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Breathing and Exchange of Gases – Class 11 Biology Chapter 14 Notes & NCERT PDF

Class 11BiologyChapter 14NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 14 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 14: Breathing and Exchange of Gases

Introduction

Organisms use oxygen (O₂) to break down simple molecules such as glucose, amino acids and fatty acids and obtain energy; carbon dioxide (CO₂), which is harmful, is released. The exchange of O₂ from the atmosphere with CO₂ produced by cells is called breathing, commonly known as respiration. This chapter studies the human respiratory system, the mechanism of breathing, and the exchange and transport of gases.

Key Concepts

14.1 Respiratory Organs

Mechanisms vary with habitat and level of organisation:

  • Sponges, coelenterates, flatworms: simple diffusion over the whole body surface.
  • Earthworms: moist cuticle. Insects: tracheal tubes.
  • Aquatic arthropods and molluscs; fishes: gills (branchial respiration).
  • Terrestrial forms: lungs (pulmonary respiration) — amphibians, reptiles, birds, mammals. Frogs can also respire through moist skin (cutaneous respiration).

    • 14.1.1 Human respiratory system:

  • External nostrils → nasal chamber → pharynx (common passage for food and air) → larynx (cartilaginous sound box) → trachea.
  • During swallowing, the epiglottis (thin elastic cartilaginous flap) covers the glottis to stop food entering the larynx.
  • The trachea divides at the level of the 5th thoracic vertebra into right and left primary bronchi → secondary and tertiary bronchi → bronchioles → terminal bronchioles. Trachea, bronchi and initial bronchioles are supported by incomplete cartilaginous rings.
  • Each terminal bronchiole gives rise to thin, irregular-walled, vascularised bags — the alveoli. Bronchi, bronchioles and alveoli form the lungs.
  • Lungs are covered by a double-layered pleura with pleural fluid, which reduces friction.
  • Conducting part (nostrils to terminal bronchioles): transports air, clears foreign particles, humidifies it and brings it to body temperature. Exchange (respiratory) part: alveoli and their ducts — actual diffusion of O₂ and CO₂.
  • Lungs lie in the air-tight thoracic chamber: vertebral column (dorsal), sternum (ventral), ribs (lateral), dome-shaped diaphragm (below). Any change in thoracic volume is reflected in the lungs.

    • Steps of respiration: (i) breathing / pulmonary ventilation; (ii) diffusion of gases across the alveolar membrane; (iii) transport of gases by blood; (iv) diffusion between blood and tissues; (v) utilisation of O₂ by cells (cellular respiration).

    14.2 Mechanism of Breathing

    Air moves because of a pressure gradient between lungs and atmosphere.

  • Inspiration: occurs when intra-pulmonary pressure is less than atmospheric. The diaphragm contracts (volume ↑ in the antero-posterior axis) and external intercostal muscles contract, lifting ribs and sternum (volume ↑ in the dorso-ventral axis). Thoracic and pulmonary volume increase, pressure falls, air rushes in.
  • Expiration: diaphragm and intercostals relax, volumes decrease, intra-pulmonary pressure rises slightly above atmospheric, air is expelled.
  • Abdominal muscles help increase the strength of inspiration and expiration.
  • A healthy human breathes 12–16 times per minute. Volumes are measured with a spirometer.

    • 14.2.1 Respiratory volumes and capacities:

  • Tidal Volume (TV): air inspired or expired in a normal breath — about 500 mL; about 6000–8000 mL per minute.
  • Inspiratory Reserve Volume (IRV): extra air inspired by forcible inspiration — 2500–3000 mL.
  • Expiratory Reserve Volume (ERV): extra air expired by forcible expiration — 1000–1100 mL.
  • Residual Volume (RV): air left in lungs even after forcible expiration — 1100–1200 mL.
  • Inspiratory Capacity (IC) = TV + IRV.
  • Expiratory Capacity (EC) = TV + ERV.
  • Functional Residual Capacity (FRC) = ERV + RV.
  • Vital Capacity (VC) = ERV + TV + IRV — maximum air breathed in after forced expiration (or out after forced inspiration).
  • Total Lung Capacity (TLC) = RV + ERV + TV + IRV = VC + RV.

    • 14.3 Exchange of Gases

    Alveoli are the primary site; exchange also occurs between blood and tissues. Gases move by simple diffusion along pressure/concentration gradients; solubility and membrane thickness also matter. Pressure of an individual gas in a mixture is its partial pressure (pO₂, pCO₂).

    Table 14.1 — Partial pressures (mm Hg):

  • Atmospheric air: pO₂ 159, pCO₂ 0.3
  • Alveoli: pO₂ 104, pCO₂ 40
  • Blood (deoxygenated): pO₂ 40, pCO₂ 45
  • Blood (oxygenated): pO₂ 95, pCO₂ 40
  • Tissues: pO₂ 40, pCO₂ 45

    • So O₂ moves alveoli → blood → tissues, and CO₂ moves tissues → blood → alveoli. CO₂ is 20–25 times more soluble than O₂, so much more CO₂ diffuses per unit difference in partial pressure. The diffusion membrane has three layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance between them; its total thickness is much less than a millimetre.

    14.4 Transport of Gases

  • O₂: about 97% carried by RBCs; 3% dissolved in plasma.
  • CO₂: about 20–25% by RBCs (as carbamino-haemoglobin); 70% as bicarbonate; about 7% dissolved in plasma.

    • 14.4.1 Transport of oxygen. Haemoglobin (red, iron-containing pigment of RBCs) binds O₂ reversibly to form oxyhaemoglobin; each haemoglobin molecule carries at most 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 oxygen dissociation curve.

  • In alveoli (high pO₂, low pCO₂, less H⁺, lower temperature) → oxyhaemoglobin forms.
  • In tissues (low pO₂, high pCO₂, high H⁺, higher temperature) → O₂ dissociates.
  • Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues.

    • 14.4.2 Transport of carbon dioxide. Haemoglobin carries CO₂ as carbamino-haemoglobin (20–25%): more binds where pCO₂ is high and pO₂ low (tissues), and it dissociates in alveoli. RBCs contain a very high concentration of carbonic anhydrase (minute amounts in plasma), which catalyses, in both directions:

    CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

    At tissues (high pCO₂) CO₂ forms HCO₃⁻ and H⁺; at alveoli (low pCO₂) the reaction reverses to release CO₂ and H₂O. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

    14.5 Regulation of Respiration

  • Respiratory rhythm centre in the medulla — primarily responsible.
  • Pneumotaxic centre in the pons — moderates the rhythm centre; can reduce the duration of inspiration and alter the respiratory rate.
  • Chemosensitive area next to the rhythm centre — highly sensitive to CO₂ and H⁺; activates the rhythm centre to remove them.
  • Receptors on the aortic arch and carotid artery also detect CO₂ and H⁺ changes.
  • The role of oxygen in regulating the rhythm is quite insignificant.

    • 14.6 Disorders of the Respiratory System

  • Asthma: difficulty in breathing with wheezing due to inflammation of bronchi and bronchioles.
  • Emphysema: chronic disorder in which alveolar walls are damaged, reducing respiratory surface; a major cause is cigarette smoking.
  • Occupational respiratory disorders: in industries involving grinding or stone-breaking, long exposure to dust causes inflammation leading to fibrosis and serious lung damage; workers should wear protective masks.

    • Diagrams to Know

  • Human respiratory system with a sectional view of the left lung (Figure 14.1).
  • Mechanism of breathing: (a) inspiration (b) expiration (Figure 14.2).
  • Exchange of gases at alveolus and tissues (Figure 14.3); section of alveolus with pulmonary capillary (Figure 14.4).
  • Oxygen dissociation curve (Figure 14.5).

    • Key-Facts Sheet

  • Trachea divides at the 5th thoracic vertebra.
  • Breathing rate: 12–16 per minute.
  • TV 500 mL; IRV 2500–3000; ERV 1000–1100; RV 1100–1200 mL.
  • VC = IRV + TV + ERV; TLC = VC + RV.
  • O₂: 97% by RBCs, 3% in plasma. CO₂: 20–25% by RBCs, 70% as bicarbonate, 7% in plasma.
  • 100 mL blood: gives 5 mL O₂ to tissues; delivers 4 mL CO₂ to alveoli.
  • Rhythm centre — medulla; pneumotaxic centre — pons.

    • Exam Focus

    The exercises ask you to define vital capacity and its significance; state the volume of air remaining in the lungs after normal expiration (FRC); explain why diffusion occurs only in the alveolar part; name the major transport mechanisms for CO₂; compare pO₂ and pCO₂ of alveolar air with atmospheric air; explain the process of inspiration under normal conditions; describe how respiration is regulated; state the effect of pCO₂ on oxygen transport; describe what happens to respiration when a person climbs a hill; state where the partial pressure of O₂ is highest; and distinguish IRV vs ERV, VC vs TLC, and TV vs RV. Practise calculating capacities from the given volumes.

    Key Terms

  • Breathing (श्वासोच्छ्वास)
  • Alveoli (कूपिकाएँ)
  • Diaphragm (मध्यपट / डायाफ्राम)
  • Intercostal muscles (अंतरापर्शुक पेशियाँ)
  • Tidal volume (ज्वारीय आयतन)
  • Vital capacity (जैव क्षमता)
  • Residual volume (अवशिष्ट आयतन)
  • Partial pressure (आंशिक दाब)
  • Oxyhaemoglobin (ऑक्सीहीमोग्लोबिन)
  • Oxygen dissociation curve (ऑक्सीजन वियोजन वक्र)
  • Carbonic anhydrase (कार्बोनिक एनहाइड्रेज़)
  • Pleura (फुप्फुसावरण)
  • Emphysema (वातस्फीति)

    • Common Mistakes

  • Saying expiration needs muscle contraction under normal conditions — normal expiration is due to relaxation of the diaphragm and intercostals.
  • Mixing up VC and TLC: TLC also includes the residual volume.
  • Writing that most CO₂ travels as carbamino-haemoglobin — about 70% travels as bicarbonate.
  • Thinking oxygen strongly controls breathing — CO₂ and H⁺ are the main chemical signals; O₂'s role is insignificant.
  • Placing the pneumotaxic centre in the medulla — it is in the pons.

    • 💡 Key Learning Points

    • ✓Compare respiratory organs across animal groups and trace the human air passage
    • ✓Explain inspiration and expiration in terms of muscle action and pressure gradients
    • ✓Define respiratory volumes and derive the pulmonary capacities
    • ✓Use partial pressures to explain gas exchange at alveoli and tissues
    • ✓Describe transport of O₂ and CO₂ and the neural regulation of breathing

    👨‍🏫 Teaching Tips

    • →Build a bell-jar lung model with balloons and a rubber-sheet diaphragm
    • →Measure students' vital capacity with a simple balloon or bottle-displacement spirometer
    • →Write Table 14.1 values on the board and draw arrows showing gas gradients
    • →Sketch the sigmoid oxygen dissociation curve and discuss shifts with pCO₂, H⁺ and temperature
    • →Have students count breaths per minute at rest and after exercise and relate to CO₂ levels
    • →Discuss masks for stone-crushing workers and the dangers of smoking (emphysema)

    📋 Assessment Questions

    1. Define vital capacity and give the volumes it includes.
    2. What is the functional residual capacity?
    3. Why does diffusion of gases occur only in the alveolar region?
    4. State the three ways CO₂ is transported in blood, with percentages.
    5. How does an increase in CO₂ and H⁺ in blood change the breathing rate?

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    Keep learning

    All chapters in Biology - Class 11

    1. 1. The Living World
    2. 2. Biological Classification
    3. 3. Plant Kingdom
    4. 4. Animal Kingdom
    5. 5. Morphology of Flowering Plants
    6. 6. Anatomy of Flowering Plants
    7. 7. Structural Organisation in Animals
    8. 8. Cell: The Unit of Life
    9. 9. Biomolecules
    10. 10. Cell Cycle and Cell Division
    11. 11. Photosynthesis in Higher Plants
    12. 12. Respiration in Plants
    13. 13. Plant Growth and Development
    14. 14. Breathing and Exchange of Gases
    15. 15. Body Fluids and Circulation
    16. 16. Excretory Products and their Elimination
    17. 17. Locomotion and Movement
    18. 18. Neural Control and Coordination
    19. 19. Chemical Coordination and Integration