NCERT Solutions for Class 11 Biology Chapter 14: Breathing and Exchange of Gases – Free PDF Download

Chapter 14, Breathing and Exchange of Gases, explains how humans take in oxygen and remove carbon dioxide through the respiratory system, covering the mechanism of breathing (based on Boyle’s law and pressure gradients), respiratory volumes and capacities, the transport of O₂ and CO₂ in blood, and the neural regulation of respiration by the medulla and pons. It also introduces common respiratory disorders such as asthma and emphysema, which are frequently asked about in board exams.

Last Updated: September 23, 2026

Exercises

Q1. Define vital capacity. What is its significance? — Vital capacity (VC) is the maximum volume of air that a person can exhale after a…

Vital capacity (VC) is the maximum volume of air that a person can exhale after a maximum forced inspiration. It is roughly 3.5 to 4.5 litres in a healthy adult human and is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume (VC = TV + IRV + ERV).

Significance: Vital capacity indicates how efficiently a person can take in fresh air and expel used air, which directly affects the rate of gaseous exchange between the alveoli and the blood. A higher vital capacity generally means better lung function and is often measured in athletes and used clinically to assess respiratory diseases such as asthma, emphysema, and fibrosis.

Q2. State the volume of air remaining in the lungs after a normal breathing — The volume of air remaining in the lungs after a normal (quiet) expiration is called the…

The volume of air remaining in the lungs after a normal (quiet) expiration is called the functional residual capacity (FRC). It is the sum of the expiratory reserve volume (ERV) and the residual volume (RV):

FRC = ERV + RV ≈ 1000–1100 mL + 1100–1500 mL ≈ 2500–3000 mL

This air ensures that gaseous exchange in the alveoli continues even between two breaths, preventing the alveoli from collapsing.

Q3. Diffusion of gases occurs in the alveolar region only and not in the other parts of the respiratory system. Why? — Gaseous exchange is restricted to the alveoli because only the alveolar walls and the…

Gaseous exchange is restricted to the alveoli because only the alveolar walls and the surrounding blood capillaries are extremely thin, moist, and made of a single layer of squamous epithelium, offering minimal resistance to diffusion. This thin alveolar-capillary membrane, combined with the enormous surface area created by millions of alveoli, allows O₂ and CO₂ to diffuse rapidly along their partial pressure gradients (from a region of higher partial pressure to lower partial pressure).

The other parts of the respiratory tract — nasal passage, pharynx, larynx, trachea, and bronchi — are lined with thicker, often ciliated and mucus-secreting epithelium whose main job is to clean, warm, and humidify air, not to exchange gases. Their walls are too thick and non-vascular for efficient diffusion, so no significant gas exchange occurs there; they simply act as a conducting pathway for air to reach the alveoli.

Q4. What are the major transport mechanisms for CO₂? Explain.

Carbon dioxide produced by tissues is transported to the lungs in three ways:

1. Dissolved in plasma (about 7%): A small fraction of CO₂ dissolves directly in the plasma and is carried in this simple dissolved form.

2. As carbaminohaemoglobin (about 20–25%): CO₂ binds reversibly with the amino groups of the globin part of haemoglobin (not the haem/iron site used by O₂) to form carbaminohaemoglobin. This binding is favoured by a low pO₂, as occurs at the tissues.

3. As bicarbonate ions (about 70%): This is the major route. CO₂ diffuses into red blood cells, where the enzyme carbonic anhydrase rapidly catalyses its combination with water to form carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺):

CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺

The bicarbonate ions diffuse out into the plasma in exchange for chloride ions entering the RBC to maintain electrical neutrality — a process called the chloride shift (Hamburger shift). At the alveoli, the entire process reverses: bicarbonate re-enters the RBC, combines with H⁺ to reform H₂CO₃, which breaks down into CO₂ and H₂O, and CO₂ diffuses out into the alveolar air to be exhaled.

Q5. What will be the pO₂ and pCO₂ in the atmospheric air compared to those in the alveolar air?

(i) pO₂ lesser, pCO₂ higher
(ii) pO₂ higher, pCO₂ lesser
(iii) pO₂ higher, pCO₂ higher
(iv) pO₂ lesser, pCO₂ lesser

Answer: (ii) pO₂ higher, pCO₂ lesser.

Atmospheric air has a higher partial pressure of oxygen (pO₂ ≈ 159 mm Hg) and a much lower partial pressure of carbon dioxide (pCO₂ ≈ 0.3 mm Hg) compared to alveolar air (pO₂ ≈ 104 mm Hg, pCO₂ ≈ 40 mm Hg). This is because alveolar air is a mixture of fresh incoming air and residual air already loaded with CO₂ from continuous gaseous exchange with pulmonary blood, plus water vapour, which lowers the effective pO₂.

Q6. Explain the process of inspiration under normal conditions — Inspiration (inhalation) is the process by which air moves from the atmosphere into the…

Inspiration (inhalation) is the process by which air moves from the atmosphere into the lungs and occurs when the intrapulmonary (intra-alveolar) pressure becomes lower than atmospheric pressure — following Boyle’s law, which states that pressure is inversely proportional to volume at constant temperature.

Under normal, quiet breathing, this negative pressure is achieved as follows: the diaphragm contracts and flattens, moving downward toward the abdominal cavity and increasing the thoracic volume along the antero-posterior axis. Simultaneously, the external intercostal muscles contract, lifting the ribs and sternum outward and upward, which increases the thoracic volume along the dorso-ventral axis. This overall increase in thoracic cavity volume causes a corresponding increase in pulmonary volume (since the lungs are attached to the thoracic wall via the pleura). As pulmonary volume increases, the intra-pulmonary pressure drops below atmospheric pressure, and air rushes into the lungs until the pressures equalise — completing inspiration.

Q7. How is respiration regulated? — Respiration in humans is regulated by a neural system centred in the brainstem, with…

Respiration in humans is regulated by a neural system centred in the brainstem, with chemical feedback fine-tuning the rate and depth of breathing.

The respiratory rhythm centre, located in the medulla oblongata, is the principal centre that generates the basic rhythm of breathing by sending signals for inspiration and expiration. The pneumotaxic centre in the pons can moderate the functions of the respiratory rhythm centre, reducing the duration of inspiration and thereby altering the respiratory rate.

A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO₂ and H⁺ ion concentration; an increase in either stimulates this centre to increase the rate and depth of breathing to expel excess CO₂. Additionally, specialised chemoreceptors located in the aortic arch and carotid bodies detect changes in CO₂ and H⁺ levels in blood and send signals to the medulla to make necessary adjustments. Interestingly, changes in oxygen levels have only a weak direct effect on this regulation compared to CO₂ and H⁺.

Q8. What is the effect of pCO₂ on oxygen transport?

The partial pressure of carbon dioxide has a strong influence on how readily haemoglobin binds or releases oxygen, a relationship known as the Bohr effect. At the alveoli, where pCO₂ is low and pO₂ is high, haemoglobin’s affinity for oxygen increases, favouring the formation of oxyhaemoglobin. At the tissues, where pCO₂ is high (and pH is correspondingly lower due to more H⁺ ions) and pO₂ is low, haemoglobin’s affinity for oxygen decreases, favouring the dissociation of oxygen from oxyhaemoglobin so that it can diffuse into the tissue cells. In short, a rise in pCO₂ promotes O₂ unloading, while a fall in pCO₂ promotes O₂ loading — ensuring oxygen is picked up in the lungs and released exactly where it is needed most.

Q9. What happens to the respiratory process in a man going up a hill? — As altitude increases, atmospheric pressure falls, and so does the partial pressure of…

As altitude increases, atmospheric pressure falls, and so does the partial pressure of oxygen (pO₂), even though the percentage of oxygen in air stays the same. A person climbing a hill therefore inhales less oxygen with each breath, which lowers the oxygen saturation of blood. In response, the respiratory rate and depth increase to compensate, and the heart rate rises to circulate blood — and whatever oxygen it carries — more quickly to the tissues. This immediate response is called acclimatisation; with prolonged stay at high altitude, the body further adapts by increasing red blood cell (and haemoglobin) production to improve oxygen-carrying capacity, a condition sometimes called high-altitude or mountain sickness if it is severe.

Q10. What is the site of gaseous exchange in an insect? — In insects, gaseous exchange occurs through an independent network of air-filled tubes…

In insects, gaseous exchange occurs through an independent network of air-filled tubes called the tracheal system, not through blood. Air enters through small openings on the body surface called spiracles, which lead into branching tubes called tracheae, which further divide into fine tracheoles that penetrate directly between and into individual cells. Oxygen diffuses straight from the tracheoles into the cells, and carbon dioxide diffuses out from the cells into the tracheoles and is expelled through the spiracles — so the actual site of gas exchange is at the tracheole-tissue interface, bypassing the circulatory system entirely.

Q11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern? — The oxygen dissociation curve (oxyhaemoglobin dissociation curve) is a graph that plots…

The oxygen dissociation curve (oxyhaemoglobin dissociation curve) is a graph that plots the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (pO₂). It shows how readily haemoglobin picks up oxygen at the lungs (high pO₂) and releases it at the tissues (low pO₂).

The curve is sigmoid (S-shaped) rather than a straight line or simple curve because haemoglobin is a tetramer with four haem groups, and the binding of the first oxygen molecule to one haem group causes a conformational change that increases the affinity of the remaining haem groups for oxygen — a phenomenon called positive cooperative binding. This makes oxygen loading proceed slowly at first, then very rapidly, and finally level off as haemoglobin approaches full saturation, producing the characteristic S-shape.

Q12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends — Hypoxia is a condition in which body tissues do not receive an adequate supply of oxygen…

Hypoxia is a condition in which body tissues do not receive an adequate supply of oxygen to meet their metabolic needs. It can arise from several distinct causes:

Hypoxic (hypoxaemic) hypoxia: reduced oxygen content of arterial blood due to low pO₂, as occurs at high altitude or in lung disease.

Anaemic hypoxia: reduced oxygen-carrying capacity of blood due to low haemoglobin concentration or defective haemoglobin.

Stagnant (ischaemic) hypoxia: adequate oxygen in blood but poor or slow circulation delivering it to tissues, e.g., due to heart failure, shock, or prolonged cold exposure.

Histotoxic hypoxia: normal oxygen delivery, but tissues are unable to use it, as in cyanide or carbon monoxide poisoning, which block cellular respiration.

Common symptoms include breathlessness, rapid breathing and heart rate, confusion, fatigue, and bluish discolouration of the skin (cyanosis) in severe cases.

Q13. Distinguish between — (a) IRV and ERV

(a) IRV and ERV

(b) Inspiratory Capacity and Expiratory Capacity

(c) Vital Capacity and Total Lung Capacity

Q14. What is tidal volume? Find out the tidal volume (approximate value) for a healthy human in an hour — Tidal volume (TV) is the volume of air inspired or expired during a single normal, quiet…

Tidal volume (TV) is the volume of air inspired or expired during a single normal, quiet breath. In a healthy adult, it is approximately 500 mL per breath.

To estimate the volume of air moved in an hour, we use the total (minute) ventilation rate, which for a healthy adult breathing 12–16 times per minute works out to roughly 6000 to 8000 mL of air per minute. Over an hour:

Volume per hour = 6000–8000 mL/minute × 60 minutes = 3.6 × 10⁵ mL to 4.8 × 10⁵ mL

So a healthy human moves approximately 360 to 480 litres of air through the lungs every hour during normal, resting breathing.

Class 11 Biology Chapter 14 – Notes and Extra Questions

The Chapter 14 exercise contains 14 questions in total (Q13 has three distinguish-between sub-parts: IRV/ERV, IC/EC, and VC/TLC), and this structure has remained unchanged through the 2023 rationalisation — only the chapter number shifted from the old Chapter 17 to the current Chapter 14 in the 19-chapter Class 11 Biology syllabus, with no questions added or removed. Students should be thorough with the definitions and normal values of all respiratory volumes and capacities (TV, IRV, ERV, RV, VC, IC, EC, TLC, FRC), as these appear repeatedly in both board exams and NEET. Diagram-based recall of the inspiration/expiration mechanism, a clear grasp of the Bohr effect and the three CO₂ transport routes (dissolved, carbaminohaemoglobin, bicarbonate via the chloride shift), and the roles of the medullary respiratory centre, pneumotaxic centre, and chemoreceptors are high-yield topics for both short-answer and numerical-style questions.

Frequently Asked Questions

What is Boyle’s law, and how does it explain the mechanics of breathing?

Boyle’s law states that at a constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume. During inspiration, contraction of the diaphragm and external intercostal muscles increases thoracic and pulmonary volume, which lowers intrapulmonary pressure below atmospheric pressure, so air flows in. During expiration, these muscles relax, thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure, and air flows out. This pressure-volume relationship, governed by Boyle’s law, is the physical basis of pulmonary ventilation.

What is the Bohr effect and why is it important for oxygen delivery to tissues?

The Bohr effect describes how a rise in pCO₂ (and the accompanying fall in pH) reduces haemoglobin’s affinity for oxygen, while a fall in pCO₂ increases it. This is important because it ensures oxygen binds strongly to haemoglobin in the oxygen-rich, CO₂-poor environment of the alveoli, and is released efficiently in metabolically active tissues where CO₂ is high — precisely matching oxygen delivery to tissue demand.

What is the difference between breathing and respiration?

Breathing (pulmonary ventilation) is the purely mechanical process of moving air in and out of the lungs through inspiration and expiration. Respiration is a broader term covering the entire process of gas exchange and utilisation of oxygen — including breathing, diffusion of gases at the alveoli (external respiration), transport of gases in blood, and cellular respiration, where oxygen is used to break down glucose and release energy inside cells.

What are some common disorders of the human respiratory system?

Two disorders highlighted in this chapter are asthma, a condition causing difficulty in breathing due to inflammation and narrowing of the bronchi and bronchioles, and emphysema, a chronic disorder in which the walls of the alveoli are damaged (often due to cigarette smoking), reducing the surface area available for gaseous exchange and causing breathlessness. Other respiratory conditions students should be aware of include occupational lung disorders caused by long-term exposure to dust, such as silicosis in miners and stone-cutters.

Written by Satish

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