How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Respiration covers the respiratory system, gas exchange, and breathing mechanisms — focus on understanding the pathway and the physics of gas diffusion.
Key Concepts (Quick Recap)
Respiratory Pathway
Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Parts of the Respiratory System
| Part | Function |
|---|
| Nostrils / Nasal cavity | Filters, warms, and moistens air; mucus traps dust |
| Pharynx | Common passage for air and food (swallowing pushes epiglottis over larynx) |
| Larynx (voice box) | Contains vocal cords; produces sound |
| Epiglottis | Flap-like structure that prevents food from entering the trachea |
| Trachea (windpipe) | C-shaped cartilage rings prevent collapse; lined with ciliated epithelium |
| Bronchi | Two branches of trachea, one to each lung |
| Bronchioles | Smaller branches within the lungs |
| Alveoli | Tiny air sacs where gas exchange occurs; surrounded by capillaries |
Gas Exchange at the Alveoli
| Process | Direction | Reason |
|---|
| O₂ from alveolar air → Blood | Alveoli to blood | pO₂ higher in alveoli (104 mmHg) than in blood (40 mmHg) |
| CO₂ from blood → Alveolar air | Blood to alveoli | pCO₂ higher in blood (45 mmHg) than in alveoli (40 mmHg) |
Oxygen Transport
| Method | Percentage | Details |
|---|
| Oxyhaemoglobin (HbO₂) | ~97% | O₂ binds to haemoglobin in RBCs |
| Dissolved in plasma | ~3% | Small amount dissolved directly in blood |
Carbon Dioxide Transport
| Method | Percentage | Details |
|---|
| Bicarbonate ions (HCO₃⁻) | ~70% | CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (in RBCs) |
| Carbaminohaemoglobin | ~23% | CO₂ binds to haemoglobin (HbCO₂) |
| Dissolved in plasma | ~7% | Small amount dissolved directly |
Mechanism of Breathing
| Phase | Diaphragm | Rib Cage | Thoracic Volume | Lung Pressure | Air Flow |
|---|
| Inhalation (inspiration) | Contracts, flattens | Moves up and out | Increases | Decreases (below atmospheric) | Air rushes IN |
| Exhalation (expiration) | Relaxes, becomes dome-shaped | Moves down and in | Decreases | Increases (above atmospheric) | Air pushed OUT |
Lung Volumes and Capacities
| Volume/Capacity | Definition | Approximate Value |
|---|
| Tidal volume (TV) | Air inhaled/exhaled in normal breathing | ~500 mL |
| Inspiratory reserve volume (IRV) | Extra air that can be inhaled forcefully | ~2500-3000 mL |
| Expiratory reserve volume (ERV) | Extra air that can be exhaled forcefully | ~1000-1200 mL |
| Residual volume (RV) | Air remaining in lungs after forceful exhalation | ~1100-1200 mL |
| Vital capacity (VC) | Maximum air that can be exhaled after maximum inhalation | TV + IRV + ERV ≈ 4000-4500 mL |
| Total lung capacity (TLC) | Total air in lungs after maximum inhalation | VC + RV ≈ 5000-5700 mL |
Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|
| Oxygen | Required | Not required |
| Location | Mitochondria | Cytoplasm |
| Products | CO₂ + H₂O + 38 ATP | Lactic acid + 2 ATP (muscles) or Ethanol + CO₂ + 2 ATP (yeast) |
| Energy | 38 ATP per glucose | 2 ATP per glucose |
| Efficiency | High | Low |
| Equation | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP | C₆H₁₂O₆ → 2C₃H₆O₃ + 2 ATP (muscles) |
I. Choose the Best Answer (MCQs)
Q1. Gas exchange in the lungs occurs at the (a) Trachea (b) Bronchi (c) Bronchioles (d) Alveoli
Answer(d) Alveoli — Alveoli are tiny air sacs at the end of bronchioles, surrounded by a dense network of blood capillaries. Their thin walls (one cell thick) and large surface area allow efficient diffusion of O₂ into blood and CO₂ out of blood.
Q2. The percentage of oxygen transported as oxyhaemoglobin is (a) 3% (b) 7% (c) 23% (d) 97%
Answer(d) 97% — About 97% of oxygen is transported bound to haemoglobin as oxyhaemoglobin (HbO₂). The remaining 3% is dissolved in blood plasma.
Q3. The structure that prevents food from entering the trachea is (a) Glottis (b) Epiglottis (c) Larynx (d) Pharynx
Answer(b) Epiglottis — The epiglottis is a flap-like structure that covers the glottis (opening of the larynx) during swallowing, preventing food from entering the trachea.
Q4. During inhalation, the diaphragm (a) Relaxes and becomes dome-shaped (b) Contracts and flattens (c) Remains unchanged (d) Moves upward
Answer(b) Contracts and flattens — During inhalation, the diaphragm contracts and moves downward (flattens), increasing the volume of the thoracic cavity. This decreases the pressure inside the lungs, and air rushes in from outside.
Q5. Vital capacity is (a) Tidal volume + Residual volume (b) Tidal volume + IRV + ERV (c) Total lung capacity - Residual volume (d) Both (b) and (c)
Answer(d) Both (b) and (c) — Vital capacity = TV + IRV + ERV. It is also equal to Total Lung Capacity (TLC) minus Residual Volume (RV). It represents the maximum volume of air a person can exhale after a maximum inhalation.
II. Fill in the Blanks
1. The respiratory pathway is: Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli. 2. The flap that prevents food from entering the trachea is the epiglottis. 3. The voice box is called the larynx. 4. The windpipe is called the trachea. 5. Gas exchange occurs in the alveoli. 6. Oxygen is transported mainly as oxyhaemoglobin (HbO₂). 7. Carbon dioxide is transported mainly as bicarbonate ions (HCO₃⁻). 8. During inhalation, the diaphragm contracts and flattens. 9. During exhalation, the diaphragm relaxes and becomes dome-shaped. 10. The air remaining in the lungs after forceful exhalation is called residual volume. 11. The maximum volume of air that can be exhaled after maximum inhalation is vital capacity. 12. Aerobic respiration produces 38 ATP per glucose molecule.
III. True or False (with corrections for false statements)
Q1. Breathing and respiration are the same process.
AnswerFalse — Correction: Breathing and respiration are different. Breathing is the physical process of inhaling and exhaling air. Respiration is the chemical process of releasing energy from food using oxygen. Breathing is a physical process; respiration is a biochemical process.
Q2. The trachea is supported by C-shaped cartilage rings.
AnswerTrue — The trachea has C-shaped cartilage rings that prevent it from collapsing and keep the airway open. The open part of the C faces the oesophagus, allowing food to pass through the oesophagus without obstruction.
Q3. Carbon dioxide is transported mainly as carbaminohaemoglobin.
AnswerFalse — Correction: CO₂ is transported mainly as bicarbonate ions (HCO₃⁻) (~70%). Only about 23% is transported as carbaminohaemoglobin (HbCO₂), and 7% is dissolved in plasma.
Q4. Residual volume can be measured by a spirometer.
AnswerFalse — Correction: Residual volume (RV) cannot be measured by a spirometer because it is the air remaining in the lungs after forceful exhalation — it can never be exhaled and measured. It can only be estimated using other techniques (e.g., helium dilution method).
Q5. Anaerobic respiration produces more energy than aerobic respiration.
AnswerFalse — Correction: Aerobic respiration produces 38 ATP per glucose molecule. Anaerobic respiration produces only 2 ATP per glucose molecule. Aerobic respiration is far more efficient because it completely oxidises glucose.
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Trachea | (a) Air sacs for gas exchange | (b) Windpipe with C-shaped cartilage |
| 2. Alveoli | (b) Windpipe | (a) Tiny air sacs for gas exchange |
| 3. Epiglottis | (c) Voice production | (d) Prevents food entering trachea |
| 4. Larynx | (d) Prevents food entering trachea | (c) Voice box, produces sound |
| 5. Diaphragm | (e) Maximum air exhaled after max inhalation | (f) Muscular sheet separating thorax and abdomen |
| 6. Vital capacity | (f) Muscular sheet | (e) TV + IRV + ERV |
V. Answer Briefly (2-mark questions)
Q1. Distinguish between breathing and respiration.
Answer
| Feature | Breathing | Respiration | |---|---|---| | Nature | Physical process | Biochemical process | | Location | Respiratory tract | Mitochondria (aerobic) / Cytoplasm (anaerobic) | | Process | Inhalation and exhalation of air | Oxidation of glucose to release energy | | Energy | No energy released | Energy released (38 ATP in aerobic) | | Enzymes | Not involved | Enzymes required |
Q2. What are alveoli? How are they adapted for gas exchange?
Answer
Alveoli are tiny, balloon-like air sacs at the end of bronchioles in the lungs. They are adapted for gas exchange by: 1. Thin walls: Single-cell thick walls allow rapid diffusion 2. Large surface area: ~300 million alveoli provide ~70 m² surface area 3. Rich blood supply: Surrounded by dense capillary network 4. Moist lining: Thin film of water allows gases to dissolve before diffusing 5. Elastic walls: Allow alveoli to expand during inhalation and recoil during exhalation
Q3. What is vital capacity? State its components.
Answer
Vital capacity is the maximum volume of air a person can exhale after a maximum inhalation. Components: Vital Capacity = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV) Approximate value: 4000-4500 mL in a healthy adult.
Q4. How is carbon dioxide transported in the blood?
Answer
CO₂ is transported in three ways: 1. As bicarbonate ions (HCO₃⁻): ~70% — CO₂ enters RBCs, combines with water (catalysed by carbonic anhydrase) to form carbonic acid, which dissociates into H⁺ and HCO₃⁻ 2. As carbaminohaemoglobin (HbCO₂): ~23% — CO₂ binds directly to haemoglobin 3. Dissolved in plasma: ~7% — CO₂ dissolves directly in blood plasma
VI. Answer in Detail (5-mark questions)
Q1. Describe the mechanism of breathing (inhalation and exhalation).
Answer
Inhalation (Inspiration): 1. The diaphragm contracts and flattens (moves downward) 2. The external intercostal muscles contract, moving the rib cage upward and outward 3. The thoracic cavity volume increases 4. The lungs expand as the pressure inside them decreases (below atmospheric pressure) 5. Air rushes into the lungs through the nostrils 6. This is an active process (requires energy) Exhalation (Expiration): 1. The diaphragm relaxes and returns to its dome shape (moves upward) 2. The external intercostal muscles relax, and the rib cage moves downward and inward 3. The thoracic cavity volume decreases 4. The lungs are compressed and the pressure inside them increases (above atmospheric pressure) 5. Air is pushed out of the lungs 6. Normal exhalation is a passive process (no energy required); forced exhalation uses internal intercostal muscles and abdominal muscles Key point: Breathing is driven by pressure differences. Air always flows from higher pressure to lower pressure. During inhalation, lung pressure drops below atmospheric pressure. During exhalation, lung pressure rises above atmospheric pressure.
Q2. Explain the transport of oxygen and carbon dioxide in the blood.
Answer
Oxygen Transport: O₂ is transported in two ways: 1. As oxyhaemoglobin (HbO₂) — ~97%: In the lungs (high pO₂), O₂ binds to haemoglobin (Hb) in RBCs to form oxyhaemoglobin. In tissues (low pO₂), oxyhaemoglobin dissociates and releases O₂. - Haemoglobin + O₂ → Oxyhaemoglobin (in lungs) - Oxyhaemoglobin → Haemoglobin + O₂ (in tissues) 2. Dissolved in plasma — ~3%: A small amount of O₂ dissolves directly in blood plasma. Factors affecting O₂ binding to haemoglobin: - pO₂: Higher pO₂ (in lungs) promotes binding; lower pO₂ (in tissues) promotes release - pCO₂: Higher pCO₂ decreases O₂ binding (Bohr effect) - Temperature: Higher temperature decreases O₂ binding - pH: Lower pH (more acidic) decreases O₂ binding Carbon Dioxide Transport: CO₂ is transported in three ways: 1. As bicarbonate ions (HCO₃⁻) — ~70%: - CO₂ enters RBCs and combines with H₂O (catalysed by carbonic anhydrase) - CO₂ + H₂O → H₂CO₃ (carbonic acid) → H⁺ + HCO₃⁻ - HCO₃⁻ diffuses into plasma and is transported to the lungs - In lungs, the reaction reverses: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O (exhaled) 2. As carbaminohaemoglobin (HbCO₂) — ~23%: - CO₂ binds to haemoglobin at a different site than O₂ - Hb + CO₂ → HbCO₂ (in tissues) - HbCO₂ → Hb + CO₂ (in lungs, CO₂ is released) 3. Dissolved in plasma — ~7%: CO₂ dissolves directly in blood plasma.
Q3. Compare aerobic and anaerobic respiration.
Answer
| Feature | Aerobic Respiration | Anaerobic Respiration | |---|---|---| | Oxygen | Required | Not required | | Location | Mitochondria | Cytoplasm | | Glucose breakdown | Complete | Incomplete | | Products | CO₂ + H₂O | Lactic acid (muscles) or Ethanol + CO₂ (yeast) | | Energy released | 38 ATP per glucose | 2 ATP per glucose | | Efficiency | High (~40%) | Low (~2.1%) | | Equation (muscles) | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP | C₆H₁₂O₆ → 2C₃H₆O₃ + 2 ATP | | Equation (yeast) | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP | | Occurs in | Most organisms always | Muscles during intense exercise; yeast during fermentation | | Stages | Glycolysis → Krebs cycle → ETC | Glycolysis only | Oxygen Debt: During intense exercise, muscles respire anaerobically, producing lactic acid. After exercise, extra O₂ is needed to break down lactic acid — this is called oxygen debt. This is why we breathe heavily after exercise.
Common Mistakes to Avoid
- Confusing breathing and respiration: Breathing = physical process (inhalation/exhalation). Respiration = chemical process (glucose → energy). They are NOT the same.
- Saying CO₂ is mainly transported as carbaminohaemoglobin: CO₂ is mainly transported as bicarbonate ions (HCO₃⁻) — about 70%. Only 23% is as carbaminohaemoglobin.
- Confusing inhalation and exhalation diaphragm action: Inhalation = diaphragm contracts and flattens (moves DOWN). Exhalation = diaphragm relaxes and becomes dome-shaped (moves UP).
- Saying residual volume can be measured: Residual volume CANNOT be measured by spirometry because it is the air that remains in the lungs after maximum exhalation — it can never be exhaled.
- Confusing aerobic and anaerobic respiration products: Aerobic = CO₂ + H₂O + 38 ATP. Anaerobic in muscles = Lactic acid + 2 ATP. Anaerobic in yeast = Ethanol + CO₂ + 2 ATP.
- Forgetting the epiglottis function: The epiglottis prevents FOOD from entering the trachea during swallowing. Without it, food would enter the trachea and cause choking.
Exam Checklist
- [ ] Can you trace the path of air from nostrils to alveoli?
- [ ] Can you explain gas exchange at the alveoli (partial pressure differences)?
- [ ] Can you describe the mechanism of inhalation and exhalation (diaphragm, rib cage, pressure)?
- [ ] Can you explain oxygen transport (oxyhaemoglobin and dissolved O₂)?
- [ ] Can you explain CO₂ transport (bicarbonate, carbaminohaemoglobin, dissolved)?
- [ ] Can you define tidal volume, vital capacity, residual volume, and total lung capacity?
- [ ] Can you distinguish between breathing and respiration?
- [ ] Can you compare aerobic and anaerobic respiration?
- [ ] Can you explain the role of the epiglottis, C-shaped cartilage rings, and alveolar adaptations?
- [ ] Can you explain oxygen debt and lactic acid accumulation?
