes from cells.
- Distribution of hormones and enzymes.
- Regulation of body temperature.
- Protection against infections (white blood cells, antibodies).
2. The Heart
The HEART is a MUSCULAR ORGAN that pumps blood throughout the body.
Position and Size
- Located in the CHEST (thoracic cavity), between the lungs.
- Slightly LEFT of the midline.
- About the size of a CLENCHED FIST.
Chambers of the Heart — FOUR Chambers
| Chamber | Receives Blood From | Pumps Blood To |
|---|---|---|
| Right Atrium (upper) | Body (via VENA CAVA) | Right Ventricle |
| Right Ventricle (lower) | Right Atrium | Lungs (via PULMONARY ARTERY) |
| Left Atrium (upper) | Lungs (via PULMONARY VEIN) | Left Ventricle |
| Left Ventricle (lower) | Left Atrium | Body (via AORTA) |
Valves of the Heart
Valves PREVENT BACKFLOW of blood:
- Tricuspid valve: Between right atrium and right ventricle.
- Bicuspid (Mitral) valve: Between left atrium and left ventricle.
- Semilunar valves: At the openings of pulmonary artery and aorta.
Key Features
- The LEFT VENTRICLE has the THICKEST walls (pumps blood to the entire body).
- The heart beats about 72 times per minute (at rest).
- Heartbeat can be heard with a STETHOSCOPE.
3. Blood Vessels
| Vessel | Direction | Walls | Valves | Function |
|---|---|---|---|---|
| Arteries | Away from heart (heart → body) | THICK, ELASTIC (muscular) | NO | Carry OXYGENATED blood (except pulmonary artery) |
| Veins | Toward heart (body → heart) | THINNER, less muscular | YES | Carry DEOXYGENATED blood (except pulmonary vein) |
| Capillaries | Connect arteries and veins | ONE-CELL THICK | NO | SITE of substance exchange with tissues |
Important Exceptions
- Pulmonary artery: Carries DEOXYGENATED blood from heart to lungs.
- Pulmonary vein: Carries OXYGENATED blood from lungs to heart.
4. Blood Composition
Blood is a CONNECTIVE TISSUE consisting of PLASMA and BLOOD CELLS.
Components
| Component | Percentage | Function |
|---|---|---|
| Plasma (liquid) | ~55% | Transports nutrients, hormones, wastes. Contains water, proteins, salts. |
| Red Blood Cells (RBCs) | ~44% | Carry OXYGEN (contain HAEMOGLOBIN). Biconcave, NO nucleus. |
| White Blood Cells (WBCs) | <1% | FIGHT infections. Have nucleus. Types: phagocytes (engulf germs), lymphocytes (make antibodies). |
| Platelets | <1% | BLOOD CLOTTING (release clotting factors). |
Blood Groups (ABO System)
| Blood Group | Can Give To | Can Receive From |
|---|---|---|
| A | A, AB | A, O |
| B | B, AB | B, O |
| AB | AB ONLY | ALL (universal recipient) |
| O | ALL (universal donor) | O ONLY |
5. Double Circulation
Humans have DOUBLE CIRCULATION — blood passes through the heart TWICE for one complete cycle.
Pulmonary Circulation (Heart → Lungs → Heart)
- Right ventricle pumps DEOXYGENATED blood to lungs via pulmonary artery.
- In lungs: CO₂ is released, O₂ is picked up.
- OXYGENATED blood returns to left atrium via pulmonary vein.
Systemic Circulation (Heart → Body → Heart)
- Left ventricle pumps OXYGENATED blood to body via AORTA.
- In body tissues: O₂ and nutrients are delivered, CO₂ and wastes are picked up.
- DEOXYGENATED blood returns to right atrium via VENA CAVA.
Advantage of Double Circulation
- OXYGENATED and DEOXYGENATED blood are KEPT SEPARATE.
- Ensures HIGH PRESSURE oxygen supply to body tissues.
- More EFFICIENT than single circulation.
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Heart chambers and their function | 3 marks | Very High |
| Arteries vs Veins (differences) | 2-3 marks | Very High |
| Blood composition (components) | 2-3 marks | High |
| Double circulation pathway | 3 marks | Very High |
| Blood groups | 2 marks | Medium |
Common Mistakes
- Saying all arteries carry oxygenated blood (pulmonary artery carries DEOXYGENATED blood).
- Confusing atrium and ventricle (atria RECEIVE, ventricles PUMP OUT).
- Writing 'RBCs have a nucleus' (they do NOT in mammals — more space for haemoglobin).
- Thinking the right side of the heart has oxygenated blood (RIGHT side has deoxygenated blood).
Self-Test (5 Questions)
Q1. How many chambers does the human heart have? (1 mark)
- A) 2
- B) 3
- C) 4
- D) 5
Q2. 'Which blood vessel carries blood from heart to lungs?' (1 mark)
- A) Aorta
- B) Pulmonary vein
- C) Pulmonary artery
- D) Vena cava
Q3. 'Name the component of blood that carries oxygen.' (1 mark)
Q4. 'Draw a labelled diagram showing double circulation.' (3 marks)
Q5. 'Why is the left ventricle thicker than the right ventricle?' (2 marks)
Answers
A1. C) 4 chambers (right atrium, right ventricle, left atrium, left ventricle). A2. C) Pulmonary artery (carries deoxygenated blood from right ventricle to lungs). A3. Red Blood Cells (RBCs) containing haemoglobin. A4. Diagram should show: heart in centre, pulmonary circulation (right ventricle → lungs → left atrium), systemic circulation (left ventricle → body → right atrium). A5. The left ventricle pumps blood to the ENTIRE BODY (systemic circulation), requiring HIGH pressure. The right ventricle pumps blood only to the LUNGS (pulmonary circulation), which is a shorter distance and requires LESS pressure.
