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Q3:
Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
Solution :
1. Anatomical and Physiological Determinants of Gas Exchange
The exchange of gases ($O_2$ and $CO_2$) in the human respiratory system is governed by Fick’s Law of Diffusion, which dictates that the rate of diffusion is directly proportional to the surface area available for exchange and the concentration gradient, and inversely proportional to the thickness of the membrane [Per Fick’s Law: $J = -D \cdot A \cdot \frac{dc}{dx}$].
2. The Diffusion Membrane Structure
The respiratory membrane, where gas exchange occurs, is extremely thin, typically measuring less than $0.5 \, \mu m$ in thickness. This structure is composed of three primary layers:
- Alveolar Epithelium: A single layer of squamous epithelial cells.
- Endothelial Lining of Alveolar Capillaries: A single layer of endothelial cells.
- Basement Membrane: A thin, non-cellular layer sandwiched between the epithelium and the endothelium.
3. Structural Comparison: Alveoli vs. Conducting Zones
| Feature | Conducting Zone (Trachea, Bronchi, Bronchioles) | Respiratory Zone (Alveoli) |
|---|---|---|
| Wall Thickness | Thick; reinforced with cartilage and smooth muscle. | Extremely thin (squamous epithelium). |
| Function | Conduction, humidification, and filtration of air. | Passive diffusion of gases. |
| Surface Area | Low total surface area relative to volume. | Massive (approx. $70-100 \, m^2$ in adults). |
4. Why Diffusion is Restricted to the Alveoli
Diffusion is an inefficient process over long distances; it is only effective when the diffusion path ($dx$) is minimal. The conducting parts of the respiratory system (nose, pharynx, larynx, trachea, bronchi, and bronchioles) serve structural and protective roles:
- Structural Integrity: The presence of cartilage rings and thick muscular walls prevents airway collapse but simultaneously acts as an impenetrable barrier to gas molecules.
- Diffusion Barrier: The thickness of the conducting walls creates a distance ($dx$) so great that the rate of diffusion ($J$) approaches zero [By Fick’s Law, as $dx \to \infty, J \to 0$].
- Absence of Proximity: In the conducting zones, blood capillaries are not in immediate contact with the airway lumen, preventing the establishment of the necessary partial pressure gradients required for gas movement.
5. Conclusion
The alveolar region is the only site of gas exchange because it is the only component of the respiratory system that combines a massive surface area with a minimal diffusion path (thin membrane), allowing for the rapid attainment of equilibrium between alveolar air and capillary blood as dictated by the partial pressure differences ($PO_2 = 104 \, mmHg$ in alveoli vs $40 \, mmHg$ in deoxygenated blood).
Final Solution: The diffusion of gases is restricted to the alveolar region because it possesses the necessary histological characteristics—specifically, an extremely thin barrier and high surface-to-volume ratio—required to facilitate gas movement via passive diffusion, whereas the conducting airways are structurally designed for transport rather than exchange.
More Questions from Class 11 Biology Breathing and Exchange of Gases EXERCISES
- Q1: Define vital capacity. What is its significance?
- Q2: State the volume of air remaining in the lungs after a normal breathing.
- Q4: What are the major transport mechanisms for $CO_2$? Explain.
- Q5: What will be the $pO_2$ and $pCO_2$ in the atmospheric air compared to those in the alveolar air ?
- Q6: Explain the process of inspiration under normal conditions.
- Q7: How is respiration regulated?
- Q8: What is the effect of $pCO_2$ on oxygen transport?
- Q9: What happens to the respiratory process in a man going up a hill?
- Q10: What is the site of gaseous exchange in an insect?
- Q11: Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
- Q12: Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.
- Q13(a): Distinguish between (a) IRV and ERV
- Q13(b): Distinguish between (b) Inspiratory capacity and Expiratory capacity.
- Q13(c): Distinguish between (c) Vital capacity and Total lung capacity.
- Q14: What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
CBSE Solutions for Class 11 Biology Breathing and Exchange of Gases
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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