Singapore Lower Secondary Science — Complete Study Notes
1. Overview — What Is the Circulatory System?
Purpose
The circulatory system transports substances around the body:
Oxygen and glucose to respiring cells
Carbon dioxide and other waste products away from cells
Hormones, antibodies, and heat
Components
The circulatory system consists of:
Heart — muscular pump that keeps blood moving
Blood vessels — arteries, veins, and capillaries (the "pipes")
Blood — the fluid that carries dissolved substances
Key point: Humans have a double circulatory system — blood passes through the heart twice for each complete circuit of the body.
2. Structure of the Heart
Four Chambers
Chamber
Side
Receives blood from
Sends blood to
Right atrium
Right
Vena cava (superior & inferior)
Right ventricle
Right ventricle
Right
Right atrium
Pulmonary artery → lungs
Left atrium
Left
Pulmonary vein (from lungs)
Left ventricle
Left ventricle
Left
Left atrium
Aorta → rest of body
Valves
Valve
Location
Function
Tricuspid valve
Between right atrium & right ventricle
Prevents backflow of blood into right atrium when right ventricle contracts
Bicuspid (mitral) valve
Between left atrium & left ventricle
Prevents backflow of blood into left atrium when left ventricle contracts
Semilunar valves
At the base of the aorta & pulmonary artery
Prevent backflow of blood into the ventricles when the ventricles relax
The bicuspid valve is also called the mitral valve. Both names are acceptable in exams.
Septum
The septum is a muscular wall that separates the left and right sides of the heart. It prevents oxygenated blood and deoxygenated blood from mixing.
Wall Thickness Adaptation
The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood to the entire body (systemic circulation), whereas the right ventricle only pumps blood to the nearby lungs (pulmonary circulation).
Exam favourite: "Explain why the left ventricle has a thicker wall than the right ventricle." — It must generate much higher pressure to push blood all the way around the body.
Heart Diagram
Note: Blue = deoxygenated blood Red = oxygenated blood Green lines = valves
3. Pathway of Blood Through the Heart
Follow the path step by step:
Vena cava (superior & inferior) carries deoxygenated blood into the right atrium
Right atrium contracts → blood passes through the tricuspid valve into the right ventricle
Right ventricle contracts → blood is pumped through the semilunar valve into the pulmonary artery
Pulmonary artery carries blood to the lungs, where gas exchange occurs (CO₂ removed, O₂ absorbed)
Oxygenated blood returns via the pulmonary vein to the left atrium
Left atrium contracts → blood passes through the bicuspid (mitral) valve into the left ventricle
Left ventricle contracts → blood is pumped through the semilunar valve into the aorta
The aorta carries oxygenated blood to the rest of the body
Exception alert! The pulmonary artery carries deoxygenated blood (the only artery that does). The pulmonary vein carries oxygenated blood (the only vein that does). Arteries are defined by direction of flow (away from heart), NOT by oxygen content.
4. Double Circulation
Humans have a double circulatory system — blood passes through the heart twice for each full circuit:
Pulmonary Circulation
Right side of heart → lungs → back to left side
Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium
Left side of heart → body organs → back to right side
Left ventricle → aorta → body organs → vena cava → right atrium
Purpose: Deliver oxygen and nutrients to cells; remove CO₂ and waste
Advantage of double circulation: Blood can be pumped at higher pressure to the body after receiving oxygen in the lungs. This ensures oxygen reaches respiring tissues faster and more efficiently.
5. Blood Vessels
Comparison Table
Feature
Artery
Vein
Capillary
Function
Carries blood away from the heart
Carries blood towards the heart
Exchange of materials between blood and cells
Wall thickness
Thick (muscle + elastic fibres)
Thin (less muscle + elastic fibres)
One cell thick (endothelium only)
Lumen size
Small lumen
Large lumen
Tiny lumen (just fits RBCs)
Valves
None
Present (prevent backflow)
None
Pressure
High
Low
Falling (high → low)
Pulse
Felt (pulsatile)
Not felt (steady flow)
Not felt
Why do arteries have thick, elastic walls? To withstand and smooth out the high pressure of blood pumped by the ventricles. The elastic fibres stretch and recoil to maintain steady blood flow.
Why do veins have valves? Blood in veins is at low pressure. Valves prevent backflow and ensure blood returns to the heart, especially in the limbs where blood must travel against gravity.
Why are capillary walls one cell thick? To provide a short diffusion distance for efficient exchange of oxygen, carbon dioxide, glucose, and waste products between blood and cells.
6. Blood — Components & Functions
Component
Description
Function
Red blood cells (erythrocytes)
Biconcave disc shape; no nucleus; contain haemoglobin
Transport oxygen (haemoglobin binds O₂ to form oxyhaemoglobin). Biconcave shape increases surface area for gas exchange. No nucleus → more room for haemoglobin.
White blood cells (leucocytes)
Have a nucleus; larger than RBCs; fewer in number
Defend against disease. Phagocytes engulf pathogens (phagocytosis). Lymphocytes produce antibodies.
Platelets
Small cell fragments; no nucleus
Involved in blood clotting — seal wounds to prevent blood loss and entry of pathogens
Biconcave disc — increases surface-area-to-volume ratio for faster oxygen diffusion
No nucleus — more space for haemoglobin molecules
Flexible membrane — can squeeze through narrow capillaries
7. Coronary Heart Disease (CHD)
What Is CHD?
Coronary heart disease occurs when the coronary arteries (which supply the heart muscle with oxygen and glucose) become narrowed or blocked by fatty deposits (atheroma). This reduces blood flow to the heart muscle, causing chest pain (angina) or a heart attack (myocardial infarction).
Causes & Risk Factors
Factor
How It Contributes
High-fat diet (saturated fats)
Increases blood cholesterol → fatty deposits (atheroma) build up on artery walls
Reduces cardiovascular fitness; contributes to obesity and high blood pressure
Obesity
Increases workload on the heart; linked to high blood pressure and diabetes
Excessive stress / alcohol
Raises blood pressure; damages blood vessel walls
Genetic predisposition
Family history increases risk
Prevention
Balanced diet — reduce saturated fats and cholesterol; increase fibre
Regular aerobic exercise
Avoid smoking
Maintain healthy body weight
Manage stress
The coronary arteries are the heart's own blood supply. If they are blocked, the heart muscle is starved of oxygen and can die — this is a heart attack.
8. Test Yourself!
Section A — Multiple Choice (10 questions)
Q1. Which blood vessel carries blood away from the heart?
A. Vena cava B. Pulmonary vein C. Aorta D. Capillary
Answer: C. Aorta Arteries carry blood away from the heart. The aorta is the main artery. The vena cava and pulmonary vein are veins (carry blood towards the heart).
Q2. Which chamber of the heart has the thickest muscular wall?
A. Right atrium B. Right ventricle C. Left atrium D. Left ventricle
Answer: D. Left ventricle The left ventricle pumps blood to the entire body (systemic circulation) at high pressure, so it needs a thick muscular wall. The right ventricle only pumps to the nearby lungs.
Q3. The pulmonary artery carries:
A. Oxygenated blood to the body B. Deoxygenated blood to the lungs C. Oxygenated blood to the heart D. Deoxygenated blood to the body
Answer: B. Deoxygenated blood to the lungs The pulmonary artery is the only artery that carries deoxygenated blood. It goes from the right ventricle to the lungs. "Artery" means away from the heart, NOT oxygenated.
Q4. What is the function of valves in veins?
A. Speed up blood flow B. Prevent backflow of blood C. Filter waste products D. Produce red blood cells
Answer: B. Prevent backflow of blood Blood in veins is at low pressure. Valves ensure one-way flow back to the heart, particularly important in the limbs where blood must travel against gravity.
Q5. Which component of blood is responsible for clotting?
A. Red blood cells B. Plasma C. Platelets D. White blood cells
Answer: C. Platelets Platelets are cell fragments that initiate blood clotting at wound sites, sealing the wound to prevent blood loss and entry of pathogens.
Q6. Why do red blood cells have a biconcave disc shape?
A. To look larger B. To increase surface area for oxygen diffusion C. To store more water D. To move faster
Answer: B. To increase surface area for oxygen diffusion The biconcave shape gives a large surface-area-to-volume ratio, allowing oxygen to diffuse in and out rapidly.
Q7. In double circulation, blood passes through the heart:
A. Once B. Twice C. Three times D. Four times
Answer: B. Twice In one complete circuit, blood passes through the heart twice — once during pulmonary circulation (right side → lungs → left side) and once during systemic circulation (left side → body → right side).
Q8. The bicuspid valve is located between:
A. Right atrium and right ventricle B. Left atrium and left ventricle C. Right ventricle and pulmonary artery D. Left ventricle and aorta
Answer: B. Left atrium and left ventricle The bicuspid (mitral) valve is on the left side of the heart. The equivalent valve on the right side is the tricuspid valve.
Q9. Which of the following is a risk factor for coronary heart disease?
A. Low blood pressure B. Regular exercise C. A diet high in saturated fats D. High fibre intake
Answer: C. A diet high in saturated fats High saturated fat intake raises blood cholesterol, leading to fatty deposits (atheroma) narrowing the coronary arteries. Regular exercise and high fibre are protective factors.
Q10. Which statement about capillaries is correct?
A. They have thick walls to withstand high pressure B. They have valves to prevent backflow C. Their walls are one cell thick for efficient exchange D. They carry blood away from the heart
Answer: C. Their walls are one cell thick for efficient exchange Capillaries are the site of material exchange. Their thin walls (one cell thick) provide a short diffusion distance for oxygen, CO₂, glucose, and waste products.
Section B — Structured Questions (5 questions)
Q11. Describe the pathway of blood from the vena cava to the aorta. Name all chambers and valves the blood passes through.
Answer:
Vena cava → right atrium → tricuspid valve → right ventricle → semilunar valve → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid (mitral) valve → left ventricle → semilunar valve → aorta.
Key points: Blood is deoxygenated from vena cava to lungs. It becomes oxygenated in the lungs. From pulmonary vein onwards, blood is oxygenated.
Q12. Explain why the left ventricle has a thicker muscular wall than the right ventricle.
Answer:
The left ventricle pumps blood through the aorta to the entire body (systemic circulation), which requires high pressure to overcome the resistance of the long network of blood vessels throughout the body. The right ventricle only pumps blood through the pulmonary artery to the lungs, which are nearby, so a lower pressure is sufficient. The thicker muscular wall allows the left ventricle to contract more forcefully and generate this higher pressure.
Q13. Compare the structure of an artery and a vein. Give three differences and explain the reason for each.
Answer:
1. Arteries have thicker walls (more muscle and elastic fibres) — to withstand and smooth out the high pressure of blood pumped by the heart. Veins have thinner walls as blood is at lower pressure.
2. Arteries have a smaller lumen — helps maintain high blood pressure. Veins have a larger lumen to reduce resistance to low-pressure blood flow.
3. Veins have valves; arteries do not — valves in veins prevent backflow because blood pressure is low. Arteries do not need valves because the high pressure ensures blood only flows in one direction (away from the heart).
Q14. State two adaptations of red blood cells and explain how each adaptation helps their function.
Answer:
1. Biconcave disc shape — increases the surface-area-to-volume ratio, allowing faster diffusion of oxygen into and out of the cell.
2. No nucleus — creates more space inside the cell for haemoglobin molecules, increasing the oxygen-carrying capacity of each red blood cell.
(Also acceptable: Flexible membrane — allows red blood cells to squeeze through capillaries that are narrower than the cell's diameter.)
Q15. Explain how coronary heart disease develops and state two ways it can be prevented.
Answer: Development: Fatty deposits (atheroma) build up on the inner walls of the coronary arteries, narrowing them. This reduces blood flow to the heart muscle, meaning less oxygen and glucose reach the cardiac muscle. This can cause chest pain (angina). If an artery becomes completely blocked, the heart muscle is starved of oxygen and may die — this is a heart attack (myocardial infarction).
Prevention (any two):
- Eat a balanced diet low in saturated fats and cholesterol to reduce fatty deposit build-up
- Exercise regularly to strengthen the heart and improve circulation
- Avoid smoking — smoking damages blood vessel lining and raises blood pressure
- Maintain a healthy body weight to reduce strain on the heart
— End of Study Notes —
Aligned with Singapore MOE Lower Secondary Science Syllabus