CBSE Class 11 Biology – Breathing and Exchange of Gases | NCERT-Based Study Module & Revision Notes

Course Details

• Course: CBSE Class 11 Biology
• Prescribed By: Central Board of Secondary Education
• Based On: NCERT Textbook

Unit V: Human Physiology

Chapter 17: Breathing and Exchange of Gases

CBSE Board Examination Relevance

• Unit V carries high weightage in CBSE Class 11 Biology theory papers
• Frequently tested through:
  • Very Short Answer Questions (VSA)
  • Short Answer Questions (SA)
  • Assertion–Reason questions
  • Case-based questions
• Important for Class 12 Human Physiology foundation and competitive exams (NEET, CUET)

Introduction to Breathing and Exchange of Gases

Respiration is a vital physiological process through which living organisms obtain energy necessary for survival. In humans, respiration involves breathing, the physical process of inhalation and exhalation, and exchange of gases, where oxygen and carbon dioxide are exchanged between the body and the environment.

This chapter explains:

• Structure and function of respiratory organs
• Mechanism of breathing
• Transport of respiratory gases in blood

Understanding these concepts is essential for linking respiration with circulation, metabolism, and cellular respiration.

1. Respiratory Organs

The human respiratory system is a complex network of organs and structures designed to facilitate gaseous exchange.

1.1 Types of Respiration

• External Respiration: Exchange of gases between lungs and blood
• Internal Respiration: Exchange of gases between blood and tissues
• Cellular Respiration: Oxidation of food within cells to release energy

1.2 Human Respiratory System – Components

The respiratory system consists of:

(a) External Nostrils

• Openings that allow air to enter the nasal cavity
• Lined with hair to filter dust particles

(b) Nasal Cavity

• Divided by nasal septum
• Warms, moistens, and filters incoming air
• Richly supplied with blood vessels

(c) Pharynx

• Common passage for air and food
• Connects nasal cavity to larynx

(d) Larynx (Voice Box)

• Produces sound
• Prevents food from entering trachea with the help of epiglottis

(e) Trachea

• Tube supported by C-shaped cartilaginous rings
• Prevents collapse during breathing

(f) Bronchi and Bronchioles

• Trachea divides into right and left bronchi
• Bronchi subdivide into bronchioles inside lungs

(g) Alveoli

• Thin-walled, sac-like structures
• Primary site of gaseous exchange
• Approximately 300 million alveoli in human lungs
• Provide a large surface area for diffusion

1.3 Structure of Lungs

• Paired, spongy organs located in thoracic cavity
• Covered by pleural membrane
• Right lung has three lobes, left lung has two lobes
• Pleural fluid reduces friction during breathing

2. Mechanism of Breathing

Breathing involves rhythmic movements that bring about inhalation and exhalation.

2.1 Definition of Breathing

Breathing is the mechanical process of taking in oxygen-rich air and expelling carbon dioxide-rich air.

2.2 Inspiration (Inhalation)

Inspiration is an active process involving muscle contraction.

Steps in Inspiration

• Diaphragm contracts and flattens
• External intercostal muscles contract
• Rib cage moves upward and outward
• Thoracic cavity volume increases
• Intrapulmonary pressure decreases
• Air enters lungs

2.3 Expiration (Exhalation)

Expiration is usually a passive process.

Steps in Expiration

• Diaphragm relaxes and becomes dome-shaped
• Intercostal muscles relax
• Rib cage moves downward and inward
• Thoracic volume decreases
• Intrapulmonary pressure increases
• Air is forced out of lungs

2.4 Breathing Movements and Pressures

• Intrapulmonary Pressure: Pressure inside lungs
• Intrapleural Pressure: Pressure between pleural membranes
• Air moves from higher pressure to lower pressure

2.5 Lung Volumes and Capacities

Understanding lung volumes is essential for CBSE exams.

Major Lung Volumes

• Tidal Volume (TV): Volume of air inhaled or exhaled during normal breathing (~500 mL)
• Inspiratory Reserve Volume (IRV): Extra air inhaled forcefully
• Expiratory Reserve Volume (ERV): Extra air exhaled forcefully
• Residual Volume (RV): Air remaining after maximum expiration

Lung Capacities

• Inspiratory Capacity (IC)
• Functional Residual Capacity (FRC)
• Vital Capacity (VC)
• Total Lung Capacity (TLC)

2.6 Regulation of Respiration

Breathing is regulated by:

• Medullary respiratory centers
• Pneumotaxic center in pons
• Chemoreceptors sensitive to:
  • Carbon dioxide concentration
  • Hydrogen ion concentration

Increase in CO₂ stimulates faster breathing.

3. Exchange of Gases

Exchange of gases occurs by simple diffusion.

3.1 Exchange at Alveoli

• Oxygen diffuses from alveoli to blood
• Carbon dioxide diffuses from blood to alveoli
• Driven by partial pressure gradients

Partial Pressure

• Pressure exerted by individual gases in a mixture
• Represented as:
  • pO₂ for oxygen
  • pCO₂ for carbon dioxide

3.2 Factors Affecting Exchange of Gases

• Partial pressure difference
• Surface area of alveoli
• Thickness of respiratory membrane
• Solubility of gases

3.3 Exchange at Tissues

• Oxygen diffuses from blood to tissues
• Carbon dioxide diffuses from tissues to blood

4. Transport of Gases

Once exchanged, gases must be transported efficiently in blood.

4.1 Transport of Oxygen

Oxygen is transported in two forms:

(a) As Oxyhaemoglobin (≈97%)

• Oxygen binds reversibly with haemoglobin
• Forms oxyhaemoglobin

(b) Dissolved in Plasma (≈3%)

4.2 Oxygen Dissociation Curve

• Graph showing relationship between pO₂ and haemoglobin saturation
• Sigmoid (S-shaped) curve

Factors Affecting Oxygen Binding

• Partial pressure of oxygen
• Partial pressure of carbon dioxide
• Temperature
• pH (Bohr effect)

4.3 Transport of Carbon Dioxide

Carbon dioxide is transported in three forms:

(a) As Bicarbonate Ions (≈70%)

• Formed in RBCs with the help of carbonic anhydrase

(b) As Carbaminohaemoglobin (≈20–25%)

• CO₂ binds with haemoglobin

(c) Dissolved in Plasma (≈5–7%)

4.4 Chloride Shift

• Exchange of bicarbonate and chloride ions between plasma and RBCs
• Maintains ionic balance

5. Disorders of Respiratory System (Exam-Relevant)

(a) Asthma

• Inflammation of bronchi
• Causes difficulty in breathing

(b) Emphysema

• Damage to alveolar walls
• Reduced respiratory surface

(c) Occupational Respiratory Disorders

• Caused by inhalation of dust (silica, asbestos)
• Leads to fibrosis

Quick Revision Notes (Exam-Ready Points)

• Alveoli provide large surface area for gas exchange
• Breathing is different from respiration
• Oxygen transport depends on haemoglobin
• CO₂ is mainly transported as bicarbonate ions
• Medulla oblongata controls breathing rhythm
• Partial pressure gradient drives diffusion of gases

NCERT Alignment Confirmation

✔ Content strictly based on NCERT Class 11 Biology
✔ Terminology and definitions match textbook language
✔ CBSE exam pattern fully considered
✔ No extra or out-of-syllabus concepts included

Conclusion

The chapter “Breathing and Exchange of Gases” forms a critical component of Human Physiology in CBSE Class 11 Biology. A clear understanding of respiratory organs, breathing mechanics, and gas transport not only ensures strong performance in board examinations but also builds a foundation for advanced biological concepts in Class 12 and competitive exams.