Breathing and exchange of gases is a fundamental chapter in Class 11 Biology that explains how oxygen enters the body and carbon dioxide is removed. The chapter covers the structure of the respiratory system, the mechanism of breathing, transport of respiratory gases, and regulation of respiration. It is one of the most important topics for NEET, board examinations, and competitive entrance tests.
What is respiration?
Respiration is the process by which oxygen is utilized by cells to produce energy and carbon dioxide is released as a waste product.
Types of respiration
External respiration – exchange of gases between alveoli and blood.
Internal respiration – exchange of gases between blood and tissues.
Cellular respiration – oxidation of food molecules within cells to release energy.
Human respiratory system
The human respiratory system consists of:
External nostrils
Nasal chamber
Pharynx
Larynx
Trachea
Bronchi
Bronchioles
Alveoli
Lungs
Air enters through the nostrils and ultimately reaches the alveoli, where gaseous exchange occurs.
Structure of the lungs
Humans have a pair of lungs located in the thoracic cavity.
Right lung
Three lobes
Slightly larger
Left lung
Two lobes
Contains the cardiac notch
Each lung is covered by a double-layered pleura with pleural fluid that reduces friction during breathing.
Alveoli: the respiratory surface
Alveoli are tiny balloon-like sacs that provide a large surface area for gas exchange.
Adaptations of alveoli
Thin squamous epithelial walls
Rich capillary network
Moist surface
Enormous surface area (about 70–100 m²)
These features make diffusion of gases highly efficient.
Mechanism of breathing
Breathing consists of inspiration and expiration.
Inspiration
During inspiration:
Diaphragm contracts and flattens.
External intercostal muscles contract.
Thoracic volume increases.
Intrapulmonary pressure decreases.
Air enters the lungs.
Expiration
During expiration:
Diaphragm relaxes.
External intercostal muscles relax.
Thoracic volume decreases.
Intrapulmonary pressure increases.
Air leaves the lungs.
Normal expiration is mostly a passive process.
Respiratory volumes and capacities
Respiratory volumes
| Volume | Approximate value |
|---|---|
| Tidal volume (TV) | 500 mL |
| Inspiratory reserve volume (IRV) | 2500–3000 mL |
| Expiratory reserve volume (ERV) | 1000–1100 mL |
| Residual volume (RV) | 1100–1200 mL |
Respiratory capacities
Inspiratory capacity (IC) = TV + IRV
Functional residual capacity (FRC) = ERV + RV
Vital capacity (VC) = IRV + TV + ERV
Total lung capacity (TLC) = VC + RV
Vital capacity is an important indicator of lung function.
Exchange of gases
Gas exchange occurs by simple diffusion due to differences in partial pressure.
In the alveoli
Oxygen diffuses from alveoli into blood.
Carbon dioxide diffuses from blood into alveoli.
In tissues
Oxygen diffuses from blood into tissues.
Carbon dioxide diffuses from tissues into blood.
Transport of oxygen
About 97% of oxygen is transported by hemoglobin.
Formation of oxyhemoglobin
Hemoglobin + O₂ → Oxyhemoglobin
One hemoglobin molecule can carry four oxygen molecules.
The oxygen dissociation curve is sigmoidal, showing cooperative binding.
Factors affecting oxygen release
Oxygen release increases with:
Increased CO₂
Increased H⁺ concentration
Increased temperature
Increased 2,3-BPG
This is known as the Bohr effect.
Transport of carbon dioxide
Carbon dioxide is transported in three forms:
| Form | Percentage |
|---|---|
| Dissolved in plasma | 7% |
| Carbaminohemoglobin | 20–25% |
| Bicarbonate ions (HCO₃⁻) | 70% |
Most carbon dioxide is transported as bicarbonate ions.
Chloride shift
In RBCs, bicarbonate ions move into plasma while chloride ions enter RBCs to maintain electrical neutrality.
This exchange is called the chloride shift (Hamburger phenomenon).
Regulation of respiration
Respiration is controlled by the respiratory rhythm center in the medulla oblongata.
Major control centers
Medullary respiratory center
Pneumotaxic center in the pons
Important stimulus
The most important stimulus for respiration is increased carbon dioxide concentration in blood.
Peripheral chemoreceptors are located in:
Carotid bodies
Aortic bodies
Respiratory disorders
Asthma
Inflammation of airways
Bronchoconstriction
Difficulty in breathing
Emphysema
Destruction of alveolar walls
Reduced respiratory surface area
Common in smokers
Occupational respiratory disorders
Examples include:
Silicosis
Asbestosis
These result from long-term inhalation of harmful particles.
Important NCERT and NEET facts
Normal respiratory rate: 12–16 breaths/min
Tidal volume: 500 mL
Vital capacity: 3500–4500 mL
Most oxygen is transported as oxyhemoglobin.
Most carbon dioxide is transported as bicarbonate ions.
Gas exchange occurs by simple diffusion.
The respiratory center is located in the medulla oblongata.
Conclusion
Breathing and exchange of gases ensure continuous oxygen supply and carbon dioxide removal, which are essential for cellular respiration and energy production. Understanding lung structure, breathing mechanics, respiratory volumes, transport of gases, and regulation of respiration is crucial for success in Class 11 examinations and NEET.
References
NCERT. Biology, Class XI. National Council of Educational Research and Training, New Delhi.
Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
Tortora GJ, Derrickson B. Principles of Anatomy and Physiology. 16th ed. Wiley; 2021.
Barrett KE, et al. Ganong’s Review of Medical Physiology. McGraw-Hill Education; 2019.
West JB. Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.
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