1. The Heart
The heart is a double pump — the right side pumps deoxygenated blood to the lungs (pulmonary circulation); the left side pumps oxygenated blood to the rest of the body (systemic circulation).
| Structure | Function |
|---|---|
| Right atrium | Receives deoxygenated blood from body via vena cava |
| Right ventricle | Pumps blood to lungs via pulmonary artery |
| Left atrium | Receives oxygenated blood from lungs via pulmonary vein |
| Left ventricle | Pumps blood to body via aorta (thickest wall — highest pressure) |
| Atrioventricular valves | Prevent backflow from ventricles to atria |
| Semi-lunar valves | Prevent backflow from arteries into ventricles |
| Coronary arteries | Supply the heart muscle itself with oxygenated blood |
The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood at higher pressure all the way around the body, not just to the nearby lungs.
2. Blood Vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction | Away from heart | Towards heart | Between arteries and veins |
| Blood pressure | High | Low | Very low |
| Wall thickness | Thick (muscle + elastic) | Thin | One cell thick |
| Valves | None (except semi-lunar at heart) | Present — prevent backflow | None |
| Lumen | Narrow | Wide | Very narrow (one RBC at a time) |
| Function | Carry blood under pressure | Return blood; aided by muscle action | Exchange of substances |
Arteries carry blood AWAY from the heart — this is usually oxygenated. The ONE exception is the pulmonary artery which carries deoxygenated blood from the right ventricle to the lungs. Similarly, the pulmonary vein carries oxygenated blood. Know these exceptions.
3. Blood Components
| Component | Structure | Function |
|---|---|---|
| Red blood cells | Biconcave, no nucleus, contain haemoglobin | Transport oxygen as oxyhaemoglobin |
| White blood cells | Nucleated; various types | Immune defence — phagocytosis and antibody production |
| Platelets | Cell fragments, no nucleus | Blood clotting — form platelet plug; release clotting factors |
| Plasma | Straw-coloured liquid (~90% water) | Transport of CO₂, glucose, amino acids, urea, hormones, heat |
Haemoglobin and oxygen transport
Haemoglobin + O₂ ⇌ Oxyhaemoglobin (in lungs, high O₂ concentration → right shift; in tissues, low O₂ concentration → left shift, releasing O₂)
Blood clotting
Damaged vessel → platelets accumulate → release clotting factors → prothrombin → thrombin (enzyme) → fibrinogen → fibrin (mesh of fibres trapping RBCs → clot).
4. Lymphatic System
Tissue fluid (plasma that has leaked from capillaries) bathes cells, delivering nutrients and collecting waste. Most is reabsorbed into capillaries; the remainder drains into lymph capillaries as lymph.
- Lymph vessels carry lymph back to the blood via the thoracic duct into the subclavian vein.
- Lymph nodes filter lymph, removing bacteria and debris; lymphocytes are produced here.
- Fat absorbed from the small intestine (as chylomicrons) enters lymph vessels (lacteals) in villi.
- Arteries: away from heart, thick walls (elastic + muscle), no valves, high pressure, carry oxygenated blood (except pulmonary artery)
- Veins: towards heart, thin walls, valves, low pressure, carry deoxygenated blood (except pulmonary vein)
- Capillaries: one cell thick, site of exchange, link arteries and veins in tissues
- Blood components: red blood cells (O2), white blood cells (immunity), platelets (clotting), plasma (transport liquid)
- Heart structure: 4 chambers. Right side = pulmonary circuit (to lungs). Left side = systemic circuit (to body). Left ventricle has thickest wall.
5. Common Exam Traps
Pulmonary ARTERY: carries DEoxygenated blood (right ventricle → lungs). Pulmonary VEIN: carries OXYgenated blood (lungs → left atrium). These are the only vessels where the artery/vein = deoxygenated/oxygenated rule is reversed.
Plasma is blood minus cells and platelets. Serum is plasma minus clotting proteins. At O-Level, use "plasma" — do not write "serum" unless specifically asked.
Veins have valves because blood pressure is low — valves prevent blood from flowing backward (away from the heart) under gravity. Arteries do not have valves because blood flows under continuous high pressure from the heart.
Key Terms — Flashcard Review
Tap each card to reveal the definition.
🎯 Practice Quiz — Test Yourself
8 O Level-style questions on this topic. Select an answer to see instant feedback.
Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.