Question
Explain the mechanism of breathing. What happens during inspiration (inhalation) and expiration (exhalation)?
Solution — Step by Step
Breathing works because air flows from high pressure to low pressure. To make air flow INTO the lungs (inspiration), we lower lung pressure below atmospheric. To push air OUT (expiration), we raise lung pressure above atmospheric.
We achieve these pressure changes by changing the volume of the thoracic cavity. Volume and pressure are inversely related at constant temperature (Boyle’s Law): increase volume → decrease pressure, and vice versa.
During inspiration (breathing in):
- Diaphragm contracts → moves downward (flattens from dome shape)
- External intercostal muscles contract → ribs move upward and outward
- Thoracic volume increases in all dimensions (front-back, side-to-side, top-bottom)
- Lungs expand (they’re elastic and follow the thoracic walls)
- Intrapulmonary pressure drops below atmospheric (~2 mmHg below, i.e., ~758 mmHg)
- Air flows from atmosphere (760 mmHg) into lungs (758 mmHg) — down the pressure gradient
Inspiration is an active process requiring muscular contraction and energy (ATP).
Normal tidal volume (air breathed in each breath at rest) = ~500 mL
During expiration (breathing out) at rest:
- Diaphragm relaxes → returns to dome shape (moves up)
- External intercostal muscles relax → ribs return to resting position
- Thoracic volume decreases
- Lungs recoil elastically (like releasing a stretched rubber band)
- Intrapulmonary pressure rises above atmospheric (~762 mmHg)
- Air flows from lungs (high pressure) to atmosphere (low pressure)
Normal expiration is passive — no muscle contraction needed. Elastic recoil of lung tissue drives it.
Forced expiration (during exercise, blowing, coughing): Internal intercostal muscles and abdominal muscles contract to actively squeeze air out faster.
| Volume | Description | Value |
|---|---|---|
| Tidal Volume (TV) | Normal breath in/out | ~500 mL |
| Inspiratory Reserve Volume (IRV) | Extra air inhaled forcefully | ~3000 mL |
| Expiratory Reserve Volume (ERV) | Extra air expelled forcefully | ~1100 mL |
| Residual Volume (RV) | Air remaining after max expiration | ~1200 mL |
| Vital Capacity | TV + IRV + ERV | ~4600 mL |
| Total Lung Capacity | Vital capacity + RV | ~5800 mL |
Why This Works
The lungs have no muscle of their own — they are elastic sacs. They depend entirely on the thoracic walls and diaphragm to change their volume. The pleural fluid between lung surface and chest wall creates surface tension that keeps the lungs in contact with the chest wall, ensuring lung volume changes with thoracic volume changes.
The elegance of the system: inspiration requires active work (muscles contract), but normal expiration is free — the lungs release the elastic energy stored during inspiration. This is energy-efficient for an organ that operates continuously, ~15 times per minute, for your entire life.
For CBSE and NEET: the diaphragm moves DOWN during inspiration (contracts, flattens) and UP during expiration (relaxes, domes back). Students draw this backwards frequently. Remember: inspiration = volume UP = pressure DOWN = air in. The diaphragm moving down INCREASES thoracic volume.
Common Mistake
Students write “during inspiration, diaphragm relaxes and moves up.” This is completely backward. During INSPIRATION, the diaphragm contracts and moves DOWN — this increases thoracic volume and decreases pressure, drawing air in. The diaphragm moves UP during EXPIRATION (relaxation, passive). Getting this wrong in a 5-mark CBSE question loses 2 marks on the diagram alone.