NCERT Solutions for Class 11 Biology Chapter 14: Breathing and Exchange of Gases (NCERT 2026–27)
These Class 11 Biology Chapter 14 solutions cover Breathing and Exchange of Gases from Unit 5, Human Physiology. Every numbered NCERT exercise question is reproduced verbatim and answered in clear, exam-ready prose, along with key concepts, a partial-pressure data table, extra practice questions, MCQs and Assertion–Reason items — everything you need to master respiration for the 2026–27 session.
Class: 11Subject: BiologyUnit: 5 — Human PhysiologyChapter: 14Topic: Breathing and Exchange of GasesSession: 2026–27
Chapter 14 explains how the human body takes in oxygen (O2) and removes carbon dioxide (CO2), a process called breathing or respiration. It begins with the respiratory organs across animal groups and the detailed structure of the human respiratory system (nostrils → nasal chamber → pharynx → larynx → trachea → bronchi → bronchioles → alveoli). It then describes the mechanism of breathing — inspiration and expiration driven by pressure gradients created by the diaphragm and intercostal muscles — and the respiratory volumes and capacities measured by a spirometer. The chapter covers exchange of gases by diffusion along partial-pressure gradients at the alveoli and tissues, the transport of O2 (as oxyhaemoglobin, with the sigmoid oxygen dissociation curve) and CO2 (mainly as bicarbonate), the neural regulation of respiration by centres in the medulla and pons, and common respiratory disorders like asthma and emphysema.
Key Concepts & Definitions
Breathing (pulmonary ventilation): the exchange of O2 from the atmosphere with CO2 produced by cells; involves inspiration (drawing air in) and expiration (releasing alveolar air out).
Conducting part vs respiratory part: the conducting part (nostrils to terminal bronchioles) transports, filters, humidifies and warms air; the respiratory/exchange part (alveoli and their ducts) is the actual site of O2–CO2 diffusion.
Tidal volume (TV): volume of air inspired or expired during normal respiration, approx. 500 mL.
Vital capacity (VC): the maximum volume of air a person can breathe in after a forced expiration (or breathe out after a forced inspiration) = ERV + TV + IRV.
Partial pressure (pO2, pCO2): the pressure contributed by an individual gas in a mixture; diffusion of gases follows these gradients.
Oxygen dissociation curve: the sigmoid curve obtained when percentage saturation of haemoglobin with O2 is plotted against pO2.
Carbonic anhydrase: the RBC enzyme that catalyses CO2 + H2O ↔ H2CO3 ↔ HCO3− + H+ in both directions, enabling bicarbonate transport of CO2.
Partial Pressures (in mm Hg) at Different Parts Involved in Diffusion (Table 14.1)
Respiratory Gas
Atmospheric Air
Alveoli
Blood (Deoxygenated)
Blood (Oxygenated)
Tissues
O2
159
104
40
95
40
CO2
0.3
40
45
40
45
NCERT Exercises — Full Solutions
1. Define vital capacity. What is its significance?
ANSWERVital capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration, or the maximum volume of air a person can breathe out after a forced inspiration. It is the sum of the Expiratory Reserve Volume, Tidal Volume and Inspiratory Reserve Volume (VC = ERV + TV + IRV).Significance: A higher vital capacity means a person can take in and give out a greater quantity of air in a single breath, which is useful for vigorous physical activity. Clinically, vital capacity is an important index of pulmonary (lung) function; athletes and people living at high altitudes tend to have a higher VC, while it is reduced in lung disorders.
2. State the volume of air remaining in the lungs after a normal breathing.
ANSWERThe volume of air remaining in the lungs after a normal (quiet) expiration is the Functional Residual Capacity (FRC). It is the sum of the Expiratory Reserve Volume and the Residual Volume (FRC = ERV + RV).Taking ERV ≈ 1000–1100 mL and RV ≈ 1100–1200 mL, the FRC is approximately 2100–2300 mL of air.
3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
ANSWERThe alveoli are specially adapted for gaseous exchange, whereas the rest of the respiratory tract only conducts air. The alveolar wall is extremely thin and the diffusion membrane consists of just three minute layers — the squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the thin basement substance between them — with a total thickness much less than a millimetre.Alveoli are richly supplied with a network of blood capillaries and provide a very large surface area in close contact with blood. The other parts (nostrils, pharynx, larynx, trachea, bronchi and bronchioles) form the conducting part; their walls are relatively thick and not vascularised for exchange, so diffusion of O2 and CO2 takes place efficiently only in the alveolar region.
4. What are the major transport mechanisms for CO2? Explain.
ANSWERCO2 is transported by the blood in three ways:(i) As bicarbonate (about 70%): The main mechanism. At the tissues, CO2 diffuses into the RBCs and plasma where the enzyme carbonic anhydrase rapidly converts it: CO2 + H2O ↔ H2CO3 ↔ HCO3− + H+. The bicarbonate is carried to the alveoli, where the reaction reverses and CO2 is released out.(ii) As carbamino-haemoglobin (about 20–25%): CO2 binds with haemoglobin. This binding depends on partial pressures — when pCO2 is high and pO2 is low (as in tissues) more CO2 binds; when pCO2 is low and pO2 is high (as in alveoli) it dissociates.(iii) In dissolved state (about 7%): A small amount of CO2 is carried dissolved directly in the plasma. Every 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli.
5. What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air?
ANSWERThe correct option is (ii) pO2 higher, pCO2 lesser.In atmospheric air the pO2 is about 159 mm Hg, which is higher than the alveolar pO2 of about 104 mm Hg. The pCO2 of atmospheric air is only about 0.3 mm Hg, which is much lesser than the alveolar pCO2 of about 40 mm Hg. Hence, compared with alveolar air, atmospheric air has higher pO2 and lesser pCO2.
6. Explain the process of inspiration under normal conditions.
ANSWERInspiration is the process of drawing atmospheric air into the lungs. It occurs when the pressure within the lungs (intra-pulmonary pressure) becomes less than the atmospheric pressure.It is initiated by the contraction of the diaphragm, which flattens and increases the volume of the thoracic chamber along the antero-posterior axis. At the same time, the external intercostal muscles contract, lifting up the ribs and the sternum and increasing the thoracic volume in the dorso-ventral axis.This overall increase in thoracic volume causes a corresponding increase in pulmonary (lung) volume. The increased pulmonary volume lowers the intra-pulmonary pressure to below the atmospheric pressure, creating a negative pressure gradient. As a result, air from outside rushes into the lungs — this is inspiration.
7. How is respiration regulated?
ANSWERRespiration is regulated by the neural system so that the respiratory rhythm can be moderated to suit the needs of the body tissues.Respiratory rhythm centre: located in the medulla region of the brain, it is primarily responsible for maintaining the respiratory rhythm.Pneumotaxic centre: present in the pons region, it can moderate the functions of the rhythm centre; its signals can reduce the duration of inspiration and so alter the respiratory rate.Chemosensitive area: situated adjacent to the rhythm centre, it is highly sensitive to CO2 and hydrogen ions. A rise in these activates it, and it signals the rhythm centre to make adjustments to eliminate the excess. Receptors associated with the aortic arch and carotid artery also detect changes in CO2 and H+ and send signals to the rhythm centre. The role of oxygen in this regulation is quite insignificant.
8. What is the effect of pCO2 on oxygen transport?
ANSWERThe partial pressure of carbon dioxide (pCO2) is one of the factors that interferes with the binding of O2 to haemoglobin.In the alveoli, where pCO2 is low (along with high pO2, low H+ concentration and lower temperature), conditions favour the formation of oxyhaemoglobin, so O2 binds readily to haemoglobin.In the tissues, where pCO2 is high (along with low pO2, high H+ concentration and higher temperature), conditions favour the dissociation of O2 from oxyhaemoglobin, so oxygen is readily released to the tissues. Thus a high pCO2 promotes the unloading of oxygen where it is needed.
9. What happens to the respiratory process in a man going up a hill?
ANSWERAs a person climbs a hill (high altitude), the atmospheric pressure — and therefore the partial pressure of oxygen (pO2) — in the air decreases. The reduced pO2 means less oxygen is available for the formation of oxyhaemoglobin, leading to a tendency towards oxygen shortage (hypoxia).To compensate, the body increases the rate of breathing (and the heart rate) so that more air is taken in and more oxygen reaches the blood. With time the body also produces more RBCs to capture more oxygen. Hence the respiratory process becomes faster and deeper as a man goes up a hill.
10. What is the site of gaseous exchange in an insect?
ANSWERIn insects, gaseous exchange takes place through a network of air tubes called tracheal tubes (the tracheal system).Atmospheric air enters through openings called spiracles and is carried directly to the tissues by the branching tracheal tubes, so O2 and CO2 are exchanged directly between the air in the finest tracheoles and the body cells, without involving blood for gas transport.
11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
ANSWEROxygen dissociation curve: the curve obtained when the percentage saturation of haemoglobin with O2 is plotted against the partial pressure of oxygen (pO2). It is highly useful in studying the effect of factors such as pCO2 and H+ concentration on the binding of O2 with haemoglobin.Reason for the sigmoid (S-shaped) pattern: Each haemoglobin molecule can carry a maximum of four molecules of O2. The binding of the first O2 molecule changes the shape of haemoglobin in a way that makes it easier for the next O2 molecules to bind (cooperative binding). Therefore at low pO2 binding is slow, at intermediate pO2 it rises steeply, and at high pO2 it levels off as haemoglobin nears full saturation — giving the characteristic S-shaped, or sigmoid, curve.
12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.
ANSWERHypoxia is a condition in which the body tissues do not receive an adequate supply of oxygen, or are unable to use it properly. It results in symptoms such as breathlessness, headache, dizziness, bluish skin (cyanosis), confusion and fatigue.It can be caused by reduced oxygen at high altitudes (high-altitude hypoxia), respiratory disorders that reduce gas exchange (such as emphysema or asthma), anaemia (too few RBCs to carry oxygen), or blockage of blood flow. Discussing such real-life examples with friends helps connect the chapter to everyday health and adventure activities like mountaineering. (A discussion-based question; the above is model information.)
13. Distinguish between
(a) IRV and ERV (b) Inspiratory capacity and Expiratory capacity (c) Vital capacity and Total lung capacity.
ANSWER(a) IRV vs ERV:
Inspiratory Reserve Volume (IRV)
Expiratory Reserve Volume (ERV)
Additional volume of air a person can inspire by a forcible inspiration over the tidal volume.
Additional volume of air a person can expire by a forcible expiration beyond a normal expiration.
Averages 2500–3000 mL.
Averages 1000–1100 mL.
(b) Inspiratory Capacity vs Expiratory Capacity:
Inspiratory Capacity (IC)
Expiratory Capacity (EC)
Total volume of air a person can inspire after a normal expiration.
Total volume of air a person can expire after a normal inspiration.
IC = TV + IRV.
EC = TV + ERV.
(c) Vital Capacity vs Total Lung Capacity:
Vital Capacity (VC)
Total Lung Capacity (TLC)
Maximum volume of air a person can breathe in after a forced expiration (or breathe out after a forced inspiration).
Total volume of air accommodated in the lungs at the end of a forced inspiration.
VC = ERV + TV + IRV. It does not include the residual volume.
TLC = RV + ERV + TV + IRV, i.e. Vital Capacity + Residual Volume.
14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
ANSWERTidal volume (TV): the volume of air inspired or expired during a single normal respiration. Its approximate value is 500 mL.Tidal volume in an hour: A healthy human breathes about 12–16 times per minute. Taking an average of about 16 breaths per minute:Air per minute = 500 mL × 16 = 8000 mL.Air in one hour (60 minutes) = 8000 mL × 60 = 4,80,000 mL = 480 litres (approximately). (Using a lower rate of about 12–13 breaths/minute gives roughly 3,60,000–4,00,000 mL, i.e. about 360–400 litres per hour.)
Extra Practice Questions
Short Answer Type Questions
Q1. Name the two centres of the brain that regulate respiration and state where each is located.
ANSWERThe respiratory rhythm centre is located in the medulla, and the pneumotaxic centre is located in the pons region of the brain.
Q2. Why is the solubility of CO2 important for its exchange across the diffusion membrane?
ANSWERThe solubility of CO2 is about 20–25 times higher than that of O2. Therefore, for the same difference in partial pressure, much more CO2 can diffuse through the diffusion membrane than O2, making CO2 removal efficient.
Q3. What is the function of the epiglottis?
ANSWERThe epiglottis is a thin, elastic cartilaginous flap that covers the glottis during swallowing, preventing the entry of food into the larynx and trachea.
Q4. List the three layers of the diffusion membrane in the alveoli.
ANSWER(i) The thin squamous epithelium of the alveoli, (ii) the endothelium of the alveolar capillaries, and (iii) the basement substance lying in between them.
Q5. State the percentages in which oxygen is transported by the blood.
ANSWERAbout 97% of O2 is transported by RBCs as oxyhaemoglobin, while the remaining about 3% is carried dissolved in the plasma.
Long Answer Type Questions
Q1. Describe the mechanism of breathing, explaining both inspiration and expiration.
ANSWERBreathing involves two stages driven by pressure gradients between the lungs and the atmosphere. During inspiration, the diaphragm contracts and flattens, increasing thoracic volume along the antero-posterior axis, while the external intercostal muscles contract to lift the ribs and sternum, increasing it in the dorso-ventral axis. The rise in thoracic volume increases pulmonary volume and lowers the intra-pulmonary pressure below atmospheric pressure, so outside air flows in. During expiration, the diaphragm and intercostal muscles relax, returning the diaphragm and sternum to their normal positions; this reduces thoracic and pulmonary volume, raising intra-pulmonary pressure slightly above atmospheric pressure, so air is expelled. Normally a healthy human breathes 12–16 times per minute, and additional abdominal muscles can strengthen forced breathing. The air volumes can be measured with a spirometer.
Q2. Explain how O2 and CO2 are exchanged at the alveoli and at the tissues.
ANSWERExchange of gases occurs by simple diffusion along partial-pressure gradients. At the alveoli, the pO2 (104 mm Hg) is higher than that of deoxygenated blood (40 mm Hg), so O2 diffuses from alveoli into the blood; the pCO2 of deoxygenated blood (45 mm Hg) is higher than alveolar pCO2 (40 mm Hg), so CO2 diffuses from blood into the alveoli to be exhaled. At the tissues, oxygenated blood has a higher pO2 (95 mm Hg) than the tissues (40 mm Hg), so O2 diffuses into the tissues; tissue pCO2 (45 mm Hg) is higher than that of oxygenated blood (40 mm Hg), so CO2 diffuses from tissues into the blood. The thin diffusion membrane, large alveolar surface area and the high solubility of CO2 all make this exchange efficient.
Q3. Describe the transport of oxygen by blood, including the oxygen dissociation curve.
ANSWERAbout 97% of oxygen is transported as oxyhaemoglobin, formed when O2 binds reversibly with the iron-containing pigment haemoglobin in RBCs; each haemoglobin molecule carries up to four O2 molecules. Binding depends mainly on pO2, and also on pCO2, H+ concentration and temperature. A graph of percentage saturation of haemoglobin against pO2 gives a sigmoid oxygen dissociation curve. In the alveoli (high pO2, low pCO2, low H+, lower temperature) oxyhaemoglobin forms readily; in the tissues (low pO2, high pCO2, high H+, higher temperature) oxygen dissociates and is released. Every 100 mL of oxygenated blood delivers about 5 mL of O2 to the tissues under normal conditions.
MCQs
1. The exchange of gases (O2 and CO2) in the lungs occurs at the:
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Diffusion of gases occurs mainly in the alveolar region of the lungs.
Reason: The alveolar diffusion membrane is very thin and richly supplied with capillaries.
A-R 2. Assertion: During inspiration, air moves into the lungs.
Reason: Contraction of the diaphragm and external intercostal muscles increases thoracic volume and lowers intra-pulmonary pressure.
A-R 3. Assertion: Most of the carbon dioxide is transported in the blood as bicarbonate.
Reason: Carbon dioxide is completely insoluble in blood plasma.
A-R 4. Assertion: A man climbing a hill breathes faster.
Reason: The partial pressure of oxygen decreases at high altitude.
A-R 5. Assertion: Oxygen readily dissociates from oxyhaemoglobin at the tissues.
Reason: The tissues have low pO2, high pCO2, high H+ concentration and higher temperature.
Answer key: 1-(A), 2-(A), 3-(C), 4-(A), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing respiratory volumes (TV, IRV, ERV, RV) with capacities (IC, EC, FRC, VC, TLC) — capacities are sums of two or more volumes.
Writing that vital capacity includes the residual volume — it does not; only TLC includes RV.
Stating that oxygen, not CO2, mainly regulates respiration — the role of oxygen in regulating respiratory rhythm is insignificant; CO2 and H+ are the key stimuli.
Mixing up the locations of the respiratory rhythm centre (medulla) and the pneumotaxic centre (pons).
Forgetting the three CO2 transport percentages: about 70% as bicarbonate, 20–25% as carbamino-haemoglobin, about 7% dissolved.
Reversing the partial-pressure gradients — O2 moves alveoli → blood → tissues, while CO2 moves tissues → blood → alveoli.
Exam tips for this chapter
Memorise Table 14.1 (partial pressures) — it is the basis for almost every gas-exchange question. Learn the volume/capacity definitions and their formulas (VC = ERV + TV + IRV; TLC = VC + RV; IC = TV + IRV; FRC = ERV + RV). For the oxygen dissociation curve, always mention the four factors (pO2, pCO2, H+, temperature) and the cooperative binding that explains the sigmoid shape. Use the carbonic anhydrase equation (CO2 + H2O ↔ H2CO3 ↔ HCO3− + H+) when answering CO2-transport questions to score full marks.
Frequently Asked Questions
What is Class 11 Biology Chapter 14 about?
Chapter 14, Breathing and Exchange of Gases, explains the human respiratory system, the mechanism of breathing (inspiration and expiration), respiratory volumes and capacities, the exchange of O2 and CO2 by diffusion, the transport of these gases in blood, the neural regulation of respiration, and common respiratory disorders.
How is most carbon dioxide transported in the blood?
About 70% of CO2 is carried as bicarbonate (HCO3−) formed with the help of carbonic anhydrase, about 20–25% as carbamino-haemoglobin bound to haemoglobin, and about 7% dissolved directly in the plasma.
What is vital capacity and why is it important?
Vital capacity is the maximum volume of air a person can breathe in after a forced expiration (or breathe out after a forced inspiration), equal to ERV + TV + IRV. It is an important clinical index of lung function and is higher in athletes and people at high altitudes.
Are these Class 11 Biology Chapter 14 solutions free?
Yes. All solutions are free and follow the official NCERT Biology textbook for the 2026–27 session.
