Complete Solutions and Summary of Breathing and Exchange of Gases – NCERT Class 11, Biology, Chapter 14 – Summary, Questions, Answers, Extra Questions
Summary of respiratory organs, breathing mechanism, gas exchange, transport, control, and disorders with key NCERT questions.
Updated: 8 months ago

Breathing and Exchange of Gases
Chapter 14: Biology - Ultimate Study Guide | NCERT Class 11 Notes, Questions, Examples & Quiz 2025
Full Chapter Summary & Detailed Notes - Breathing and Exchange of Gases Class 11 NCERT
Overview & Key Concepts
- Chapter Goal: Understand the respiratory system, mechanisms of breathing, gas exchange, transport, regulation, and disorders. Exam Focus: Human respiratory anatomy, partial pressures, hemoglobin binding, volumes/capacities. 2025 Updates: Emphasis on pollution-related disorders, climate impact on respiration. Fun Fact: Alveoli surface area equals a tennis court (~70 m²). Core Idea: Respiration ensures O₂ supply for energy and CO₂ removal. Real-World: Asthma from urban pollution, emphysema from smoking.
- Wider Scope: Links to circulation (Ch 15), cellular respiration (Ch 12), neural control (Ch 18).
14.1 Respiratory Organs
Respiration is the exchange of O₂ from atmosphere with CO₂ from cells, essential for energy via catabolism of glucose, amino acids, fatty acids. O₂ utilization indirectly breaks down nutrients; CO₂, a harmful byproduct, must be expelled. Place hands on chest to feel breathing movement.
- Variations Across Animals: Depends on habitat/organization. Lower invertebrates (sponges, coelenterates, flatworms) use simple diffusion over body surface. Earthworms: Moist cuticle. Insects: Tracheal tubes for air transport. Aquatic arthropods/molluscs: Gills (branchial respiration). Terrestrial: Lungs (pulmonary). Vertebrates: Fishes (gills), amphibians/reptiles/birds/mammals (lungs); frogs also cutaneous via moist skin.
- Human Respiratory System (Detailed Anatomy): Paired external nostrils above upper lip lead to nasal chamber via nasal passage. Nasal chamber opens to pharynx (common food-air passage). Pharynx to larynx (sound box, cartilaginous) to trachea (straight tube to mid-thoracic cavity, divides at 5th thoracic vertebra into right/left primary bronchi). Bronchi branch into secondary/tertiary bronchi, bronchioles, terminal bronchioles. Supported by incomplete C-shaped cartilaginous rings. Terminal bronchioles form thin, irregular-walled, vascularized alveoli (bag-like). Branching network = lungs (two, covered by double-layered pleura with pleural fluid reducing friction). Outer pleura contacts thoracic lining; inner with lung surface.
- Conducting vs Respiratory Parts: External nostrils to terminal bronchioles = conducting (transports air, filters foreign particles, humidifies, warms to body temp). Alveoli/ducts = respiratory/exchange part (diffusion site). Thoracic chamber (air-tight): Vertebral column (dorsal), sternum (ventral), ribs (lateral), dome-shaped diaphragm (inferior). Volume changes in thorax reflect in lungs, essential for breathing.
- Steps of Respiration: (i) Pulmonary ventilation (breathing: in O₂, out CO₂). (ii) Diffusion O₂/CO₂ across alveolar membrane. (iii) Blood transport of gases. (iv) Diffusion between blood/tissues. (v) Cellular respiration (O₂ use, CO₂ release – Ch 12).
Figure 14.1: Diagrammatic view of human respiratory system (left lung sectional).
14.2 Mechanism of Breathing
Two stages: Inspiration (air in) and expiration (air out) via pressure gradient between lungs/atmosphere. Inspiration: Intra-pulmonary pressure < atmospheric (negative). Expiration: Intra-pulmonary > atmospheric.
- Muscles Involved: Diaphragm contracts (antero-posterior axis increase), external intercostals lift ribs/sternum (dorso-ventral increase). Overall thoracic volume up → pulmonary volume up → pressure down → air forced in. Relaxation: Diaphragm/intercostals return to normal, volume down → pressure up → air expelled. Additional abdominal muscles for forced breathing. Average: 12-16 breaths/min. Measured by spirometer for clinical assessment.
- Respiratory Volumes and Capacities (Detailed):
- Tidal Volume (TV): Normal breath air (~500 mL); 6000-8000 mL/min.
- Inspiratory Reserve Volume (IRV): Forced inspiration beyond TV (2500-3000 mL).
- Expiratory Reserve Volume (ERV): Forced expiration beyond TV (1000-1100 mL).
- Residual Volume (RV): Air left after forced expiration (1100-1200 mL).
- Capacities: Inspiratory Capacity (IC = TV + IRV). Expiratory Capacity (EC = TV + ERV). Functional Residual Capacity (FRC = ERV + RV). Vital Capacity (VC = ERV + TV + IRV). Total Lung Capacity (TLC = VC + RV).
Figures 14.2a/b: Inspiration/expiration mechanisms.
14.3 Exchange of Gases
Primary sites: Alveoli (lungs) and tissues. Simple diffusion via pressure/concentration gradient. Factors: Solubility (CO₂ 20-25x > O₂), membrane thickness (<1 mm).
- Partial Pressures: pO₂ (individual gas contribution). Table 14.1: Atmosphere (O₂ 159 mmHg, CO₂ 0.3); Alveoli (104, 40); Deoxygenated blood (40, 45); Oxygenated blood (95, 40); Tissues (40, 45). Gradients: O₂ alveoli → blood → tissues; CO₂ tissues → blood → alveoli.
- Diffusion Membrane: Three layers – alveolar squamous epithelium, capillary endothelium, basement substance. Thin for efficient diffusion. Figure 14.3: Gas exchange diagram. Figure 14.4: Alveolus-capillary section.
14.4 Transport of Gases
Blood medium: 97% O₂ by RBCs, 3% dissolved plasma. CO₂: 20-25% RBCs, 70% bicarbonate, 7% dissolved plasma.
- 14.4.1 Transport of Oxygen: Hemoglobin (Fe-pigment in RBCs) binds reversibly to oxyhemoglobin (4 O₂ max/molecule). Binding: pO₂ primary; affected by pCO₂, H⁺, temperature. Sigmoid oxygen dissociation curve (Fig 14.5): High pO₂/low pCO₂/low H⁺/low temp in alveoli → binding; opposite in tissues → dissociation. 100 mL oxygenated blood delivers ~5 mL O₂.
- 14.4.2 Transport of Carbon Dioxide: 20-25% carbaminohemoglobin (pCO₂-related; low pO₂ aids binding in tissues, high pO₂ aids release in alveoli). 70% bicarbonate via carbonic anhydrase: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (RBCs/plasma). High pCO₂ in tissues → bicarbonate formation; low in alveoli → reverse. 100 mL deoxygenated blood delivers ~4 mL CO₂.
14.5 Regulation of Respiration
Neural control maintains rhythm per tissue demands. Respiratory rhythm center (medulla) primary; pneumotaxic center (pons) moderates inspiration duration/rate. Chemosensitive area (medulla) sensitive to CO₂/H⁺ → signals rhythm center. Aortic/carotid receptors detect CO₂/H⁺ changes → signals. O₂ role minor.
14.6 Disorders of Respiratory System
- Asthma: Wheezing from bronchi/bronchioles inflammation/narrowing.
- Emphysema: Chronic; alveolar walls damaged → reduced surface. Major cause: Smoking.
- Occupational: Dust in grinding/stone-breaking → inflammation/fibrosis/lung damage. Use masks.
Summary
Cells use O₂ for metabolism, produce CO₂. Evolved mechanisms for transport/removal. Human system: Lungs/air passages. Steps: Ventilation, diffusion, transport, tissue exchange, cellular respiration. Volumes/capacities via spirometer. Diffusion via gradients/solubility/thinness. O₂ as oxyhemoglobin; CO₂ as bicarbonate/carbamino. Rhythm: Medulla/pneumotaxic/chemosensitive centers.
Why This Guide Stands Out
Complete coverage: All subtopics, diagrams explained, Q&A (NCERT + extras), quiz. Exam-ready for 2025. Free & ad-free.
Key Themes & Tips
- Respiration Essentials: O₂ in, CO₂ out for energy.
- Human Focus: Anatomy, volumes, transport curves.
- Tip: Memorize partial pressures table; draw dissociation curve.
Exam Case Studies
Questions on volumes (e.g., VC calculation), disorders (smoking effects).
Project & Group Ideas
- Measure TV via balloon; discuss pollution impact on lungs.



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