CBSE Class 11 Biology Revision Notes Chapter 14 Breathing and Exchange of Gases

Breathing and Exchange of Gases explains how oxygen enters the body and carbon dioxide is removed from it. In CBSE Class 11 Biology, this chapter covers respiratory organs, human breathing, gas exchange, gas transport and respiratory disorders.

Breathing and Exchange of Gases explains how organisms take in oxygen and release carbon dioxide. Cells use oxygen to break down molecules such as glucose, amino acids and fatty acids for energy. Carbon dioxide produced during these reactions must be removed from the body continuously.

Use CBSE Class 11 Biology Revision Notes Chapter 14 for 2026–27 to revise respiratory organs, the human respiratory system, mechanism of breathing, respiratory volumes and capacities, exchange of gases, transport of oxygen and carbon dioxide, regulation of respiration and respiratory disorders.

Key Takeaways

• Breathing: The exchange of oxygen from the atmosphere with carbon dioxide produced by cells.
• Human respiratory system: Includes nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles and lungs.
• Gas exchange: Occurs mainly in alveoli by simple diffusion.
• Gas transport: Oxygen is mainly transported by haemoglobin, while carbon dioxide is mainly transported as bicarbonate.

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Breathing and Exchange of Gases Class 11 Biology Notes: Chapter Overview

Oxygen is required by cells for energy release, while carbon dioxide produced during metabolism must be eliminated. The process of taking in oxygen and releasing carbon dioxide is called breathing or pulmonary ventilation.

Breathing supports cellular respiration, but both are different processes. Breathing is a physical process. Cellular respiration is a biochemical process inside cells.

Respiratory Organs in CBSE Class 11 Biology Notes Chapter 14

Different organisms use different respiratory organs based on habitat and body organisation.

Lower invertebrates exchange gases by simple diffusion over the body surface. More complex animals have specialised respiratory structures.

Human Respiratory System in Class 11 Biology Chapter 14 Notes

The human respiratory system has air passages and lungs. It brings air to the alveoli, where exchange of gases occurs.

The larynx helps in sound production. The epiglottis covers the glottis during swallowing and prevents food from entering the larynx.

Conducting Part and Exchange Part

The respiratory system has a conducting part and an exchange part.

The conducting part does not perform gas exchange. The exchange part is the actual site of diffusion.

Lungs and Thoracic Chamber

Humans have two lungs covered by a double-layered pleura. Pleural fluid is present between the two layers and reduces friction on the lung surface.

The lungs are present in an air-tight thoracic chamber. Any change in thoracic volume changes pulmonary volume.

Steps Involved in Respiration

Respiration includes multiple steps, beginning with breathing and ending with oxygen use by cells.

Cellular respiration is covered separately in the chapter on respiration in plants.

Mechanism of Breathing in Breathing and Exchange of Gases Class 11 Notes

Breathing has two stages: inspiration and expiration. Air movement depends on pressure differences between lungs and atmosphere.

Air moves from a region of higher pressure to lower pressure.

Inspiration

Inspiration occurs when intra-pulmonary pressure becomes less than atmospheric pressure.

Inspiration is an active process under normal conditions.

Expiration

Expiration occurs when intra-pulmonary pressure becomes higher than atmospheric pressure.

Normal expiration is generally passive.

Breathing Rate

A healthy human breathes about 12 to 16 times per minute. Breathing volume can be measured using a spirometer. This helps in clinical assessment of pulmonary functions.

Respiratory Volumes in Class 11 Biology Chapter 14 Notes

Respiratory volumes refer to the amount of air involved in breathing movements.

Tidal volume helps calculate the volume of air moved per minute. A healthy person can inspire or expire about 6000 to 8000 mL of air per minute.

Respiratory Capacities in CBSE Class 11 Biology Notes Chapter 14

Respiratory capacities are calculated by adding different respiratory volumes.

These values are useful in clinical diagnosis.

Exchange of Gases in Breathing and Exchange of Gases Class 11 Biology Notes

Alveoli are the primary sites of gas exchange. Gas exchange also occurs between blood and tissues.

Exchange of gases occurs by simple diffusion.

Factors Affecting Gas Exchange

Carbon dioxide diffuses faster than oxygen because it is 20 to 25 times more soluble than oxygen.

Partial Pressure of Oxygen and Carbon Dioxide

Partial pressure is the pressure contributed by an individual gas in a gas mixture.

These gradients support oxygen diffusion from alveoli to blood and blood to tissues. They also support carbon dioxide diffusion from tissues to blood and blood to alveoli.

Diffusion Membrane in Alveoli

The diffusion membrane is very thin and helps rapid gas exchange.

The total thickness is less than a millimetre, making diffusion efficient.

Transport of Gases in Class 11 Biology Chapter 14 Notes

Blood transports oxygen and carbon dioxide between lungs and tissues.

Transport of Oxygen

About 97% of oxygen is transported by RBCs. The remaining 3% is transported in dissolved form through plasma.

Haemoglobin is a red, iron-containing pigment present in RBCs. Oxygen binds reversibly with haemoglobin to form oxyhaemoglobin.

Oxygen Binding with Haemoglobin

Each haemoglobin molecule can carry up to four oxygen molecules.

In the alveoli, high pO₂ and low pCO₂ favour oxygen binding with haemoglobin.

Oxygen Release in Tissues

Oxygen is released from oxyhaemoglobin in tissues.

Every 100 mL of oxygenated blood can deliver about 5 mL of oxygen to tissues under normal physiological conditions.

Oxygen Dissociation Curve

The oxygen dissociation curve is obtained by plotting percentage saturation of haemoglobin with oxygen against pO₂.

The sigmoid shape is linked with the cooperative binding of oxygen to haemoglobin.

Transport of Carbon Dioxide

Carbon dioxide is transported in three forms.

Carbon dioxide transport depends on pCO₂ and pO₂.

Carbon Dioxide as Carbamino-Haemoglobin

Carbon dioxide binds with haemoglobin to form carbamino-haemoglobin.

This helps transport carbon dioxide from tissues to alveoli.

Carbon Dioxide as Bicarbonate

Most carbon dioxide is transported as bicarbonate ions.

RBCs contain a high concentration of carbonic anhydrase. This enzyme helps the reaction occur in both directions.

Every 100 mL of deoxygenated blood delivers about 4 mL of carbon dioxide to the alveoli.

Regulation of Respiration in CBSE Class 11 Biology Revision Notes Chapter 14

Respiration is regulated by the neural system. The body adjusts breathing rate and depth according to tissue demands.

An increase in carbon dioxide or hydrogen ion concentration activates the chemosensitive area. It signals the rhythm centre to adjust breathing.

The role of oxygen in regulating respiratory rhythm is relatively insignificant.

Disorders of Respiratory System

Respiratory disorders affect breathing and gas exchange.

Asthma

Asthma causes wheezing and difficulty in breathing. It occurs due to inflammation of bronchi and bronchioles.

Emphysema

Emphysema reduces respiratory surface area because alveolar walls are damaged. Cigarette smoking is a major cause.

Occupational Respiratory Disorders

Workers in industries involving grinding or stone-breaking may inhale large amounts of dust. Long exposure can cause inflammation, fibrosis and serious lung damage. Protective masks should be used in such workplaces.

Breathing and Respiration Difference

Breathing supports cellular respiration by supplying oxygen and removing carbon dioxide.

Conducting Part and Exchange Part Difference

Inspiration and Expiration Difference

Oxygen and Carbon Dioxide Transport Difference

Important Terms from Breathing and Exchange of Gases Class 11 Notes

NCERT-Based Exam Points

• Oxygen is used by cells to break down molecules and release energy.
• Carbon dioxide produced during catabolism must be removed.
• Breathing is the exchange of oxygen from atmosphere with carbon dioxide produced by cells.
• Lower invertebrates exchange gases through body surface.
• Earthworms use moist cuticle for gas exchange.
• Insects use tracheal tubes.
• Most aquatic arthropods and molluscs use gills.
• Terrestrial animals mainly use lungs.
• Amphibians can respire through moist skin.
• The larynx is called the sound box.
• Epiglottis prevents food entry into the larynx.
• Alveoli are the main sites of gas exchange.
• Conducting part extends from nostrils to terminal bronchioles.
• Exchange part includes alveoli and their ducts.
• Inspiration occurs when intra-pulmonary pressure is less than atmospheric pressure.
• Expiration occurs when intra-pulmonary pressure is more than atmospheric pressure.
• Diaphragm and intercostal muscles help create pressure gradients.
• A healthy human breathes about 12 to 16 times per minute.
• Tidal volume is about 500 mL.
• Residual volume is about 1100 to 1200 mL.
• Gas exchange occurs by simple diffusion.
• Carbon dioxide is 20 to 25 times more soluble than oxygen.
• About 97% oxygen is transported by RBCs.
• Haemoglobin can bind four oxygen molecules.
• Oxygen dissociation curve is sigmoid.
• About 70% carbon dioxide is transported as bicarbonate.
• About 20 to 25% carbon dioxide is transported as carbamino-haemoglobin.
• Respiratory rhythm centre is present in medulla.
• Pneumotaxic centre is present in pons.
• Asthma causes wheezing due to inflamed bronchi and bronchioles.
• Emphysema damages alveolar walls.
• Occupational respiratory disorders may occur due to long dust exposure.

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