CBSE Class 11 Biology Chapter 17: Breathing and Exchange of Gases NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 11 PDF

This chapter delves into the crucial biological process of breathing and the exchange of gases in living organisms. The NCERT Solutions for Class 11 Biology, Chapter 17, provide detailed explanations and step-by-step answers to various questions related to the respiratory system. Students will explore the mechanisms of inspiration and expiration, the role of different respiratory organs, gas transport in the blood, and the regulation of respiration. These solutions cover topics such as the structure of the lungs, the function of alveoli, the partial pressures of gases, and the effects of pollutants like carbon monoxide on gas exchange. By studying these solutions, students can gain a deeper understanding of how organisms obtain oxygen and eliminate carbon dioxide, essential for cellular respiration and survival. This resource is designed to aid students in their exam preparation by clarifying complex concepts and providing accurate answers to practice problems.

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Board	CBSE
Class	Class 11
Subject	Biology Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 17

Chapter summary

Chapter 17 of the Class 11 Biology NCERT Solutions focuses on Breathing and Exchange of Gases. It covers the physiological processes involved in taking in oxygen and releasing carbon dioxide. The solutions explain the mechanics of breathing, including the roles of the diaphragm and intercostal muscles, gas diffusion across respiratory surfaces like alveoli, and the transport of respiratory gases (O2 and CO2) by blood. It also touches upon the regulation of breathing and common issues related to gas exchange.

Learning outcomes

Understand the mechanism of breathing (inspiration and expiration).
Explain the process of gas exchange in the lungs and tissues.
Describe the transport of oxygen and carbon dioxide in the blood.
Identify the role of respiratory organs in gas exchange.
Analyze the effects of harmful substances like carbon monoxide on respiration.

Topics covered

Paper topics

Respiratory Organs in Animals
Mechanism of Breathing
Gas Exchange
Transport of Respiratory Gases
Carbon Monoxide Poisoning
Regulation of Respiration
Disorders related to Respiration

Important topics

Mechanism of Breathing (Inspiration & Expiration)
Gas Exchange in Alveoli
Oxygen and Carbon Dioxide Transport
Effects of Carbon Monoxide

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Questions and Solutions

Question 1

Q. 1 Respiration in insects is called direct because

the cells exchange O2/CO2 directly with the air in the tubes
the tissues exchange O2/CO2 directly with coelomic fluid

the tissues exchange O2/CO2 directly with the air outside through body surface
tracheal tubes exchange <math>O_2/CO_2</math> directly with the haemocoel which then exchange with tissues

Solution: The correct option is (a). Insects possess a specialized respiratory system consisting of a network of fine tubes called tracheae. These tracheae branch extensively throughout the body, reaching close to individual cells. The openings of these tubes to the exterior are called spiracles. Because the tracheal system directly connects the external air to the internal tissues, cells can exchange oxygen and carbon dioxide directly with the air present within the tracheoles, making the process 'direct respiration'. Other options are incorrect as insects do not rely on coelomic fluid or direct exchange with the external environment through the body surface for gas exchange, nor do tracheal tubes primarily exchange gases with the haemocoel for subsequent tissue exchange.

Question 2

Q. 2 Which of the following does not occur during breathing?

Brings the air to body temperature (b) Warms up the air
Diffusion of gases (d) Cleans up the air

Solution: The correct option is (c). Breathing, also known as ventilation, is the mechanical process of moving air into and out of the lungs. During this process, the air is warmed to body temperature, humidified, and filtered to remove dust and other particles by the respiratory passages (nasal cavity, pharynx, larynx, trachea, bronchi). However, the diffusion of gases (the actual exchange of O2 and CO2 between the alveoli and the blood, and between the blood and the tissues) is a separate physiological process that occurs *after* or *during* the air reaching the respiratory surfaces, not as part of the mechanical act of breathing itself.

Question 3

Q. 3 A person suffers punctures in his chest cavity in an accident, without any damage to the lungs. Its effect could be

reduced breathing rate (b) rapid increase in breathing rate
no change in respiration (d) cessation of breathing

Solution: The correct option is (d). Breathing relies on maintaining a specific pressure gradient between the atmosphere and the inside of the lungs (intrapleural and intrapulmonary pressure). The chest cavity is sealed, and the lungs expand and contract due to the action of the diaphragm and intercostal muscles, which creates negative pressure allowing air to flow in. If the chest cavity is punctured, air can rush into the pleural space, equalizing the pressure between the chest cavity and the atmosphere. This loss of the pressure gradient prevents the lungs from expanding and contracting properly, leading to a failure of ventilation and potentially cessation of breathing, even if the lungs themselves are undamaged.

Question 4

Q. 4 It is known that exposure to carbon monoxide is harmful to animals because

it reduces CO2 transport (b) it reduces O2 transport
it increases CO2 transport (d) it increases O2 transport

Solution: The correct option is (b). Haemoglobin (Hb) in red blood cells is responsible for transporting oxygen (O2) from the lungs to the tissues. It forms a reversible bond with oxygen. Carbon monoxide (CO) is a toxic gas that binds to haemoglobin much more strongly than oxygen does, forming carboxyhaemoglobin (HbCO). The affinity of haemoglobin for CO is about 250 times greater than its affinity for O2. This binding is also essentially irreversible under normal physiological conditions. Consequently, when CO is present, it occupies the O2 binding sites on haemoglobin, drastically reducing the amount of oxygen that can be transported by the blood. This leads to oxygen deprivation in the body's tissues, which can be fatal.

The reactions involved are:

$Hb + O_2 ↔ HbO_2 \quad \text{(Oxyhaemoglobin)}$

$Hb + CO → HbCO \quad \text{(Carboxyhaemoglobin)}$

The formation of HbCO effectively removes Hb from its role in O2 transport.

Question 5

Q. 5 Mark the true statement among the following with reference to normal breathing

inspiration is a passive process whereas expiration is active
inspiration is a active process whereas expiration is passive
inspiration and expiration are active processes
inspiration and expiration are passive processes

Solution: The correct option is (b). In normal, quiet breathing, inspiration is an active process. It involves the contraction of the diaphragm and the external intercostal muscles. The contraction of the diaphragm flattens it, increasing the vertical dimension of the thoracic cavity. The contraction of the external intercostals lifts the ribs up and outwards, increasing the anteroposterior and lateral dimensions of the thoracic cavity. This expansion of the thoracic cavity reduces the intrapleural and intrapulmonary pressures below atmospheric pressure, causing air to flow into the lungs. Expiration, under normal resting conditions, is a passive process. It occurs when the diaphragm and external intercostal muscles relax. The elastic recoil of the lungs and chest wall causes the thoracic cavity to decrease in volume, increasing the intrapulmonary pressure above atmospheric pressure, and forcing air out of the lungs. Forced breathing (e.g., during exercise) involves active expiration using internal intercostal and abdominal muscles.

Common mistakes

Confusing breathing (ventilation) with respiration (gas exchange at cellular level).
Incorrectly identifying inspiration or expiration as solely active or passive processes.
Misunderstanding the role of pressure gradients in gas movement.
Underestimating the impact of substances like carbon monoxide on oxygen transport.

Revision tips

Draw diagrams of the respiratory system and label all parts.
Create flowcharts for the process of inspiration and expiration.
Summarize the differences between O2 and CO2 transport mechanisms.
Relate the concepts of partial pressure to gas diffusion.

Practice MCQs

Q1. Respiration in insects is considered direct because:

Q2. Which of the following processes does NOT occur during the act of breathing?

Q3. If a person's chest cavity is punctured without lung damage, what could be the immediate effect?

Q4. Exposure to carbon monoxide (CO) is harmful because:

Q5. Which statement accurately describes normal breathing?

Frequently asked questions

What is the primary function of breathing and exchange of gases?

The primary function is to supply oxygen to the body's cells for cellular respiration and to remove carbon dioxide, a waste product of metabolism, from the body.

How does breathing occur in insects?

Insects respire through a network of tubes called tracheae, which open to the outside through spiracles. Air enters these tubes, and gas exchange occurs directly between the air and the body tissues.

Why is carbon monoxide dangerous?

Carbon monoxide binds very strongly to haemoglobin, forming carboxyhaemoglobin. This prevents haemoglobin from carrying oxygen, leading to oxygen deprivation in tissues.

Is inspiration or expiration more active?

In normal breathing, inspiration is an active process requiring muscle contraction, while expiration is generally passive, relying on the elastic recoil of the lungs and chest wall.

What happens if the chest cavity is punctured?

A puncture disrupts the pressure balance needed for breathing. Air can enter the chest cavity, causing the lungs to collapse and potentially leading to cessation of breathing.

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