NCERT NCERT Solutions Class 11 Biology Chapter 14 Breathing and Exchange of Gases

The class 11 biology NCERT solutions chapter 14 Breathing and Exchange of Gases cover every back-exercise question, according to the latest 2026-27 CBSE syllabus, and help students prepare for the CBSE Boards, NEET and CUET. Each answer is worked step by step, from how breathing moves air using pressure gradients to how the respiratory volumes add up, how gases cross the diffusion membrane at the alveoli, and how oxygen and carbon dioxide are carried in the blood.

This chapter opens the Human Physiology unit, and the ideas of partial pressure and diffusion you meet here return in circulation, excretion and every organ-system chapter that follows.

• CBSE Weightage: about 3 to 4 marks, part of the Human Physiology unit that carries the largest share of the Class 11 Biology paper.
• Topics covered: respiratory organs, mechanism of breathing, respiratory volumes and capacities, exchange and transport of O₂ and CO₂, the oxygen dissociation curve, and the regulation of respiration.
• Exercise count: 14 back-exercise questions, a mix of definitions, numericals on lung volumes, and reasoning on diffusion and transport.

These class 11 biology NCERT solutions chapter 14 Breathing and Exchange of Gases are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board and NEET papers.

Why Breathing and Exchange of Gases Matters and What the Chapter Covers

Every cell in the body needs a steady supply of oxygen to release energy from food, and it must get rid of the carbon dioxide that the process produces. This chapter follows a single molecule of oxygen from the air outside to a working cell, and carbon dioxide on the return trip. Master the four base volumes and the partial-pressure table here, and most of the chapter becomes arithmetic and reasoning rather than memory.

• Everyday role: the same physiology explains why a swimmer trains lung volume, why climbers acclimatise, and why a doctor reads a spirometry report.
• Core skill: using a partial pressure gradient to predict which way a gas will diffuse across the alveolar membrane.
• Why it is central: the gradients and transport chemistry introduced here are used again in Body Fluids and Circulation, where blood carries these very gases.

Nearly every numerical in this chapter is built from just four measured volumes and one short table of partial pressures. Students who learn those two data sets first tend to answer the rest by substitution. The class 11 biology NCERT solutions chapter 14 Breathing and Exchange of Gases below follow the same order as the NCERT textbook so students can check their working line by line.

Respiratory Volumes and Capacities Explained

NCERT separates respiratory volumes, which are measured directly, from pulmonary capacities, which are found by adding two or more volumes. Four questions in the exercise, on vital capacity, on the air left after normal breathing, on tidal volume, and on distinguishing the capacities, all rest on the same four base volumes. Learn these four numbers and every capacity follows.

Vital capacity is the maximum air moved in one breath, while the air left after a normal breath is the functional residual capacity, ERV plus RV. A common slip is to answer "residual volume" for the air remaining after a normal breath, but a normal expiration is not forced, so the expiratory reserve volume is still inside the lungs and must be added. For tidal volume, a healthy human at 12 to 16 breaths per minute moves 500 × (12 to 16) = 6000 to 8000 mL per minute, which over an hour comes to 360000 to 480000 mL, that is about 360 to 480 litres. Refer to Exercise Q1, Q2, Q13 and Q14 for the full working.

The Mechanism of Breathing Under Normal Conditions

NCERT gives the rule of breathing plainly: air moves from a region of higher pressure to one of lower pressure, and inspiration occurs when the pressure inside the lungs (intra-pulmonary pressure) falls below atmospheric pressure. The whole process is one chain, muscles change volume, pressure follows volume, and air follows pressure. The one thing to keep straight is that air is never pushed into the lungs, it is drawn in once the lungs make room.

• Diaphragm contracts: the muscular floor of the thorax flattens and moves down, raising thoracic volume along the antero-posterior axis.
• External inter-costal muscles contract: the ribs and sternum are lifted, raising thoracic volume along the dorso-ventral axis.
• Lungs follow the chest wall: the pleural membranes couple the lung surface to the thoracic lining, so pulmonary volume rises with thoracic volume.

The larger space lowers the intra-pulmonary pressure below atmospheric, and about 500 mL of air flows in through the nostrils, nasal chamber, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli. Inspiration is active, requiring muscle contraction, whereas a normal expiration is passive, needing only the diaphragm and inter-costal muscles to relax. When they relax, thoracic and pulmonary volume shrink, intra-pulmonary pressure rises above atmospheric, and air leaves. Under normal conditions the diaphragm and external inter-costal muscles are enough, though additional abdominal muscles can strengthen a forceful breath.

Why Diffusion of Gases Occurs Only in the Alveolar Region

Diffusion across a biological surface needs four things to be present together: a thin membrane, a large surface area, a fluid on the far side to receive the gas, and a partial pressure gradient across it. NCERT states the first three explicitly. Only the alveoli meet all four, which is why the trachea, bronchi and bronchioles simply conduct air and cannot exchange gases.

• A thin membrane: the alveolar diffusion membrane has three layers, the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries and the basement substance between them, together less than a millimetre thick.
• A huge surface and a blood supply: millions of thin, vascularised alveoli give an enormous area, each wrapped in pulmonary capillaries so blood waits on the far side.
• A steep gradient: alveolar air at pO₂ 104 mm Hg faces deoxygenated blood at 40 mm Hg, a difference of 64 mm Hg driving oxygen inward.

The conducting tubes fail every test, they have thick cartilage-supported walls, no capillary bed against their lining, and no facing gradient. An insect shows the same principle from the other side: its site of gas exchange is the network of tracheal tubes, and the actual exchange happens at the fine, fluid-tipped tracheoles that end on the tissues, with air entering through spiracles. In an insect, blood plays no part in transporting the gases, since air is piped straight to the cells. The alveolus and the tracheole are two answers to one problem, chosen by body size, as worked in Exercise Q3 and Q10.

Partial Pressures and the Exchange of Gases

Partial pressure is the pressure contributed by an individual gas in a mixture, written pO₂ for oxygen and pCO₂ for carbon dioxide. A gas always diffuses down its own partial pressure gradient, so the direction of every exchange in the chapter can be read from NCERT Table 14.1. The exercise asks students to compare atmospheric air with alveolar air, which is settled by two subtractions.

Reading the table, atmospheric pO₂ (159 mm Hg) is higher than alveolar pO₂ (104 mm Hg) by 55 mm Hg, while atmospheric pCO₂ (0.3 mm Hg) is lesser than alveolar (40 mm Hg) by 39.7 mm Hg, so the correct option is pO₂ higher, pCO₂ lesser. Alveolar air is oxygen-depleted and carbon-dioxide-enriched because it mixes with the residual air already in the lungs and exchanges gases with the blood. A quick way to remember the whole table is two lines: O₂ runs 159 → 104 → 40 across atmosphere, alveoli and tissues, and CO₂ runs 0.3 → 40 → 45. Oxygen falls all the way in, carbon dioxide rises all the way in, and every diffusion question is a comparison of two values from these lines.

Transport of Oxygen and the Oxygen Dissociation Curve

About 97 per cent of oxygen is carried by RBCs as oxyhaemoglobin, and the remaining 3 per cent travels dissolved in plasma. Binding of oxygen with haemoglobin is primarily related to pO₂, while pCO₂, hydrogen ion concentration and temperature are the other factors that affect it. A graph of the percentage saturation of haemoglobin against pO₂ gives the S-shaped oxygen dissociation curve.

• Why it is sigmoid: each haemoglobin molecule binds a maximum of four O₂ molecules, and binding is co-operative, so the first O₂ attaches with difficulty (the slow foot), each bound O₂ makes the next easier (the steep middle), and near-full occupancy leaves little room (the plateau).
• Loading at the lungs: alveolar pO₂ of 104 mm Hg sits on the plateau, so blood leaves the lungs nearly fully saturated.
• Unloading at the tissues: tissue pO₂ of about 40 mm Hg sits on the steep middle, so a small fall releases a large amount of oxygen.

Carbon dioxide tunes this release. A high pCO₂ favours the dissociation of oxygen from oxyhaemoglobin and shifts the curve to the right, so oxygen comes off exactly at the tissues (pCO₂ 45 mm Hg) that are working hardest, while a low pCO₂ at the alveoli (40 mm Hg) favours the formation of oxyhaemoglobin. Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues under normal conditions. The mechanism runs through H⁺ produced by the carbonic anhydrase reaction, so carbon dioxide loosens haemoglobin's grip on oxygen and delivery is matched to demand automatically.

Transport of Carbon Dioxide by Blood

Blood is the medium of transport for both gases, and NCERT gives three routes for carbon dioxide with figures. The unifying idea is that every route is reversible and is switched by the local pCO₂, so carbon dioxide is loaded at the tissues and unloaded at the alveoli. Together these routes deliver about 4 mL of CO₂ per 100 mL of deoxygenated blood.

The bicarbonate route dominates because converting CO₂ into HCO₃^- clears it from the dissolved pool, keeping the gradient open so more CO₂ keeps diffusing in. The three shares add to roughly 100 per cent, as they must. The same conditions that load carbon dioxide onto haemoglobin at the tissues also unload oxygen from it, which is why the questions on CO₂ transport and on the effect of pCO₂ on oxygen transport so often appear together, as detailed in Exercise Q4 and Q8.

Regulation of Respiration and the Effect of Altitude

Human beings can moderate the respiratory rhythm to suit the demands of the tissues, and this is done by the neural system. Three neural structures and one chemical signal do the work, and the surprising point that NCERT stresses is that the role of oxygen in this regulation is quite insignificant. The system watches carbon dioxide, not oxygen, because CO₂ changes fast and in proportion to how hard the tissues are working.

• Respiratory rhythm centre: in the medulla, it sets the basic rhythm of breathing on its own.
• Pneumotaxic centre: in the pons, it moderates the rhythm centre and can reduce the duration of inspiration.
• Chemosensitive area and receptors: a chemosensitive area next to the rhythm centre, plus receptors on the aortic arch and carotid artery, detect a rise in CO₂ and H⁺ and signal the rhythm centre to increase breathing.

Altitude tests this system. As a person climbs, atmospheric pressure falls, so pO₂ falls even though air is still about 21 per cent oxygen. Alveolar pO₂ drops, the gradient driving oxygen into blood shrinks, less oxyhaemoglobin forms, and the tissues face hypoxia. The immediate response is faster, deeper breathing and a raised heart rate, felt as breathlessness and fatigue, while a longer stay triggers acclimatisation through increased RBC production. Hypoxia can arise anywhere in the oxygen supply chain, from low pO₂ in the air, to lung disease that thickens the diffusion membrane, to anaemia or carbon monoxide poisoning that cut the blood's carrying capacity, to poor circulation that fails to deliver loaded blood.

Breathing and Exchange of Gases Exercise-wise Breakdown

The NCERT back exercise has 14 questions, blending definitions, numericals on lung volumes and reasoning on diffusion and transport. The table below maps the question blocks to their topics so students can plan practice by theme.

Because the numericals reuse one small data set, the marks sit in stating the formula, substituting the standard values and tracking the units. Learn the four base volumes and the two partial-pressure lines, then derive the rest rather than recalling it. Every question in the class 11 biology NCERT solutions chapter 14 Breathing and Exchange of Gases PDF is solved with each step shown, so students can compare their working against the model answer.

Common Mistakes Students Make in Breathing and Exchange of Gases

Most marks in this chapter are lost on small slips of wording and on reading the adjective in a question. Each mistake below costs 1 to 2 marks, so watch for it at the exact step.

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