NCERT Class 11 Science — Chapter 4: Breathing and Exchange of Gases

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Assertion (A): An increase in the volume of the thoracic cavity causes a similar increase in the pulmonary volume. Reason (R): We can directly alter the pulmonary volume by contracting the smooth muscles of the lungs.

Official previous-year question

Verified 1 Aug 2026.

Options

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is NOT the correct explanation of A
CA is true but R is false
DA is false but R is true

Did you get this right?

Solution

The correct option is

A is true but R is false

Explanation

The mechanism of breathing relies on creating pressure gradients between the lungs and the atmosphere. This is achieved by changing the volume of the thoracic cavity, which mechanically influences the pulmonary volume due to the anatomical structure of the respiratory system.

Statement-wise Analysis:

Assertion (A): Correct. The lungs are situated in the thoracic chamber, which is anatomically an air-tight compartment formed by the vertebral column, sternum, ribs, and diaphragm. Because of this close anatomical coupling, any change in the volume of the thoracic cavity is immediately reflected in the pulmonary (lung) cavity. Therefore, an increase in thoracic volume leads to a similar increase in pulmonary volume.
Reason (R): Incorrect. Humans cannot directly alter the pulmonary volume. The lungs themselves do not possess intrinsic skeletal muscles to actively expand or contract their volume. Instead, breathing movements are generated by the contraction and relaxation of the diaphragm and intercostal muscles (external and internal). These muscles alter the thoracic volume, and the lungs passively expand or recoil in response.

Key Takeaway:

Breathing is a mechanical process where pulmonary volume changes are passive consequences of thoracic volume alterations driven by the diaphragm and intercostal muscles.

Your note

More from Chapter 4: Breathing and Exchange of Gases

Consider the following statements about the larynx and epiglottis in humans: 1. Larynx is a cartilaginous box associated with sound production. 2. Epiglottis facilitates the entry of food into the larynx. 3. Epiglottis is a fibrous flap devoid of elasticity. Which of the statements given above is/are correct?

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Consider the following about diffusion of gases in the lungs: 1. It is driven mainly by partial pressure gradients. 2. Solubility of gases has no effect on the rate. 3. Membrane thickness influences the rate of diffusion. Which of the statements given above is/are correct?

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Which of the following pathways accounts for the majority of carbon dioxide transport in blood?

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Which enzyme predominantly present in RBCs catalyses the reversible formation of bicarbonate from carbon dioxide and water?

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With reference to gas exchange mechanisms among animals, consider the following statements: 1. Insects rely on gills for respiration. 2. Most aquatic arthropods depend on vascularised bags for respiration. 3. Most aquatic arthropods depend on gills for respiration. Which of the statements given above are correct?

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The correct option is

A is true but R is false

Explanation

Statement-wise Analysis:

Assertion (A): Correct. The lungs are situated in the thoracic chamber, which is anatomically an air-tight compartment formed by the vertebral column, sternum, ribs, and diaphragm. Because of this close anatomical coupling, any change in the volume of the thoracic cavity is immediately reflected in the pulmonary (lung) cavity. Therefore, an increase in thoracic volume leads to a similar increase in pulmonary volume.
Reason (R): Incorrect. Humans cannot directly alter the pulmonary volume. The lungs themselves do not possess intrinsic skeletal muscles to actively expand or contract their volume. Instead, breathing movements are generated by the contraction and relaxation of the diaphragm and intercostal muscles (external and internal). These muscles alter the thoracic volume, and the lungs passively expand or recoil in response.

Key Takeaway:

Breathing is a mechanical process where pulmonary volume changes are passive consequences of thoracic volume alterations driven by the diaphragm and intercostal muscles.