Quick answer: Air passes through the nostrils → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The trachea has rings of cartilage so the air passage does not collapse. When we breathe in, the diaphragm flattens and the ribs lift, so the chest cavity gets larger and air is sucked in. Gas exchange happens in the alveoli, which have a very large, thin, moist surface with many blood capillaries. The lungs always keep a residual volume of air so that oxygen can be absorbed and carbon dioxide released throughout the breathing cycle.
Path of air
| Part | Role |
|---|---|
| Nostrils | Fine hairs and mucus filter dust and germs |
| Pharynx and larynx | Throat and voice box |
| Trachea (windpipe) | Carries air to the lungs; held open by rings of cartilage |
| Bronchi | Two branches, one into each lung |
| Bronchioles | Smaller and smaller tubes inside the lungs |
| Alveoli | Tiny balloon-like air sacs where gases are exchanged |
Why rings of cartilage? The rings make sure that the air passage does not collapse, so air can always flow through the trachea.
Mechanism of breathing
| Feature | Breathing in (inhalation) | Breathing out (exhalation) |
|---|---|---|
| Diaphragm | Contracts and flattens | Relaxes and becomes dome-shaped |
| Ribs | Lifted up and out by rib muscles | Move down and in |
| Chest cavity | Becomes larger | Becomes smaller |
| Air pressure in lungs | Falls below outside air | Rises above outside air |
| Air flow | Air is sucked in | Air is pushed out |
Air always flows from higher pressure to lower pressure, which is why changing the size of the chest moves air in and out.
Alveoli: designed for gas exchange
- Large surface area: NCERT notes that if the alveolar surface were spread out, it would cover about 80 m².
- Thin walls: The walls are only one cell thick, so gases cross quickly.
- Rich blood supply: Each alveolus is surrounded by a network of blood capillaries.
- Moist lining: Gases dissolve in the moisture before diffusing across.
Oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli to be breathed out.
Residual volume
Even after we breathe out fully, some air stays in the lungs. This is the residual volume (about 1.2 litres in a typical adult).
Why is it needed? During the breathing cycle, when air is taken in and let out, the lungs always contain some air. This gives enough time for oxygen to be absorbed and for carbon dioxide to be released, even between breaths.
Why large animals need haemoglobin
In large animals, most cells are far from the lungs, and diffusion alone is far too slow. NCERT gives a striking estimate: if diffusion alone moved oxygen in our body, it would take about 3 years for a molecule of oxygen to reach our toes from our lungs.
Humans use the respiratory pigment haemoglobin, found in red blood cells. It has a very high affinity for oxygen, picks it up in the lungs and carries it to the tissues.
Carbon dioxide is more soluble in water than oxygen, so it is mostly transported dissolved in the blood.
Summary table
| Part or term | Key fact |
|---|---|
| Rings of cartilage | Stop the trachea collapsing |
| Diaphragm | Flattens when we breathe in |
| Alveoli | Large, thin, moist surface with capillaries |
| Alveolar surface area | About 80 m² |
| Residual volume | Air left after full exhalation; keeps gas exchange going |
| Haemoglobin | Carries oxygen in red blood cells |
| Carbon dioxide | Carried mostly dissolved in blood |
Exam tips
- When comparing inhalation and exhalation, describe both the diaphragm and the ribs.
- Use NCERT's wording for residual volume: "sufficient time for oxygen to be absorbed and for the carbon dioxide to be released".
- Draw and label the respiratory system neatly; the alveoli and bronchioles are often asked.
Common mistakes
- Saying the diaphragm moves up when we breathe in. It flattens and moves down.
- Saying the lungs empty completely when we breathe out.
- Saying carbon dioxide is carried mainly by haemoglobin. It is mostly carried dissolved in the blood.
- Calling the rings of the trachea "bone". They are cartilage.
Frequently Asked Questions
Why does the trachea have rings of cartilage?
The rings keep the trachea open and stop the air passage from collapsing.
What is residual volume?
It is the volume of air that remains in the lungs even after breathing out as fully as possible.
Why is residual volume important?
It means the lungs always contain air, so there is enough time for oxygen to be absorbed and carbon dioxide to be released throughout the breathing cycle.
How are alveoli suited for gas exchange?
They have a very large surface area, very thin walls, a moist lining and a rich supply of blood capillaries.
Why do humans need haemoglobin?
Diffusion alone is far too slow to supply oxygen to all the cells of a large body, so haemoglobin in red blood cells carries oxygen quickly to the tissues.