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

Concept. The volume of air moved in and out of the lungs is measured with a spirometer. Different volumes (tidal, inspiratory reserve, expiratory reserve, residual) can be added to obtain pulmonary capacities, which are used in clinical diagnosis. Vital capacity is the most important of these because it measures the “usable” part of the lung volume — the air that can actually be exchanged with the atmosphere.

Definition. Vital capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration, or equally the maximum volume of air a person can breathe out after a forced inspiration. It includes the expiratory reserve volume (ERV), tidal volume (TV) and inspiratory reserve volume (IRV):

VC = ERV + TV + IRV

Relationship with other capacities. Vital capacity does not include the residual volume (RV) of 1100–1200 mL that always stays in the lungs. When RV is added, the result is the total lung capacity (TLC = VC + RV).

Significance of vital capacity

1. Index of lung function. VC tells how much air a person can move in and out of the lungs in one maximal breath. A normal VC indicates healthy, elastic lungs and strong respiratory muscles.
2. Clinical diagnosis. NCERT states that pulmonary capacities are used in clinical diagnosis. A reduced VC points to conditions such as damage to alveolar walls (emphysema), fibrosis from dust exposure, obstruction of air passages or weakness of respiratory muscles.
3. Assessing the capacity for gas exchange. A larger VC means that more fresh air can be brought to the alveoli with each deep breath, which is useful for sustained physical activity.
4. Monitoring progress and fitness. VC can be measured repeatedly with a spirometer to follow the course of a disease or treatment, and it is generally higher in trained individuals such as athletes and lower in people with lung disease (general physiology knowledge).

Extra depth. Because the residual volume cannot be expelled, only the vital capacity is under voluntary control; this is why VC, and not TLC, is the measurement commonly recorded by spirometry. The total of VC and RV gives the total lung capacity of about 5100–5800 mL (derived), showing that nearly 20% of the lung volume is never exchanged.

Conclusion: Vital capacity (ERV + TV + IRV) is the maximum air that can be breathed in after a forced expiration or out after a forced inspiration; being a measure of the usable lung volume, it is clinically significant in assessing and diagnosing the state of pulmonary function.

Concept. Even in quiet breathing the lungs are never empty. After each normal expiration some air remains, and even after the most forceful expiration an irreducible amount stays behind. Two terms in NCERT describe these quantities: residual volume (air left after forcible expiration) and functional residual capacity (air left after a normal expiration).

Answer. The volume of air remaining in the lungs after a normal breathing (normal expiration) is the Functional Residual Capacity (FRC). It is the sum of the expiratory reserve volume and the residual volume:

FRC = ERV + RV

Why this air is useful. The FRC acts as a reservoir of air in the lungs. Because it is large compared with the tidal volume (500 mL), the composition of alveolar air changes only slightly with each breath, so the O₂ and CO₂ levels of blood stay steady. It also keeps the alveoli from collapsing completely and permits continuous gas exchange even between breaths.

Do not confuse. Residual volume (1100–1200 mL) is the air left after a forcible expiration, whereas FRC (about 2100–2300 mL) is the air left after a normal expiration. The question asks about normal breathing, so the answer is FRC.

Conclusion: After a normal expiration the lungs still hold the functional residual capacity, ERV + RV, which is about 2100–2300 mL; it serves as an air reservoir that keeps alveolar gas composition stable.

Concept. The respiratory system has two functional parts: the conducting part (external nostrils to terminal bronchioles) and the exchange part (alveoli and their ducts). The conducting part only transports atmospheric air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature. The alveoli are the primary sites of gas exchange. Gases diffuse across a membrane only if certain conditions are satisfied — these conditions are met in the alveoli but not elsewhere.

1. Very thin diffusion membrane. The alveolar diffusion membrane consists of just three layers — the thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary and the basement substance between them — and its total thickness is much less than a millimetre. The walls of the nasal passage, trachea and bronchi are thick and are supported by cartilage rings and muscle, so gases cannot cross them quickly.
2. Rich blood supply. Each alveolus is surrounded by a network of blood capillaries (Fig. 14.4) so blood and air are separated only by the thin membrane. The conducting passages have no such close association with capillaries for gas exchange.
3. Large surface area. The huge number of thin, bag-like alveoli provides an enormous surface for diffusion (general knowledge: tens of square metres), unlike the narrow tubes of the conducting part.
4. Maintained partial-pressure gradient. Diffusion depends on the difference in partial pressures. Alveolar air (pO₂ 104, pCO₂ 40 mm Hg) and deoxygenated blood (pO₂ 40, pCO₂ 45 mm Hg) have a steep gradient, maintained by constant ventilation and blood flow, so O₂ diffuses into blood and CO₂ into alveoli.
5. High solubility of CO₂. Since the solubility of CO₂ is 20–25 times that of O₂, CO₂ diffuses readily across the membrane even with a small pressure difference.
6. Moist, single-cell-thick wall. The alveolar surface is moist, allowing gases to dissolve before they diffuse; the conducting part is lined with mucus-secreting ciliated epithelium suited for trapping dust and not for exchange.

Extra depth. The same principle applies at the tissues: gas exchange between blood and tissue cells occurs through the thin walls of the capillaries, not through arteries or veins. Efficient diffusion everywhere in the body therefore needs thin membranes, large surfaces, steep gradients and good perfusion.

Conclusion: Gases diffuse only in the alveoli because only there is a very thin three-layered membrane in intimate contact with a dense capillary network over a large surface with a steep partial-pressure gradient; the rest of the respiratory tract merely conducts, cleans, humidifies and warms the air.

Concept. CO₂ produced by the cells diffuses into the blood and has to be carried to the lungs. Blood is the transport medium, but CO₂ is not carried in just one way. According to NCERT, nearly 70% of CO₂ is carried as bicarbonate, 20–25% by red blood cells (as carbamino-haemoglobin) and about 7% in the dissolved state in plasma. Each mechanism has its own chemistry and role.

1. As bicarbonate (about 70%). RBCs contain a very high concentration of the enzyme carbonic anhydrase (minute quantities also in plasma). At the tissues, where pCO₂ is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and, with water, forms carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and hydrogen ion (H⁺). The bicarbonate is carried in the blood to the lungs. At the alveolar site, where pCO₂ is low, the reaction runs in the opposite direction, releasing CO₂ and H₂O, and the CO₂ diffuses into the alveoli to be exhaled.
2. As carbamino-haemoglobin (20–25%). CO₂ binds to the haemoglobin of RBCs to form carbamino-haemoglobin. Binding is related to the partial pressure of CO₂, and pO₂ is a major factor affecting it: when pCO₂ is high and pO₂ low (tissues) more CO₂ binds; when pCO₂ is low and pO₂ high (alveoli) CO₂ dissociates from carbamino-haemoglobin and is delivered to the alveoli.
3. In dissolved form in plasma (about 7%). A small portion of CO₂ simply dissolves in plasma and is carried to the lungs; because CO₂ is 20–25 times more soluble than O₂, this fraction is significant compared with dissolved O₂ (3%).

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺  (carbonic anhydrase in RBC)

Extra depth. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli. The same enzyme works in both directions — the direction is decided by the partial pressure of CO₂ — which is why one enzyme can both load CO₂ in tissues and unload it in the lungs.

Conclusion: CO₂ is carried by blood mainly as bicarbonate (about 70%, via carbonic anhydrase), as carbamino-haemoglobin (20–25%) and in dissolved form in plasma (about 7%), the direction of each process being governed by the partial pressure of CO₂ in tissues and alveoli.

Concept. The partial pressure of a gas is the pressure contributed by that gas in a mixture of gases; it is written pO₂ for oxygen and pCO₂ for carbon dioxide. Diffusion of gases in the body is driven by differences in partial pressure, so comparing the values in the atmosphere and in the alveoli explains the direction of movement of the gases. Table 14.1 of NCERT gives the exact values.

Correct answer: (ii) pO₂ higher, pCO₂ lesser. Atmospheric air has pO₂ = 159 mm Hg (higher than the alveolar 104 mm Hg) and pCO₂ = 0.3 mm Hg (lesser than the alveolar 40 mm Hg).

Why each wrong option is wrong

• (i) pO₂ lesser, pCO₂ higher — this describes the alveolar air compared with atmospheric air (alveolar pO₂ 104 is lower, pCO₂ 40 is higher). It is the reverse of what the question asks, which is the atmosphere compared with the alveoli.
• (iii) pO₂ higher, pCO₂ higher — atmospheric pCO₂ is only 0.3 mm Hg, far lower than the alveolar 40 mm Hg, so “pCO₂ higher” is wrong.
• (iv) pO₂ lesser, pCO₂ lesser — atmospheric pO₂ (159) is greater than the alveolar value (104), so “pO₂ lesser” is wrong.

Why the alveolar values differ. The inspired air mixes with the large volume of air already in the lungs (the functional residual capacity of about 2100–2300 mL), so fresh air is diluted. O₂ is continuously removed from alveolar air into blood, which lowers pO₂ to 104 mm Hg, and CO₂ is continuously added from blood, raising pCO₂ to 40 mm Hg. (Humidification of inspired air also lowers its pO₂ slightly — general physiology.)

Extra depth. The values form a gradient: O₂ moves from atmosphere (159) to alveoli (104) to blood (40 → 95) to tissues (40). CO₂ moves the other way: tissues (45) → blood (45 → 40) → alveoli (40) → atmosphere (0.3). The gradient for CO₂ is small but it diffuses efficiently because CO₂ is 20–25 times more soluble than O₂.

Conclusion: Atmospheric air has a higher pO₂ (159 mm Hg) and a lesser pCO₂ (0.3 mm Hg) than alveolar air (104 and 40 mm Hg), so option (ii) is correct.

Concept. Breathing has two stages: inspiration, during which atmospheric air is drawn in, and expiration, by which alveolar air is released. Air moves in and out of the lungs only when a pressure gradient is created between the lungs and the atmosphere. Inspiration can occur only if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e. there is a negative pressure in the lungs with respect to the atmosphere. Because the lungs are located in an air-tight thoracic chamber and we cannot directly alter pulmonary volume, the change is brought about indirectly by altering the thoracic volume with the help of the diaphragm and the external intercostal muscles.

Steps of normal inspiration

1. Contraction of the diaphragm. The dome-shaped diaphragm contracts and flattens, increasing the volume of the thoracic chamber in the antero-posterior axis.
2. Contraction of the external intercostal muscles. These muscles between the ribs contract and lift up the ribs and the sternum, increasing the volume of the thoracic chamber in the dorso-ventral axis.
3. Increase in thoracic and pulmonary volume. The overall increase in thoracic volume causes a similar increase in the pulmonary (lung) volume because the lungs are attached to the thoracic wall through the pleural membranes.
4. Fall in intra-pulmonary pressure. An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure.
5. Air enters the lungs. The pressure gradient forces air from outside to flow into the lungs through the air passages until the pressures are equal — this is inspiration (Fig. 14.2a).

Extra depth. Under normal (quiet) conditions inspiration is an active process but expiration is mostly passive, resulting from relaxation of the same muscles. The strength of inspiration and expiration can be increased with the help of additional muscles in the abdomen. A healthy human breathes 12–16 times per minute, with a tidal volume of about 500 mL per breath.

Conclusion: In normal inspiration the diaphragm and external intercostal muscles contract, enlarging the thoracic and hence pulmonary volume in both antero-posterior and dorso-ventral axes; the intra-pulmonary pressure falls below atmospheric pressure and air flows into the lungs.

Concept. Human beings can maintain and moderate the respiratory rhythm to suit the demands of body tissues — breathing is faster during exercise and slower at rest. This is achieved by the neural system, which receives information on blood chemistry and adjusts the respiratory muscles accordingly.

1. Respiratory rhythm centre (medulla). A specialised centre in the medulla oblongata region of the brain is primarily responsible for regulating respiration. It generates the basic rhythm of breathing movements.
2. Pneumotaxic centre (pons). A centre in the pons region of the brain can moderate the functions of the respiratory rhythm centre. Neural signals from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
3. Chemosensitive area. A chemosensitive area situated adjacent to the rhythm centre is highly sensitive to CO₂ and hydrogen ions. An increase in these substances activates this centre, which signals the rhythm centre to make the necessary adjustments (faster, deeper breathing) so that these substances are eliminated.
4. Peripheral receptors. Receptors associated with the aortic arch and carotid artery also recognise changes in CO₂ and H⁺ concentration and send signals to the rhythm centre for remedial action.
5. Role of oxygen. The role of oxygen in the regulation of the respiratory rhythm is quite insignificant under normal conditions.

Extra depth. This is a negative-feedback loop: the product (CO₂) of the process regulates the process itself. During exercise more CO₂ is produced, the centres are stimulated, and ventilation rises to meet the increased demand.

Conclusion: Respiration is regulated by the neural system: the rhythm centre in the medulla, moderated by the pneumotaxic centre in the pons, adjusts breathing in response to CO₂ and H⁺ detected by the chemosensitive area and aortic/carotid receptors, whereas oxygen plays an insignificant role.

Concept. About 97% of oxygen is carried by haemoglobin in the RBCs as oxyhaemoglobin. The binding of O₂ to haemoglobin depends primarily on the partial pressure of O₂, but NCERT states that partial pressure of CO₂, hydrogen ion concentration and temperature are other factors that can interfere with this binding. The effect of pCO₂ is best understood from the oxygen dissociation curve, the sigmoid plot of percentage saturation of haemoglobin against pO₂.

Effect of pCO₂. A high pCO₂ reduces the affinity of haemoglobin for oxygen and favours the dissociation of oxygen from oxyhaemoglobin; a low pCO₂ increases the affinity and favours the formation of oxyhaemoglobin. On the graph, a high pCO₂ shifts the curve to the right (the same pO₂ gives a lower percentage saturation), while a low pCO₂ shifts it to the left. (This is known as the Bohr effect.)

Physiological significance. Active tissues produce more CO₂ (and acid and heat), so haemoglobin automatically releases more oxygen exactly where it is needed most. In the lungs, as CO₂ is removed, haemoglobin picks up O₂ more readily. Under normal conditions every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues.

Extra depth — the reverse effect. The binding of CO₂ to haemoglobin (carbamino-haemoglobin) is in turn influenced by pO₂: low pO₂ in the tissues favours CO₂ binding and high pO₂ in the alveoli favours its release. Thus O₂ and CO₂ transport are linked and help each other.

Conclusion: A high pCO₂ (as in tissues) lowers the affinity of haemoglobin for oxygen and promotes release of O₂, whereas a low pCO₂ (as in alveoli) promotes O₂ binding, so oxygen is loaded in the lungs and delivered in the tissues.

Concept. Atmospheric air contains about 21% oxygen at every altitude, but the atmospheric pressure falls as altitude increases. Since partial pressure = percentage × total pressure, the pO₂ of the air at high altitude is much lower than the 159 mm Hg at sea level (Table 14.1). The respiratory system of a man climbing a hill therefore faces a shortage of oxygen — a condition called hypoxia. (This question goes beyond the printed text of the chapter; the answer applies the NCERT principles of partial pressure, diffusion gradients and regulation of respiration.)

Immediate effects

1. Lower alveolar and blood pO₂. Lower atmospheric pO₂ reduces the pO₂ in alveolar air, so the diffusion gradient for O₂ from alveoli to blood becomes less steep and less O₂ diffuses into the blood.
2. Lower saturation of haemoglobin. According to the oxygen dissociation curve, a lower pO₂ means a lower percentage saturation of haemoglobin, so less oxygen is delivered to the tissues.
3. Symptoms of hypoxia (mountain sickness). Breathlessness, headache, nausea, weakness, fatigue and in severe cases confusion may occur because brain and muscles get too little O₂ for aerobic respiration.

Compensatory responses

1. Increased rate and depth of breathing. The aortic and carotid receptors sense the changes and signal the respiratory rhythm centre, so ventilation increases to bring in more air. (Normally O₂ has an insignificant role in regulation, but at low pO₂ peripheral receptors do respond.)
2. Increased heart rate to pump more blood and carry more oxygen to the tissues.
3. Increased RBC production and haemoglobin (acclimatisation over days to weeks), improving the oxygen-carrying capacity of blood; capillary networks in tissues may also increase (general physiology knowledge).
4. Adjustment of the dissociation curve: the factors that favour O₂ release in tissues (higher H⁺, pCO₂, temperature) help unload oxygen to the cells.

Conclusion: On going up a hill the falling atmospheric pressure lowers the pO₂ of inspired and alveolar air, causing hypoxia; the body compensates by faster breathing, increased heart rate and, on prolonged stay, an increased number of red blood cells.

Concept. Mechanisms of breathing vary among animal groups depending on habitat and level of organisation. NCERT notes that earthworms use their moist cuticle, aquatic arthropods and molluscs use gills, but insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.

Answer. In an insect the gaseous exchange takes place in the tracheal system, i.e. at the fine ends of the tracheal tubes (tracheoles) which reach the tissues and cells directly.

1. Spiracles. The body surface has small openings called spiracles on the sides of the thorax and abdomen through which atmospheric air enters (and leaves).
2. Tracheae. Each spiracle leads to a tube, the trachea, whose wall is kept open by chitinous thickenings, and the tracheae branch repeatedly through the body.
3. Tracheoles. The finest branches, the tracheoles, are fluid-filled and extend right up to individual cells and tissues, so oxygen diffuses directly into cells and CO₂ diffuses out.
4. Role of blood. Because air is delivered right to the tissues, the insect’s blood (haemolymph) does not carry oxygen; the exchange is directly between tracheoles and cells (general zoology knowledge).

Extra depth. The tracheal system is an extremely efficient arrangement for small active animals because diffusion through air is fast; it is one reason insects can sustain very rapid flight metabolism. Larger body movements of the abdomen help ventilate the tubes.

Conclusion: Gaseous exchange in an insect occurs through the network of tracheal tubes, whose fine tracheoles carry atmospheric air from the spiracles directly to the tissues.

Concept. Haemoglobin is a red, iron-containing pigment present in RBCs. O₂ binds with haemoglobin in a reversible manner to form oxyhaemoglobin, and each haemoglobin molecule can carry a maximum of four molecules of O₂. The extent of binding mainly depends on the partial pressure of O₂.

Definition. The oxygen dissociation curve is the sigmoid 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, H⁺ concentration and temperature on the binding of O₂ with haemoglobin.

Reason for the sigmoidal pattern. The S-shape arises because the binding of oxygen to haemoglobin is cooperative:

1. A haemoglobin molecule has four oxygen-binding sites (each of the four polypeptide chains carries a haem group with iron), and so it can carry a maximum of four O₂ molecules (NCERT).
2. At low pO₂ the first O₂ molecule binds with difficulty (the molecule is in a low-affinity, tense state), so saturation rises slowly — the flat initial part of the curve.
3. Once one O₂ binds, it changes the shape of the haemoglobin molecule and increases the affinity of the remaining sites; the second, third and fourth O₂ molecules bind progressively more easily, so saturation rises steeply over a small increase of pO₂ — the steep middle part.
4. As haemoglobin approaches full saturation, only a few vacant sites remain, so the curve flattens into a plateau at nearly 100% saturation.

Extra depth. This shape is physiologically ideal: in the alveoli (pO₂ about 104 mm Hg) haemoglobin is on the plateau, so it loads oxygen fully even if pO₂ varies a little; in the tissues (pO₂ about 40 mm Hg) it is on the steep part, so a small drop gives up a large amount of oxygen. If binding were not cooperative the curve would be a simple hyperbola with poor unloading in the tissues.

Conclusion: The oxygen dissociation curve is the sigmoid plot of percentage saturation of haemoglobin against pO₂; its S-shape results from the cooperative binding of up to four O₂ molecules to one haemoglobin molecule, each bound O₂ making the next easier to bind.

Concept. Hypoxia is a condition in which the body or a region of the body is deprived of an adequate supply of oxygen at the tissue level. It is not mentioned in the printed chapter text, but it follows directly from the principles of gas exchange and transport described in Sections 14.3–14.4. (The information below is general physiology knowledge, compiled as the question asks.)

Types and causes of hypoxia

Effects. Shortness of breath, rapid breathing and heart rate, headache, dizziness, bluish discolouration (cyanosis), fatigue, confusion and, if severe or prolonged, loss of consciousness and damage to the brain and heart, since these organs need a continuous supply of O₂ for aerobic respiration.

Link with the chapter. Hypoxia reduces the pO₂ gradient for diffusion (Section 14.3), reduces haemoglobin saturation (oxygen dissociation curve, Section 14.4), and stimulates the aortic and carotid receptors to increase breathing (Section 14.5). Chronic lung damage, such as emphysema from smoking or fibrosis from dust, causes hypoxia (Section 14.6).

Prevention and treatment. Treat the underlying cause; give supplemental oxygen; descend from high altitude and acclimatise gradually; stop smoking; wear protective masks in dusty industries; treat anaemia.

Conclusion: Hypoxia is an inadequate supply of oxygen to the tissues, arising from low environmental pO₂, lung disease, reduced haemoglobin function, poor circulation or cellular poisoning, and it is relieved by correcting the cause and providing oxygen.

Concept. Respiratory volumes are the individual amounts of air handled in breathing movements; by adding a few of them we obtain pulmonary capacities. The terms are measured with a spirometer and used in clinical diagnosis. The values are NCERT’s; sums are derived from them.

(a) IRV and ERV

(b) Inspiratory capacity and Expiratory capacity

(c) Vital capacity and Total lung capacity

Extra depth. Remember the pattern: capacities are always sums of volumes. FRC = ERV + RV; IC = TV + IRV; EC = TV + ERV; VC = ERV + TV + IRV; TLC = VC + RV. Tidal volume (500 mL) is the central volume common to IC, EC and VC.

Conclusion: IRV (2500–3000 mL) is the extra air inspired and ERV (1000–1100 mL) the extra air expired by forcible effort; IC = TV + IRV while EC = TV + ERV; and VC = ERV + TV + IRV while TLC = VC + residual volume.

Concept. Tidal volume (TV) is the volume of air inspired or expired during a normal respiration. It is approximately 500 mL. It is one of the four respiratory volumes measured with a spirometer, and it is the volume that forms the common part of the inspiratory, expiratory and vital capacities.

Calculation of tidal volume of air breathed in one hour. NCERT states that a healthy man breathes 12–16 times per minute and that he can inspire or expire approximately 6000 to 8000 mL of air per minute.

1. Tidal volume per breath = 500 mL.
2. Breaths per minute = 12 to 16.
3. Air per minute = 500 × 12 = 6000 mL (lower limit) to 500 × 16 = 8000 mL (upper limit).
4. Time = 60 minutes.
5. Air per hour = 6000 × 60 = 3,60,000 mL = 360 litres (lower limit) to 8000 × 60 = 4,80,000 mL = 480 litres (upper limit).

Volume per hour = TV × breaths per minute × 60 = 500 mL × (12–16) × 60 = 360–480 litres

Extra depth. The air actually reaching the alveoli is less than this because part of each breath stays in the conducting passages (the dead space, about 150 mL: general physiology). Also, during exercise both the rate and the depth of breathing rise, so the volume per hour is much greater. Moreover, the tidal volume is only a part of the total lung capacity (5100–5800 mL, derived), which shows how much reserve the lungs have.

Conclusion: Tidal volume is the air inspired or expired in a normal breath, about 500 mL; at 12–16 breaths per minute a healthy human therefore breathes about 6000–8000 mL per minute, i.e. approximately 360–480 litres in an hour.