Respiratory and Circulatory Systems✓ Updated 2026
Respiratory and circulatory systems explained for PSLE Science. Lungs, alveoli, heart, blood vessels, and blood components — with Singapore examples and exam tips for P5/P6.
Two Systems That Work as Partners
Your body needs oxygen constantly — every cell in your body uses it to release energy from glucose through respiration. The respiratory system (lungs and airways) is responsible for getting oxygen into your body and removing carbon dioxide. The circulatory system (heart, blood vessels, and blood) then transports that oxygen — and nutrients from digested food — to every cell.
The two systems meet at the lungs: blood arrives carrying CO₂, swaps it for oxygen, and then the heart pumps the oxygenated blood around the body. They are partners — neither can work without the other.
These Systems in Action in Singapore
During the NAPFA fitness test at school, your muscles are working hard and need much more energy. To produce this energy through respiration, they need more oxygen and produce more CO₂. Your body responds: your breathing rate increases (to get more oxygen into the blood faster) and your heart rate increases (to deliver that oxygen to the muscles faster). You feel your chest heaving and your pulse racing — that is both systems working at maximum capacity.
During the 2019–2020 haze periods, fine PM2.5 particles from Indonesian forest fires entered Singaporeans' lungs. These tiny particles penetrate deep into the alveoli and can lodge there, reducing the surface available for gas exchange and causing breathing difficulties — especially in the elderly and those with asthma. This is why the NEA issues health advisories and recommends N95 masks that filter these particles.
Blood donation drives by HSA (Health Sciences Authority) in Singapore remind us that blood is a living transport system. One donation of blood can save up to three lives — because each component (red cells, platelets, plasma) can be separated and used for different patients.
The Journey of Air into Your Body
- Nose/mouth: Air enters. Nose hairs and mucus filter out dust and bacteria. Air is warmed and moistened.
- Trachea (windpipe): Carries air down to the lungs. Rings of cartilage keep it open.
- Bronchi: The trachea splits into two bronchi, one going to each lung.
- Bronchioles: Bronchi branch into thousands of smaller tubes inside each lung.
- Alveoli: Tiny air sacs at the end of each bronchiole. This is where gas exchange occurs.
| Component | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen | ~21% | ~16% (less O₂ — absorbed by blood) |
| Carbon dioxide | ~0.04% | ~4% (more CO₂ — from respiration) |
| Nitrogen | ~78% | ~78% (unchanged) |
| Water vapour | Variable (lower) | Higher — lungs add moisture |
| Temperature | Ambient | Warmer (body temperature) |
Gas Exchange — Where Oxygen Meets Blood
Alveoli are tiny air sacs — there are about 600 million in a pair of human lungs. Despite being so small (each only 0.2mm across), together they provide a surface area of approximately 70 m² — about 40 times the surface area of your entire skin. This enormous surface area is the key to efficient gas exchange.
Each alveolus is:
- Very thin-walled — only one cell thick, so gases diffuse quickly across
- Surrounded by capillaries — tiny blood vessels in direct contact with the alveolus wall
- Moist inside — gases must dissolve in the moist lining to diffuse across
Oxygen diffuses from the alveolus (high concentration) into the blood capillary (low concentration). Carbon dioxide diffuses in the opposite direction. This happens simultaneously and continuously with every breath.
The Heart, Blood Vessels, and the Double Loop
The heart is a muscular pump that beats about 70 times per minute at rest, pumping blood in a double circulation:
- Loop 1 — Heart to lungs: Deoxygenated blood from the body enters the right side of the heart, which pumps it to the lungs. In the lungs, CO₂ is dropped off and O₂ is picked up.
- Loop 2 — Heart to body: Oxygenated blood returns to the left side of the heart, which pumps it to all organs and tissues in the body. Oxygen and nutrients are delivered; CO₂ and waste are collected.
Blood vessels:
- Arteries: Carry blood away from the heart. Thick, muscular walls to withstand high pressure. Usually carry oxygenated blood (exception: pulmonary artery carries deoxygenated blood to lungs).
- Veins: Return blood to the heart. Thinner walls, lower pressure. Usually carry deoxygenated blood (exception: pulmonary vein carries oxygenated blood from lungs).
- Capillaries: Microscopic vessels connecting arteries and veins. Walls only one cell thick — allow exchange of oxygen, nutrients, CO₂, and waste between blood and cells.
The Four Components of Blood
- Red blood cells: Contain haemoglobin, which carries oxygen. Disc-shaped, no nucleus (in mature form) — more space for haemoglobin. Most numerous component.
- White blood cells: Part of the immune system. Fight bacteria, viruses, and other pathogens. Much larger but far fewer than red blood cells.
- Platelets: Tiny cell fragments that help blood clot at wounds. Without platelets, even a small cut would bleed continuously.
- Plasma: The liquid component (~55% of blood volume). Pale yellow. Carries dissolved substances: glucose, amino acids, CO₂, hormones, urea, and proteins.
Why Are Alveoli So Numerous and So Tiny?
The total surface area of the alveoli (70 m²) is achieved not by having one large air sac, but by having 600 million tiny ones. This is mathematically much more efficient — for a given volume of lung tissue, millions of tiny spheres have dramatically more total surface area than a few large ones. This is the same principle used by villi in the small intestine and root hair cells in plant roots.
The thinness of alveolar walls minimises the distance that oxygen molecules must travel by diffusion to get from air to blood — and diffusion is much faster over shorter distances. If the walls were thicker, gas exchange would be too slow to supply the body's oxygen needs, especially during exercise.
Common Mistakes
Key Points at a Glance
- Alveoli: ~70 m² surface area, thin walls, surrounded by capillaries — efficient gas exchange
- Inhaled: ~21% O₂, ~0.04% CO₂. Exhaled: ~16% O₂, ~4% CO₂, more H₂O, warmer
- Arteries: away from heart (thick walls). Veins: to heart (thinner). Capillaries: exchange
- Blood: red cells (O₂), white cells (immunity), platelets (clotting), plasma (transport)
- Pulmonary artery: deoxygenated (heart to lungs). Pulmonary vein: oxygenated (lungs to heart)
- Heart rate and breathing rate both increase during exercise to deliver more O₂ to muscles
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Related PSLE Topics
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Model Answers — Respiratory & Circulatory System
Question 1 (2 marks)
State two adaptations of the alveoli that make gas exchange efficient.
Examiner note: "Thin walls" alone scores 0. You must link the feature to the benefit — always use "so" or "therefore" to connect structure to function.
Question 2 (3 marks)
Explain why a person breathes faster and more deeply during strenuous exercise.
Examiner note: The key cause is rising CO₂, not just "needing oxygen." Always mention CO₂ rising as the trigger.
Question 3 (2 marks)
Describe what happens to oxygen after it enters the alveolus from the inhaled air.
Examiner note: "Diffuses into the blood" alone is worth 1 mark. The second mark requires mentioning haemoglobin or transport to the body's cells.
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