The Human Respiratory System: Complete PSLE Guide

Breathing is something we do about 20,000 times a day without thinking. But in Singapore Primary Science, you need to understand exactly what happens when we breathe — which organs are involved, what gases are exchanged, and how this keeps us alive. The respiratory system is a key PSLE topic, often paired with the circulatory system and questions about gaseous exchange.

The Organs of the Respiratory System

The respiratory system consists of several organs that work together to bring oxygen into the body and remove carbon dioxide. You need to know all of them for PSLE:

• Nose and mouth — where air enters the body; the nose warms, moistens, and filters air
• Trachea (windpipe) — the tube that carries air from the throat to the lungs
• Bronchi — two branches of the trachea, one leading to each lung
• Lungs — the main organs of respiration; where gaseous exchange occurs
• Diaphragm — a dome-shaped muscle below the lungs that controls breathing by contracting and relaxing

Inhalation vs Exhalation

Understanding the differences between breathing in (inhalation) and breathing out (exhalation) is critical for PSLE. Students are frequently asked to compare what happens to the chest, diaphragm, and air composition.

Gaseous Exchange in the Lungs

Inside the lungs, there are millions of tiny air sacs called alveoli. These are surrounded by tiny blood vessels called capillaries. Gaseous exchange happens here:

1. Oxygen from the inhaled air passes through the thin walls of the alveoli into the blood
2. Carbon dioxide from the blood passes into the alveoli to be breathed out

The alveoli are designed for efficient gaseous exchange — they have thin walls for easy diffusion, a moist lining so gases can dissolve, and a rich blood supply to carry gases to and from the body.

Why Do We Need to Breathe?

Every cell in the body needs oxygen to release energy from food — this process is called cellular respiration. Without a constant supply of oxygen, cells cannot produce energy and will die. Carbon dioxide is a waste product of this process and must be removed from the body or it becomes toxic at high concentrations.

This is also why exercise makes us breathe faster: our muscles need more energy, so they demand more oxygen and produce more carbon dioxide. The brain detects the rising carbon dioxide levels and signals the respiratory system to breathe faster.

The Respiratory System and the Circulatory System

The respiratory and circulatory systems work closely together. Once oxygen enters the blood in the lungs, the heart pumps that oxygen-rich blood to all parts of the body. At the same time, carbon-dioxide-rich blood from the body is returned to the lungs to release carbon dioxide and pick up fresh oxygen.

For PSLE, you may be asked to describe this link: oxygen moves from the lungs into the blood, and the heart pumps it around the body. Carbon dioxide moves from the body cells into the blood and is carried back to the lungs to be exhaled.

Model Exam Answers

Question: Explain what happens to the diaphragm and chest when we inhale.

Model Answer: When we inhale, the diaphragm contracts and moves downward. This increases the volume of the chest cavity, causing the air pressure inside the lungs to decrease. Air from outside, which is at higher pressure, rushes into the lungs through the nose or mouth, trachea, and bronchi.

Question: State two differences between inhaled and exhaled air.

Model Answer: (1) Inhaled air contains more oxygen (about 21%) than exhaled air (about 16%). (2) Exhaled air contains more carbon dioxide (about 4%) than inhaled air (about 0.04%). Exhaled air also contains more water vapour than inhaled air.