Chapter 5 — Breathing and Respiration
Key Concepts Quick Recap
| Term | Definition |
|---|
| Breathing (ventilation) | Mechanical process of inhaling and exhaling air |
| Respiration | Biochemical process of energy release from organic molecules |
| Tidal volume (TV) | Air exchanged in normal breathing (~500 mL) |
| Vital capacity (VC) | Maximum air expelled after maximum inspiration (IRV + TV + ERV) |
| Total lung capacity (TLC) | Vital capacity + Residual volume |
| Residual volume (RV) | Air remaining after maximum expiration (~1200 mL) |
| Dead space | Air in conducting passages — not involved in gas exchange |
| Oxyhaemoglobin | Hb + O₂; formed in lungs |
| Carboxyhaemoglobin | Hb + CO; formed with carbon monoxide (poisonous) |
| Haemoglobin | Iron-containing red pigment in RBC; carries O₂ and CO₂ |
I. Choose the Best Answer (MCQs)
1. The normal breathing rate in adults is: (a) 5-10 breaths/min (b) 12-20 breaths/min (c) 25-35 breaths/min (d) 40-50 breaths/min
Answer(b) 12-20 breaths/min Normal resting breathing rate (eupnoea) = 12–20 breaths per minute. Increases during exercise, fever, and at altitude. Each breath moves ~500 mL of air (tidal volume).
2. The diaphragm contracts during: (a) Forced expiration (b) Quiet expiration (c) Inspiration (d) Both inspiration and expiration
Answer(c) Inspiration During inspiration: diaphragm contracts (flattens), external intercostal muscles contract → thoracic cavity volume increases → intrapulmonary pressure drops below atmospheric → air flows in. During expiration: diaphragm and intercostals relax → thoracic cavity decreases → air pushed out.
3. The site of gas exchange in the lungs is: (a) Bronchi (b) Bronchioles (c) Alveoli (d) Trachea
Answer(c) Alveoli Alveoli are thin-walled (simple squamous epithelium) air sacs surrounded by dense capillary network. Massive surface area (~70–80 m²). O₂ diffuses from alveoli into blood; CO₂ from blood into alveoli.
4. The oxygen-haemoglobin dissociation curve is: (a) Linear (b) Sigmoid (S-shaped) (c) Hyperbolic (d) Exponential
Answer(b) Sigmoid (S-shaped) The sigmoidal shape reflects cooperative binding of O₂ to Hb. Binding of first O₂ increases affinity for next O₂ (positive cooperativity). High pO₂ (lungs) → Hb fully saturated. Low pO₂ (tissues) → Hb releases O₂. Shift factors: temperature↑, CO₂↑, pH↓ → right shift (more O₂ released to tissues).
5. CO₂ is transported in blood mainly as: (a) Dissolved CO₂ in plasma (b) Carbaminohaemoglobin (c) Bicarbonate ions (HCO₃⁻) (d) Carbonic acid
Answer(c) Bicarbonate ions (HCO₃⁻) CO₂ transport:
- 70% as HCO₃⁻ (bicarbonate) in plasma.
- 20-25% as carbaminohaemoglobin (CO₂ + Hb → HbCO₂).
- 7% dissolved in plasma.
In RBC: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (carbonic anhydrase enzyme).
6. Which of the following is a respiratory disorder characterised by narrowing of airways? (a) Emphysema (b) Asthma (c) Pneumonia (d) Silicosis
Answer(b) Asthma Asthma: chronic inflammatory disorder → airway hypersensitivity → bronchoconstriction + mucus secretion → wheezing, breathlessness. Triggered by allergens, exercise, cold air. Treated with bronchodilators (salbutamol) and anti-inflammatories (corticosteroids).
7. Haemoglobin has a higher affinity for: (a) O₂ than CO₂ (b) CO₂ than O₂ (c) CO than O₂ (d) N₂ than O₂
Answer(c) CO than O₂ Haemoglobin has ~250 times higher affinity for CO (carbon monoxide) than O₂. CO binds to the same haem sites → forms carboxyhaemoglobin → prevents O₂ transport → death by asphyxiation. CO is produced by incomplete combustion (vehicle exhaust, faulty heaters) — colourless, odourless, deadly.
II. Short Answer Questions
Q1. Describe the respiratory volumes and capacities.
Answer
| Volume/Capacity | Definition | Value |
|---|
| Tidal Volume (TV) | Air inspired/expired in normal breathing | ~500 mL |
| Inspiratory Reserve Volume (IRV) | Extra air inspired after normal inspiration | 2500–3000 mL |
| Expiratory Reserve Volume (ERV) | Extra air expired after normal expiration | 1000–1100 mL |
| Residual Volume (RV) | Air left after maximum expiration | 1100–1200 mL |
| Inspiratory Capacity (IC) | TV + IRV | ~3000–3500 mL |
| Expiratory Capacity (EC) | TV + ERV | ~1500 mL |
| Functional Residual Capacity (FRC) | ERV + RV | ~2300 mL |
| Vital Capacity (VC) | IRV + TV + ERV | ~4500 mL |
| Total Lung Capacity (TLC) | VC + RV | ~5800 mL |
Significance of RV: Residual volume prevents lungs from collapsing completely (atelectasis). Also keeps alveoli partially open for continued gas exchange between breaths.
Q2. Describe how oxygen is transported in the blood.
1. Bound to haemoglobin (~98%):
Answer
- O₂ + Hb (deoxyhaemoglobin) → HbO₂ (oxyhaemoglobin) (in lungs)
- HbO₂ → Hb + O₂ (in tissues)
- Each Hb molecule has 4 haem groups → carries 4 O₂ molecules.
- Binding is cooperative → sigmoidal dissociation curve.
Factors favouring O₂ release in tissues (Bohr effect):
→ Shift the dissociation curve to the right (Bohr effect) → more O₂ released to active tissues.
- High CO₂ (↑ pCO₂)
- Low pH (acidic)
- High temperature
2. Dissolved in plasma (~2%):
Answer
- Very small amount dissolved directly.
3. Oxygen dissociation curve:
Answer
- At high pO₂ (100 mmHg in alveoli): Hb ~97% saturated.
- At low pO₂ (40 mmHg in tissues): Hb ~75% saturated → releases ~25% of O₂ to tissues.
- At rest: 25% O₂ released; during exercise: up to 75% released.
Q3. What is the mechanism of breathing (inspiration and expiration)?
Answer
Inspiration (active process): 1. Diaphragm contracts → flattens downward. 2. External intercostal muscles contract → ribs move up and out. 3. Thoracic cavity volume increases. 4. Lung volume increases → intrapulmonary pressure falls below atmospheric (760 → ~758 mmHg). 5. Air flows into lungs (high pressure → low pressure).
Expiration (passive at rest): 1. Diaphragm and intercostal muscles relax. 2. Thoracic volume decreases. 3. Lung recoil (elastic tissue) → intrapulmonary pressure rises above atmospheric. 4. Air flows out.
Forced expiration: Internal intercostal muscles contract; abdominal muscles push diaphragm up → more air expelled.
Neural control: Rhythmicity centre in medulla oblongata sets basic breathing rhythm. Chemoreceptors in medulla and aortic arch detect CO₂ and H⁺ levels → signal respiratory centre to adjust rate and depth.
Common Mistakes
- Saying diaphragm relaxes during inspiration — diaphragm CONTRACTS during inspiration.
- Confusing CO transport (carboxyhaemoglobin, toxic) with CO₂ transport (carbaminohaemoglobin, bicarbonate, normal).
- Saying O₂ is transported mainly dissolved in plasma — 98% is bound to Hb.
- Forgetting that Bohr effect shifts dissociation curve RIGHT (more O₂ released to tissues at low pH/high CO₂).
Exam Checklist
- Respiratory volumes: TV=500, IRV=2500-3000, ERV=1000-1100, RV=1200 mL.
- VC = IRV + TV + ERV; TLC = VC + RV.
- O₂ transport: 98% as HbO₂; sigmoidal curve; Bohr effect.
- CO₂ transport: 70% as HCO₃⁻; 20-25% as carbaminoHb; 7% dissolved.
- Gas exchange in alveoli: simple squamous epithelium; 70-80 m² surface.
- CO has 250× more affinity than O₂ for Hb → carboxyhaemoglobin.
