Quick Summary
Respiration involves breathing (gas exchange between body and environment) and cellular respiration (ATP production). The human respiratory system brings O₂ to cells and removes CO₂. Key topics: mechanics of breathing, lung volumes, haemoglobin-O₂ dissociation curve, and transport of gases. All are directly tested in NEET.
Human Respiratory System
Path of air: Nostrils → Nasal cavity → Pharynx → Larynx (voice box) → Trachea → Bronchi → Bronchioles → Alveoli
Alveoli: Thin-walled (single cell layer), moist, extensively vascular — site of gas exchange Lungs: Right lung (3 lobes), Left lung (2 lobes) Pleura: Double membrane surrounding lungs; intrapleural fluid reduces friction
Mechanism of Breathing
Inspiration (Inhalation)
- Diaphragm contracts → flattens → thoracic cavity volume increases
- External intercostal muscles contract → ribs move up and out
- Lung volume increases → pressure drops below atmospheric → air rushes in
Expiration (Exhalation)
- Diaphragm relaxes → moves up → thoracic cavity volume decreases
- Internal intercostal muscles (in forced expiration) → ribs move down and in
- Lung volume decreases → pressure above atmospheric → air pushed out
Normal breathing rate: 12–16 breaths/min at rest
Lung Volumes and Capacities
| Volume/Capacity | Abbreviation | Value | Definition |
|---|---|---|---|
| Tidal Volume | TV | 500 mL | Air per normal breath |
| Inspiratory Reserve Volume | IRV | ~3000 mL | Extra air after normal inhalation |
| Expiratory Reserve Volume | ERV | ~1100 mL | Extra air after normal exhalation |
| Residual Volume | RV | ~1200 mL | Air remaining after maximum exhalation |
| Vital Capacity | VC | ~4600 mL | TV + IRV + ERV |
| Total Lung Capacity | TLC | ~5800 mL | VC + RV |
| Functional Residual Capacity | FRC | ~2300 mL | ERV + RV |
| Inspiratory Capacity | IC | ~3500 mL | TV + IRV |
Gas Exchange
At alveoli (external respiration):
- pO₂ in alveoli (104 mmHg) > pO₂ in blood (40 mmHg) → O₂ diffuses into blood
- pCO₂ in blood (45 mmHg) > pCO₂ in alveoli (40 mmHg) → CO₂ diffuses out
At tissues (internal respiration):
- pO₂ in blood > pO₂ in tissues → O₂ diffuses to tissues
- pCO₂ in tissues > pCO₂ in blood → CO₂ diffuses into blood
Transport of Oxygen
Haemoglobin (Hb): Iron-containing protein in RBCs; 4 subunits (2α, 2β), 4 haem groups
- O₂ + Hb ⇌ HbO₂ (oxyhaemoglobin)
- O₂-Hb dissociation curve: Sigmoid (S-shaped) — cooperative binding
- Bohr effect: ↑CO₂ / ↓pH / ↑temperature → curve shifts right → more O₂ released to tissues
Transport of Carbon Dioxide
| Form | Amount | Mechanism |
|---|---|---|
| Bicarbonate (HCO₃⁻) | ~70% | CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (carbonic anhydrase); Cl⁻/HCO₃⁻ exchange (chloride shift/Hamburger phenomenon) |
| Carbaminohaemoglobin | ~23% | CO₂ + Hb → HbCO₂ |
| Dissolved in plasma | ~7% | Direct dissolution |
Regulation of Breathing
Respiratory centre: Medulla oblongata (rhythmicity centre) Factors that increase breathing rate:
- ↑CO₂ (pCO₂) — most important stimulus
- ↓O₂ (severe hypoxia)
- ↓pH
- Exercise
Respiratory Disorders
| Disorder | Description |
|---|---|
| Asthma | Inflammation and narrowing of bronchioles; wheezing |
| Emphysema | Alveolar wall damage (smoking); reduced surface area; breathlessness |
| Pneumonia | Alveoli filled with fluid (bacterial/viral); Streptococcus pneumoniae |
| Tuberculosis (TB) | Mycobacterium tuberculosis; granulomas in lungs |
| Occupational diseases | Silicosis, asbestosis — dust particles in lungs |
Important Questions
One Mark
- What is the tidal volume?
- Name the main respiratory centre in the brain.
- What is the Bohr effect?
- What percentage of CO₂ is transported as bicarbonate?
Short Answer (3 marks)
- Describe the mechanism of inspiration and expiration.
- What is vital capacity? How is it calculated? Give the normal value.
- Explain the transport of CO₂ in blood.
Long Answer (5 marks)
- Describe the transport of oxygen in blood. Explain the oxygen-haemoglobin dissociation curve and the Bohr effect.
- Write a labelled diagram of the human respiratory system and describe the process of gas exchange in the alveoli.
Common Mistakes
- Breathing rate is regulated mainly by CO₂ levels (not O₂) — CO₂ builds up faster and is the primary stimulus.
- Vital capacity does NOT include residual volume — RV remains always.
- Bohr effect: higher CO₂ → curve shifts RIGHT → O₂ MORE readily released (not retained).
- Chloride shift: HCO₃⁻ moves out of RBC into plasma; Cl⁻ moves in — occurs in capillaries near tissues.
Board Exam Tips
- Lung volumes table is a reliable 3-mark or 5-mark question — memorise all 8 values.
- "Explain CO₂ transport" is a standard 3-mark question — include all three forms with percentages.
- Mechanism of breathing (inspiration + expiration) with diaphragm role is a 3-mark question.
- NEET: O₂-Hb dissociation curve, Bohr effect, lung volumes, and CO₂ transport contribute 2–3 questions per paper.
Frequently Asked Questions
Breathing (pulmonary ventilation): physical process of inhaling and exhaling air. Respiration: biochemical process of breaking down glucose to produce ATP. Breathing supplies oxygen for respiration.