The Respiratory System — ICSE Class 9 Biology Important Questions
13 ICSE Class 9 Biology practice questions on The Respiratory System, each with a full model answer, covering 5 topics from the chapter in the question formats used in the exam.
By The Classmate AI Editorial Team
- 13
- Questions
- 5
- Topics
- 4
- Key concepts
- ₹0
- With answers
A labelled diagram of the respiratory pathway is close to guaranteed every year, so know the sequence — nose to alveoli — cold. The diaphragm/intercostal-muscle mechanism of inspiration and expiration, and the breathing-versus-respiration distinction, are the two ideas examiners most often test as short-answer reasoning rather than recall.
About The Respiratory System
Air takes a fixed route from nostrils to alveoli, and this chapter is largely about knowing that pathway cold — nasal passages, pharynx, larynx, trachea, bronchi, bronchioles — before getting into how the diaphragm and intercostal muscles actually move air in and out. The one distinction ICSE examiners keep testing: breathing is the mechanical exchange of air, while respiration is the chemical release of energy from glucose happening inside every cell.
Key concepts & formulas
Air flows: nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli of the lungs, where gaseous exchange takes place.
Inspiration: diaphragm contracts and flattens, external intercostals raise the ribs, chest volume increases, pressure falls, air rushes in. Expiration: diaphragm and intercostals relax, chest volume decreases, pressure rises, air is pushed out.
At the alveoli, oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the alveolar air, along their concentration gradients. Alveoli have thin, moist walls and a rich blood supply for rapid diffusion.
Breathing is the physical process of taking in air and giving out air (a mechanical exchange of gases). Respiration is the chemical process inside cells that oxidises glucose to release energy: C_6H_12O_6 + 6O_2 → 6CO_2 + 6H_2O + energy.
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Important questions with answers
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Multiple-choice questions (1 mark)
The tiny air sacs in the lungs where gaseous exchange occurs are called:
- (a)
Bronchi
- (b)
Bronchioles
- (c)
Alveoli
- (d)
Nephrons
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Answer: (c) Alveoli.
Alveoli are the microscopic, thin-walled air sacs of the lungs; their large surface area and rich blood supply allow rapid exchange of oxygen and carbon dioxide.
The muscular sheet that separates the chest cavity from the abdominal cavity and helps in breathing is the:
- (a)
Pleura
- (b)
Diaphragm
- (c)
Larynx
- (d)
Epiglottis
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Answer: (b) Diaphragm.
The diaphragm is a dome-shaped muscle below the lungs; its contraction and relaxation change the volume of the chest cavity during breathing.
During inspiration, the diaphragm:
- (a)
Relaxes and becomes dome-shaped
- (b)
Contracts and becomes flattened
- (c)
Moves upward
- (d)
Plays no role
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Answer: (b) Contracts and becomes flattened.
When the diaphragm contracts it flattens and moves down, increasing the volume of the chest cavity so that air is drawn into the lungs.
The percentage of oxygen in exhaled air compared with inhaled air is:
- (a)
Higher in exhaled air
- (b)
The same in both
- (c)
Lower in exhaled air
- (d)
Zero in exhaled air
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Answer: (c) Lower in exhaled air.
Some oxygen from inhaled air (21\%) is absorbed by the blood, so exhaled air has less oxygen (16\%) and much more carbon dioxide.
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Practise free with the AI tutor →Assertion–Reason questions (1 mark)
Assertion (A): The walls of the alveoli are extremely thin and richly supplied with blood capillaries.
Reason (R): This structure allows rapid diffusion of gases between the alveolar air and the blood.
- (a)
Both A and R are true and R is the correct explanation of A
- (b)
Both A and R are true but R is not the correct explanation of A
- (c)
A is true but R is false
- (d)
A is false but R is true
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Answer: (a) The thin, moist alveolar wall and its close contact with capillaries provide a short diffusion path and a steep gradient, which is exactly why gases diffuse rapidly, so R correctly explains A.
Very short answer questions (2 marks)
Differentiate between breathing and respiration.
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Breathing is a physical (mechanical) process in which air is taken into the lungs (inhalation) and given out (exhalation); no energy is released.
Respiration is a chemical process taking place inside the cells in which glucose is oxidised using oxygen to release energy, with carbon dioxide and water as by-products. Breathing supplies the oxygen needed for respiration.
State the function of (i) the epiglottis and (ii) the rings of cartilage in the trachea.
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(i) Epiglottis: a flap of cartilage that covers the opening of the larynx (glottis) during swallowing, preventing food from entering the windpipe (trachea).
(ii) Rings of cartilage: the C-shaped cartilage rings keep the trachea permanently open (prevent it from collapsing) so that air can pass freely at all times.
Short answer questions (3 marks)
Draw a well-labelled diagram of the human respiratory system showing the trachea, bronchi, lungs and diaphragm.
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Air passes through the trachea, which divides into two bronchi entering the two lungs; the lungs sit above the dome-shaped diaphragm.
Explain the process of inspiration (breathing in) in terms of the diaphragm, ribs and air pressure.
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During inspiration (inhalation):
(i) The diaphragm contracts, flattens and moves downward.
(ii) The external intercostal muscles contract, pulling the ribs upward and outward.
(iii) These movements increase the volume of the chest (thoracic) cavity.
(iv) As volume increases, the air pressure inside the lungs falls below the atmospheric pressure.
(v) Therefore air from outside rushes in through the nose and trachea into the lungs, filling the expanded alveoli.
Compare the composition of inhaled air and exhaled air for oxygen, carbon dioxide and water vapour, and explain the differences.
Show model answer
| Gas | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen | about 21\% | about 16\% |
| Carbon dioxide | about 0.04\% | about 4\% |
| Water vapour | Variable / less | Saturated (more) |
Explanation: In the alveoli, oxygen diffuses from the air into the blood, so exhaled air has less oxygen. Carbon dioxide produced by respiration diffuses from the blood into the alveoli, so exhaled air is richer in carbon dioxide. The moist alveolar surfaces add water vapour, so exhaled air is warm and saturated with moisture.
Long answer questions (5 marks)
Describe the complete mechanism of breathing, explaining both inspiration and expiration with reference to the diaphragm, intercostal muscles, chest volume and air pressure.
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Breathing is brought about by the diaphragm and the intercostal muscles, which change the volume of the chest cavity.
Inspiration (breathing in):
(i) The diaphragm contracts, flattens and moves downward.
(ii) The external intercostal muscles contract, raising the ribs upward and outward.
(iii) The volume of the chest cavity increases.
(iv) The air pressure inside the lungs decreases below atmospheric pressure.
(v) Air therefore rushes into the lungs from outside, inflating the alveoli.
Expiration (breathing out):
(i) The diaphragm relaxes and returns to its dome shape, moving upward.
(ii) The intercostal muscles relax, so the ribs move downward and inward.
(iii) The volume of the chest cavity decreases.
(iv) The air pressure inside the lungs rises above atmospheric pressure.
(v) Air is therefore pushed out of the lungs to the exterior.
Thus the rhythmic contraction and relaxation of these muscles alternately increase and decrease chest volume, drawing air in and forcing it out.
Explain how gaseous exchange takes place (i) between the alveoli and the blood in the lungs and (ii) between the blood and the body tissues, stating the direction of movement of each gas and the reason.
Show model answer
Gaseous exchange happens by diffusion - each gas moves from a region of its higher concentration to a region of its lower concentration.
(i) At the alveoli (lungs):
The alveolar air is rich in oxygen and low in carbon dioxide; the blood arriving from the body (in the pulmonary capillaries) is low in oxygen and high in carbon dioxide.
- Oxygen diffuses from the alveolar air into the blood (high to low concentration) and combines with haemoglobin to form oxyhaemoglobin.
- Carbon dioxide diffuses from the blood into the alveolar air and is breathed out.
The thin, moist alveolar walls and rich capillary network make this diffusion rapid.
(ii) At the body tissues:
The tissue cells are constantly using oxygen and producing carbon dioxide, so they are low in oxygen and high in carbon dioxide; the blood arriving from the lungs is the opposite.
- Oxygen diffuses from the blood into the tissue cells (for respiration).
- Carbon dioxide diffuses from the tissue cells into the blood, to be carried back to the lungs.
In both places the exchange occurs simply because of the concentration (diffusion) gradients of the two gases, and the blood acts as the transport medium between the lungs and the tissues.
Case-based questions (4 marks)
The apparatus known as the bell-jar model represents breathing. A glass jar has a Y-shaped tube with two balloons (lungs) inside, and a rubber sheet (diaphragm) tied across the open bottom.
| Part of model | Represents |
|---|---|
| Glass jar | Chest cavity |
| Balloons | Lungs |
| Rubber sheet | Diaphragm |
| Y-tube | Trachea and bronchi |
(i) What happens to the balloons when the rubber sheet is pulled down? Which real process does this represent?
(ii) Explain why the balloons inflate when the sheet is pulled down.
(iii) What happens when the rubber sheet is pushed up?
(iv) State one way in which this model does NOT correctly represent real breathing.
Show model answer
(i) When the rubber sheet is pulled down the balloons inflate (expand). This represents inspiration (breathing in).
(ii) Pulling the sheet down increases the volume of the jar (chest cavity), so the air pressure inside falls below atmospheric pressure. Air therefore rushes in through the Y-tube and inflates the balloons - just as lowering the diaphragm draws air into the lungs.
(iii) When the sheet is pushed up the volume of the jar decreases, the pressure inside rises, and air is forced out, so the balloons deflate (collapse) - representing expiration (breathing out).
(iv) In the model only the diaphragm (rubber sheet) moves; in real breathing the ribs and intercostal muscles also move the chest wall. (Also, real lungs are not free balloons but are attached and surrounded by pleura.)
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Frequently asked questions
Do these The Respiratory System questions follow the latest ICSE syllabus?
Yes — they are aligned to the CISCE 2026–27 syllabus for ICSE Class 9 Biology, so nothing here is outside the current course.
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