Breathing and Exchange of Gases NEET Notes 2027: Key Points and PDF

Breathing and Exchange of Gases is Chapter 14 of NCERT Class 11 Biology and part of Unit 5, Human Physiology, of the NEET UG syllabus. These notes cover respiratory organs, the human respiratory system, the mechanism of breathing, respiratory volumes and capacities, exchange and transport of gases, regulation of respiration and respiratory disorders, with a free notes PDF.

Breathing and Exchange of Gases NEET Notes

Section-wise key points from NCERT Chapter 14, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Breathing and Exchange of Gases at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What are the respiratory organs in different animals?

Breathing is the exchange of O2 from the atmosphere with CO2 produced by cells. Mechanisms vary with habitat and level of organisation (recall only for NEET).

Human Respiratory System: Key Points

Air passes from the external nostrils through the nasal chamber, pharynx, larynx and trachea into the bronchi, bronchioles and alveoli.

• The pharynx is a common passage for food and air. The larynx is a cartilaginous box that helps in sound production, so it is the sound box.
• During swallowing, the glottis is covered by the epiglottis, a thin elastic cartilaginous flap, so food does not enter the larynx.
• The trachea divides at the level of the 5th thoracic vertebra into right and left primary bronchi, which branch into secondary and tertiary bronchi, bronchioles and terminal bronchioles.
• Trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings.
• Each terminal bronchiole gives rise to many thin, irregular-walled, vascularised bag-like alveoli. Bronchi, bronchioles and alveoli make up the lungs.
• The two lungs are covered by a double-layered pleura with pleural fluid between the layers, which reduces friction. The outer pleural membrane touches the thoracic lining; the inner one touches the lung surface.
• The thoracic chamber is air-tight: vertebral column dorsally, sternum ventrally, ribs laterally and the dome-shaped diaphragm below. A change in thoracic volume is reflected in the lung volume.

Steps of Respiration

• 1. Breathing or pulmonary ventilation: air drawn in and CO2-rich alveolar air released.
• 2. Diffusion of O2 and CO2 across the alveolar membrane.
• 3. Transport of gases by the blood.
• 4. Diffusion of O2 and CO2 between blood and tissues.
• 5. Utilisation of O2 by cells for catabolic reactions and release of CO2 (cellular respiration).

How does the mechanism of breathing work?

Air moves in and out because of a pressure gradient between the lungs and the atmosphere, created by the diaphragm and the external and internal intercostal muscles.

• Additional abdominal muscles can increase the strength of inspiration and expiration.
• A healthy human breathes 12 to 16 times a minute on average.
• A spirometer estimates the volume of air in breathing and helps clinical assessment of pulmonary functions.

Respiratory Volumes and Capacities

Volumes are measured with a spirometer; adding volumes gives the pulmonary capacities used in clinical diagnosis.

Exchange of Gases at the Alveoli and Tissues

Alveoli are the primary sites of gas exchange; exchange also occurs between blood and tissues. O2 and CO2 move by simple diffusion along pressure or concentration gradients.

• The rate of diffusion depends on the partial pressure gradient, the solubility of the gas and the thickness of the membrane.
• O2 moves from alveoli to blood to tissues; CO2 moves the opposite way, from tissues to blood to alveoli.
• CO2 is 20 to 25 times more soluble than O2, so much more CO2 diffuses per unit difference in partial pressure.
• The diffusion membrane has three major layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries and the basement substance between them. Its total thickness is much less than a millimetre.

How are oxygen and carbon dioxide transported?

Blood is the medium of transport for both gases.

Transport of Oxygen

• Haemoglobin is a red, iron-containing pigment in RBCs; each molecule carries a maximum of four O2 molecules.
• Binding depends mainly on pO2; pCO2, H+ concentration and temperature also affect it.
• Plotting percentage saturation of haemoglobin against pO2 gives a sigmoid curve, the oxygen dissociation curve.
• In the alveoli (high pO2, low pCO2, less H+, lower temperature) oxyhaemoglobin forms; in the tissues (low pO2, high pCO2, high H+, higher temperature) oxygen dissociates.
• Every 100 mL of oxygenated blood delivers around 5 mL of O2 to the tissues under normal conditions.

Transport of Carbon Dioxide

• Carbamino-haemoglobin binding depends on pCO2, and pO2 is a major factor affecting it: more binding in tissues (high pCO2, low pO2), dissociation in the alveoli (low pCO2, high pO2).
• RBCs have a very high concentration of carbonic anhydrase (minute amounts in plasma), which catalyses CO2 + H2O to H2CO3 to HCO3- + H+ in both directions.
• In tissues, CO2 forms HCO3- and H+; at the alveoli the reaction reverses to release CO2 and H2O.
• Every 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli.

How is respiration regulated?

The neural system adjusts the respiratory rhythm to the demands of body tissues.

The role of oxygen in regulating the respiratory rhythm is quite insignificant.

Disorders of the Respiratory System

Most-asked Facts for NEET: Breathing and Exchange of Gases

Revise these one-line facts from the NCERT chapter just before the exam.

• Insects use tracheal tubes; earthworms use moist cuticle; frogs can also breathe through skin.
• Larynx is the sound box; the epiglottis covers the glottis during swallowing.
• Trachea divides at the level of the 5th thoracic vertebra.
• Airways up to the initial bronchioles have incomplete cartilaginous rings.
• Conducting part: nostrils to terminal bronchioles; exchange part: alveoli and their ducts.
• Diaphragm contraction increases volume antero-posteriorly; external intercostals increase it dorso-ventrally.
• Normal breathing rate: 12 to 16 times a minute.
• TV about 500 mL; IRV 2500 to 3000 mL; ERV 1000 to 1100 mL; RV 1100 to 1200 mL.
• FRC = ERV + RV; VC = ERV + TV + IRV; TLC = VC + RV.
• Alveolar pO2 104 and pCO2 40 mm Hg; atmospheric pO2 159 and pCO2 0.3 mm Hg.
• CO2 is 20 to 25 times more soluble than O2.
• 97% of O2 travels in RBCs; 70% of CO2 travels as bicarbonate.
• One haemoglobin molecule carries at most four O2 molecules; the oxygen dissociation curve is sigmoid.
• 100 mL oxygenated blood delivers about 5 mL O2; 100 mL deoxygenated blood delivers about 4 mL CO2.
• Rhythm centre in the medulla; pneumotaxic centre in the pons.
• Emphysema: damaged alveolar walls, often due to smoking.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Placing the pneumotaxic centre in the medulla. It is in the pons; the respiratory rhythm centre is in the medulla.
• Saying oxygen levels drive the respiratory rhythm. NCERT says the role of oxygen is quite insignificant; CO2 and H+ matter.
• Writing that most CO2 is carried as carbamino-haemoglobin. About 70% is carried as bicarbonate; only 20 to 25% by haemoglobin.
• Including RV in vital capacity. VC = ERV + TV + IRV; adding RV gives total lung capacity.
• Writing FRC as TV + RV. Functional residual capacity is ERV + RV.
• Swapping the axes: the diaphragm increases volume antero-posteriorly; the external intercostals increase it dorso-ventrally.
• Calling the tracheal cartilaginous rings complete. They are incomplete.
• Thinking CO2 diffuses faster because its pressure gradient is larger. Its gradient is smaller; it diffuses well because it is 20 to 25 times more soluble.
• Saying high pCO2 and H+ favour oxyhaemoglobin formation. They favour dissociation of oxygen, as in the tissues.
• Counting the alveolar ducts in the conducting part. The alveoli and their ducts form the exchange part.

Breathing and Exchange of Gases Notes PDF and Related Pages

Download the AglaSem notes for Breathing and Exchange of Gases (key notes and important questions with answers) free: View PDF

• All chapters: NEET Biology notes
• Full topic list: NEET syllabus
• Practice: NEET question papers
• Source text: NCERT Chapter 14 (NCERT, Class 11 Biology)

NEET Biology Notes: All Chapters

Botany

• The Living World
• Biological Classification
• Plant Kingdom
• Cell: The Unit of Life
• Plant Growth and Development
• Anatomy of Flowering Plants
• Cell Cycle and Cell Division
• Respiration in Plants
• Photosynthesis in Higher Plants
• Morphology of Flowering Plants
• Principles of Inheritance and Variation
• Microbes in Human Welfare
• Sexual Reproduction in Flowering Plants
• Molecular Basis of Inheritance
• Organisms and Populations
• Ecosystem
• Biodiversity and Conservation

Zoology

• Animal Kingdom
• Biomolecules
• Breathing and Exchange of Gases
• Excretory Products and their Elimination
• Body Fluids and Circulation
• Structural Organisation in Animals
• Locomotion and Movement
• Reproductive Health
• Chemical Coordination and Integration
• Neural Control and Coordination
• Evolution
• Human Health and Disease
• Human Reproduction
• Biotechnology and its Applications
• Biotechnology: Principles and Processes

All chapters with their notes PDFs are on the NEET Biology notes page.

Breathing and Exchange of Gases NEET Notes FAQs