This MCQ module is based on: Respiratory Organs
TOPIC 61 OF 90
Respiratory Organs
🎓 Class 11
Biology
CBSE
Theory
Ch 14 – Breathing and Exchange of Gases
⏱ ~14 min
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Respiratory Organs and the Human Respiratory System
Chapter 12 showed what a cell does with oxygen once it has it. This chapter answers the earlier question: how does the oxygen get there at all, and how is the carbon dioxide removed? Oxygen is utilised by organisms to indirectly break down simple molecules like glucose, amino acids and fatty acids to derive energy, and carbon dioxide, which is harmful, is also released during these catabolic reactions. So O₂ has to be continuously provided to the cells and CO₂ produced by the cells has to be released out.
Breathing. This process of exchange of O₂ from the atmosphere with CO₂ produced by the cells is called breathing, commonly known as respiration. Place your hands on your chest and you can feel it moving up and down.
14.1 Respiratory Organs
Mechanisms of breathing vary among different groups of animals, depending mainly on their habitats and levels of organisation. Read the survey below as a gradient: from no organ at all, through simple moist surfaces, to dedicated vascularised structures.
| Animal group | Structure used | Name of the mode |
|---|---|---|
| Lower invertebrates — sponges, coelenterates, flatworms | Simple diffusion over their entire body surface | — |
| Earthworms | Their moist cuticle | Cutaneous |
| Insects | A network of tubes (tracheal tubes) to transport atmospheric air within the body | Tracheal |
| Most aquatic arthropods and molluscs | Special vascularised structures called gills (branchial respiration) | Branchial |
| Terrestrial forms | Vascularised bags called lungs (pulmonary respiration) | Pulmonary |
| Fishes (vertebrates) | Gills | Branchial |
| Amphibians, reptiles, birds and mammals | Lungs | Pulmonary |
| Amphibians like frogs — in addition | Their moist skin (cutaneous respiration) | Cutaneous |
The thread running through the table. Every one of these surfaces is thin and moist, and all but the smallest animals add vascularisation — a blood supply pressed right up against the surface. A sponge needs neither, because no cell of its body is far from water. As body size and activity rise, diffusion over the general surface becomes insufficient, and a dedicated organ with a large area and a close blood supply becomes unavoidable. The insect solution is the odd one out: its tracheal tubes carry air itself to the tissues, so its blood plays almost no part in gas transport.
14.1.1 Human Respiratory System
The passage, step by step
- A pair of external nostrils opening out above the upper lips.
- These lead to a nasal chamber through the nasal passage.
- The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air.
- The pharynx opens through the larynx region into the trachea.
- Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchi.
- Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles.
- Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
Larynx is a cartilaginous box which helps in sound production and hence is called the sound box. During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called the epiglottis, to prevent the entry of food into the larynx.
Which parts are held open by cartilage. The trachea, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. Note that the rings are incomplete — open at the back, where the trachea lies against the oesophagus, so that a swallowed bolus of food has room to pass. And note where the support stops: the terminal bronchioles and alveoli have none, which is why their walls can be thin enough for gas to cross.
The lungs and their coverings
The branching network of bronchi, bronchioles and alveoli comprise the lungs. We have two lungs, which are covered by a double layered pleura, with pleural fluid between them. It reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining, whereas the inner pleural membrane is in contact with the lung surface.
Conducting part and exchange part — a distinction worth memorising
The part starting with the external nostrils up to the terminal bronchioles constitutes the conducting part, whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.
| Conducting part | Exchange (respiratory) part | |
|---|---|---|
| Extent | External nostrils → terminal bronchioles | Alveoli and their ducts |
| Function | Transports atmospheric air to the alveoli; clears it of foreign particles; humidifies it; brings the air to body temperature | Site of actual diffusion of O₂ and CO₂ between blood and atmospheric air |
| Gas exchange | None | Yes |
| Cartilage support | Present in trachea, bronchi and initial bronchioles | Absent — walls are very thin |
The conducting part is therefore not a passive pipe. It performs three services on every breath: it filters, it humidifies, and it warms. Air arriving at the alveoli is clean, saturated with water vapour and at body temperature, whatever the weather outside.
The thoracic chamber
The lungs are situated in the thoracic chamber, which is anatomically an air-tight chamber. It is formed:
- dorsally by the vertebral column,
- ventrally by the sternum,
- laterally by the ribs,
- on the lower side by the dome-shaped diaphragm.
Why the anatomy matters. The anatomical setup of lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume. There is no muscle in the lung itself. The lung is inflated and deflated entirely by moving the walls of the box it sits in — which is what Part 2 takes up.
The five steps of respiration
Respiration involves the following steps:
(i) Breathing or pulmonary ventilation, by which atmospheric air is drawn in and CO₂-rich alveolar air is released out.
(ii) Diffusion of gases (O₂ and CO₂) across the alveolar membrane.
(iii) Transport of gases by the blood.
(iv) Diffusion of O₂ and CO₂ between blood and tissues.
(v) Utilisation of O₂ by the cells for catabolic reactions and resultant release of CO₂ — cellular respiration, as dealt with in Chapter 12.
(i) Breathing or pulmonary ventilation, by which atmospheric air is drawn in and CO₂-rich alveolar air is released out.
(ii) Diffusion of gases (O₂ and CO₂) across the alveolar membrane.
(iii) Transport of gases by the blood.
(iv) Diffusion of O₂ and CO₂ between blood and tissues.
(v) Utilisation of O₂ by the cells for catabolic reactions and resultant release of CO₂ — cellular respiration, as dealt with in Chapter 12.
Notice how the chapter is organised around this list. Step (i) is Section 14.2, steps (ii) and (iv) are Section 14.3, step (iii) is Section 14.4, and step (v) you have already studied. “Breathing” is only the first of five — which is exactly why the chapter is titled Breathing and Exchange of Gases rather than simply Respiration.
📐 Activity 14.1 — Trace a breath and test the epiglottis
Part A. On a blank outline of the human trunk, label in order: external nostrils, nasal chamber, pharynx, larynx, trachea, primary bronchus, secondary and tertiary bronchi, bronchiole, terminal bronchiole, alveolus. Then draw a line across your diagram separating the conducting part from the exchange part, and mark which structures carry incomplete cartilaginous rings.
Part B. Place two fingers gently on the front of your throat and locate the hard cartilaginous box — the larynx. Now swallow, and feel it rise and fall. Then hum, and feel it vibrate.
Predict: why must the larynx move upward at the moment of swallowing? And why are the cartilage rings of the trachea incomplete rather than full circles?
Part A answers. The dividing line falls after the terminal bronchioles: everything from the external nostrils up to them is the conducting part, while the alveoli and their ducts form the exchange part. Cartilage is present in the trachea, primary, secondary and tertiary bronchi and the initial bronchioles — and absent beyond.
Part B — why the larynx rises. Because the pharynx is a common passage for food and air, and at the instant of swallowing the two must be separated. As the larynx rises, the epiglottis — a thin elastic cartilaginous flap — covers the glottis to prevent the entry of food into the larynx. If this fails, food enters the airway and the cough reflex is triggered violently. This is also why it is genuinely difficult to swallow and breathe at the same moment.
Why the rings are incomplete. The gap lies at the back of the trachea, against the oesophagus. Complete rings would hold the trachea rigidly open but would leave no room for a swallowed bolus to bulge forward as it passes down the food pipe. The incomplete ring is a compromise: enough cartilage at the front and sides to stop the airway collapsing when pressure inside falls during inspiration, and enough softness at the back to let food through.
One more thing to note from the humming. The vibration you feel confirms the larynx is the sound box — it helps in sound production, a function entirely separate from conducting air.
🎯 Interactive: Name the structure and its job
Belongs to: Conducting part
The external nostrils lead to the nasal chamber through the nasal passage. As part of the conducting part it helps clear the air of foreign particles, humidify it and bring it to body temperature before it reaches the alveoli.
🎯 Competency-Based Questions
Scenario: A patient is brought to a clinic after a chest injury has punctured the chest wall, letting air into the space between the two pleural membranes on one side. That lung collapses, although the lung tissue itself, the trachea and the bronchi are undamaged. A second patient has inhaled a peanut, which has lodged in the right primary bronchus.
Q1. Explain why the first patient's lung collapses even though the lung itself is undamaged. L4 Analyse
Because the lung has no muscle of its own and is inflated only indirectly. The thoracic chamber is anatomically an air-tight chamber, and the anatomical setup of the lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung cavity — the two are coupled through the double layered pleura with pleural fluid between them, the outer membrane in contact with the thoracic lining and the inner with the lung surface. Puncture the chest wall and air enters between the pleural layers, breaking that coupling. The chest can still move, but the movement is no longer transmitted to the lung, which recoils and collapses. As the chapter puts it, such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.
Q2. The second patient can still breathe, though with difficulty. Which part of the respiratory system is blocked, and what function is lost? L3 Apply
The right primary bronchus is part of the conducting part, which extends from the external nostrils up to the terminal bronchioles. Blocking it does not damage any exchange surface, but it cuts off transport of atmospheric air to the alveoli of that entire lung. Breathing continues through the left lung, so the patient survives, but roughly half the exchange surface is unavailable. Note the consequence of the anatomy: because the blockage is in a conducting structure rather than the exchange part, the alveoli beyond it are perfectly healthy and will work again the moment the obstruction is removed.
Q3. Fill in the blanks: The trachea divides at the level of the ______ thoracic vertebra into the right and left ______ bronchi. Each terminal bronchiole gives rise to ______. The trachea, bronchi and initial bronchioles are supported by ______ cartilaginous rings. L1 Remember
5th; primary; alveoli (very thin, irregular-walled, vascularised bag-like structures); incomplete.
Q4. An insect and a frog are both small animals, yet they use entirely different respiratory arrangements. Compare them and explain what each arrangement implies about the role of blood. L4 Analyse
Insects have a network of tubes, the tracheal tubes, to transport atmospheric air within the body. Air itself is delivered to the vicinity of the tissues, so the insect's blood plays almost no part in gas transport — the gas never has to dissolve and travel in a fluid. Frogs, being amphibians, respire through lungs and can also respire through their moist skin (cutaneous respiration). In both of those routes the gas must dissolve, cross a vascularised surface and be carried by blood to the tissues. The implication: the insect system is fast over short distances but sets a strict upper limit on body size, since a gas can only diffuse so far down a tube; the vertebrate system needs a heart and a carrier pigment but can serve a body of almost any size. The frog's dual arrangement also gives it a real advantage — it can continue gas exchange through the skin while submerged, when its lungs are useless.
Q5. “Breathing and respiration mean the same thing.” Evaluate this statement. L5 Evaluate
The statement is defensible in everyday speech and wrong in physiology, and the chapter deliberately sets up the distinction. In common usage the chapter itself concedes the point: the exchange of O₂ for CO₂ is called breathing, commonly known as respiration. But it then lists five steps, of which breathing is only the first: (i) breathing or pulmonary ventilation; (ii) diffusion of gases across the alveolar membrane; (iii) transport of gases by the blood; (iv) diffusion of O₂ and CO₂ between blood and tissues; (v) utilisation of O₂ by the cells for catabolic reactions and resultant release of CO₂. Only step (v) is cellular respiration, the energy-releasing process of Chapter 12 — and that is what a biochemist means by respiration. So the two words name processes at different levels: breathing is a bulk movement of air driven by pressure gradients; cellular respiration is a series of enzyme-catalysed oxidations in the cytoplasm and mitochondria. A plant makes the distinction obvious — it respires in every living cell but has no breathing apparatus at all, only stomata and lenticels. The precise formulation: breathing is one step that serves respiration; equating them collapses five processes into one.
🧠 Assertion–Reason Questions
For each pair choose: (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.
Assertion (A): The pleural fluid between the two pleural membranes is important for normal breathing.
Reason (R): It reduces friction on the lung surface, and it couples the lung to the thoracic wall so that changes in thoracic volume are transmitted to the lung.
Answer: A. Both are true and the reason explains the assertion. NCERT states the friction-reducing role directly, and the coupling follows from the fact that any change in thoracic volume is reflected in the pulmonary cavity.
Assertion (A): Gas exchange occurs throughout the respiratory tract from the nasal chamber onwards.
Reason (R): The part from the external nostrils up to the terminal bronchioles constitutes the conducting part of the respiratory system.
Answer: D. The assertion is false — only the alveoli and their ducts, the exchange part, are the site of actual diffusion. The reason is true and is precisely why the assertion fails: the conducting part only transports, cleans, humidifies and warms the air.
Assertion (A): The cartilaginous rings of the trachea are incomplete.
Reason (R): The larynx is a cartilaginous box that helps in sound production.
Answer: B. Both statements are true, but the reason has nothing to do with the assertion. The rings are incomplete at the back, where the trachea lies against the oesophagus, so that swallowed food can pass; the sound-producing role of the larynx is a separate fact.
Coming next. Part 2 takes up Section 14.2 — how inspiration and expiration are produced by pressure gradients, the roles of the diaphragm and the intercostal muscles, and the full set of respiratory volumes and capacities with their values.
Frequently Asked Questions - Respiratory Organs and the Human Respiratory System
What is breathing and how does it differ from respiration?
Breathing is the process of exchange of O2 from the atmosphere with CO2 produced by the cells. It is only the first of the five steps of respiration; the fifth step, utilisation of O2 by cells for catabolic reactions, is cellular respiration. In common usage breathing is called respiration, but physiologically they are different levels of the same overall process.
How do different animals breathe?
Sponges, coelenterates and flatworms exchange gases by simple diffusion over the entire body surface. Earthworms use a moist cuticle and insects a network of tracheal tubes. Most aquatic arthropods and molluscs use gills, and terrestrial forms use lungs. Among vertebrates fishes use gills while amphibians, reptiles, birds and mammals use lungs, and frogs can also respire through moist skin.
Trace the path of air in the human respiratory system.
External nostrils to the nasal chamber through the nasal passage, then the pharynx, then through the larynx into the trachea. The trachea divides at the 5th thoracic vertebra into right and left primary bronchi, which divide repeatedly into secondary and tertiary bronchi and bronchioles, ending in terminal bronchioles, each of which gives rise to alveoli.
What is the difference between the conducting part and the exchange part?
The conducting part runs from the external nostrils up to the terminal bronchioles; it transports air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature. The exchange or respiratory part is the alveoli and their ducts, the site of actual diffusion of O2 and CO2 between blood and atmospheric air.
Why is the larynx called the sound box?
Because it is a cartilaginous box, situated between the pharynx and the trachea, which helps in sound production. During swallowing the glottis can be covered by the epiglottis, a thin elastic cartilaginous flap, to prevent entry of food into the larynx.
What is the function of the pleura and pleural fluid?
The two lungs are covered by a double layered pleura with pleural fluid between the layers, which reduces friction on the lung surface. The outer membrane is in close contact with the thoracic lining and the inner with the lung surface, which couples the lung to the chest wall.
How is the thoracic chamber formed and why does its structure matter?
It is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm, and is anatomically air-tight. Because any change in thoracic volume is reflected in the pulmonary cavity, this arrangement is essential for breathing, as pulmonary volume cannot be altered directly.
What are the five steps of respiration?
Breathing or pulmonary ventilation; diffusion of O2 and CO2 across the alveolar membrane; transport of gases by the blood; diffusion of O2 and CO2 between blood and tissues; and utilisation of O2 by the cells for catabolic reactions with release of CO2, that is cellular respiration.
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