Breathing & Exchange of Gases
NCERT Line-by-Line Breakdown for NEET 2026
Unit 5: Human PhysiologyAs per the NEET 2026 syllabus, this unit begins with the respiratory system. We need oxygen for the breakdown of nutrients (like glucose) to derive energy. In this process, CO₂ is released. [cite_start]This chapter covers the Respiratory Organs, the Mechanism of Breathing, and how Gases are Transported in our body [cite: 335-336].
1. Respiratory Organs
A. Diversity in Breathing (Recall Only)
[cite_start]Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organization[cite: 335].
- General Body Surface: Sponges, Coelenterates, Flatworms.
- Moist Cuticle (Skin): Earthworms, Frogs (Cutaneous respiration).
- Tracheal Tubes: Insects.
- Gills (Branchial): Aquatic Arthropods, Molluscs, Fish.
- Lungs (Pulmonary): Reptiles, Birds, Mammals.
B. Human Respiratory System
Path of Air: Nostrils → Nasal chamber → Pharynx (Common passage) → Larynx (Sound box) → Trachea → Bronchi → Bronchioles → Alveoli.
[Image of human respiratory system]- Epiglottis: Cartilaginous flap covering Glottis to prevent food entry.
- Trachea: Supported by incomplete C-shaped cartilaginous rings (up to initial bronchioles).
- Alveoli: Thin, irregular-walled, vascularized bags (Site of exchange).
- Pleura: Double-layered membrane covering lungs. Pleural fluid reduces friction.
2. Mechanism of Breathing
Breathing involves two stages: Inspiration and Expiration. It is driven by pressure gradients generated by the Diaphragm and Intercostal Muscles.
| Feature | Inspiration (Active) | Expiration (Passive) |
|---|---|---|
| Diaphragm | Contracts (Flattens). Volume increases in Antero-posterior axis. | Relaxes (Dome-shaped). Volume decreases. |
| External Intercostal Muscles | Contracts (Ribs move Out & Up). Volume increases in Dorso-ventral axis. | Relaxes (Ribs move In & Down). |
| Intra-pulmonary Pressure | Decreases (Less than atmospheric). | Increases (More than atmospheric). |
Respiratory Volumes & Capacities [cite: 336]
- Tidal Volume (TV): Normal breath (~500 mL).
- Inspiratory Reserve Volume (IRV): Forcible inspiration (~2500-3000 mL).
- Expiratory Reserve Volume (ERV): Forcible expiration (~1000-1100 mL).
- Residual Volume (RV): Air remaining after forcible expiration (~1100-1200 mL).
- Vital Capacity (VC): ERV + TV + IRV (Max air breathed in after forced expiration).
- Total Lung Capacity (TLC): VC + RV.
3. Exchange of Gases
Primary site: Alveoli. Exchange occurs by simple diffusion based on pressure/concentration gradient.
🌫️ Diffusion Membrane (Thickness < 1mm)
- Squamous epithelium of Alveoli.
- Endothelium of alveolar capillaries.
- Basement substance in between.
Partial Pressures (mm Hg)
| Gas | Atmosphere | Alveoli | Deoxygenated Blood | Oxygenated Blood | Tissues |
|---|---|---|---|---|---|
| O₂ | 159 | 104 | 40 | 95 | 40 |
| CO₂ | 0.3 | 40 | 45 | 40 | 45 |
Solubility of CO₂ is 20-25 times higher than O₂.
4. Transport of Gases
A. Transport of Oxygen
- 97%: Bound to Haemoglobin (RBCs) as Oxyhaemoglobin.
- 3%: Dissolved in Plasma.
Oxygen Dissociation Curve: Sigmoid shape.
[Image of oxygen dissociation curve]Low pO₂, High pCO₂, High H⁺ (Low pH), High Temperature. (Occurs in Tissues).
B. Transport of Carbon Dioxide
- 20-25%: As Carbamino-haemoglobin (bound to Amine group).
- 70%: As Bicarbonate (HCO₃⁻). Enzyme: Carbonic Anhydrase (very high conc. in RBC).
- 7%: Dissolved in Plasma.
5. Regulation & Disorders
A. Regulation of Respiration
Neural System modulates breathing:
- Respiratory Rhythm Center (Medulla): Primarily responsible.
- Pneumotaxic Center (Pons): Moderates the functions. Can reduce duration of inspiration → alter respiratory rate.
- Chemosensitive Area: Adjacent to rhythm center. Highly sensitive to CO₂ and H⁺ (Not O₂).
B. Disorders [cite: 337]
Difficulty in breathing causing wheezing. Due to inflammation of bronchi and bronchioles.
Alveolar walls are damaged → Respiratory surface decreased. Major cause: Cigarette Smoking.
Silicosis, Asbestosis. Long exposure to dust leads to inflammation and fibrosis (proliferation of fibrous tissues).
📝 Rapid Fire MCQs
Q1. The pneumotaxic center is located in:
- A) Cerebrum
- B) Medulla oblongata
- C) Pons varolii
- D) Cerebellum
Click to check Answer
Answer: C) Pons varolii.
Q2. Which of the following conditions shifts the oxyhaemoglobin dissociation curve to the right?
- A) High pO₂
- B) Low pCO₂
- C) High H⁺ concentration
- D) Low Temperature
Click to check Answer
Answer: C) High H⁺ concentration (Low pH).