Question
Explain the mechanism of breathing in humans, covering both inspiration (inhalation) and expiration (exhalation). What role do the diaphragm and intercostal muscles play?
Solution — Step by Step
Breathing works on a simple pressure principle: air moves from high pressure to low pressure. Our respiratory muscles don’t pull air in — they create a low-pressure zone inside the thoracic cavity, and atmospheric air rushes in to equalise.
During inspiration, two sets of muscles contract simultaneously:
- Diaphragm contracts and flattens downward
- External intercostal muscles contract, pulling the ribs upward and outward
This increases the volume of the thoracic cavity in all three dimensions — top-to-bottom (diaphragm) and side-to-side (ribs). By Boyle’s Law, volume ↑ means pressure ↓.
The intrapulmonary pressure (pressure inside lungs) drops to about 2-3 mmHg below atmospheric pressure. Since the outside air is now at higher pressure, it flows into the lungs through the respiratory tract.
Normal quiet inspiration moves about 500 mL of air — this is the tidal volume.
During quiet expiration, the diaphragm and external intercostals simply relax. The elastic recoil of the lung tissue and the thoracic wall restores the original volume.
As volume decreases, intrapulmonary pressure rises to 2-3 mmHg above atmospheric, and air is pushed out.
During exercise or forced breathing, expiration becomes active. The internal intercostal muscles and abdominal muscles contract to further compress the thoracic cavity, driving out more air.
This is why singers and athletes train breathing muscles — passive recoil isn’t enough for high-demand situations.
Why This Works
The entire mechanism is an application of Boyle’s Law ( at constant temperature). When thoracic volume increases, pressure inside drops. When it decreases, pressure rises. The lungs behave like elastic balloons suspended in this pressure chamber — they don’t have their own muscles.
The pleural fluid between the visceral and parietal pleurae plays a critical supporting role. It creates surface tension that keeps the lungs adhered to the thoracic wall, so when the thorax expands, the lungs are pulled along with it.
Inspiration: air flows in
Expiration: air flows out
Alternative Method — Remembering via “CAVE”
For NEET MCQs where you need to recall which muscles do what, use this framework:
| Action | Muscles | Volume Change |
|---|---|---|
| Quiet Inspiration | Diaphragm + External intercostals | ↑ |
| Quiet Expiration | Relaxation only (passive) | ↓ |
| Forced Expiration | Internal intercostals + Abdominals | ↓↓ |
NEET frequently asks: “Which muscles are involved in normal quiet expiration?” The answer is none — it’s passive. Students who write “internal intercostals” for quiet expiration lose easy marks. Internal intercostals only activate during forced expiration.
Common Mistake
Students often write that the lungs expand because the muscles “pull” them open. This is wrong — the lungs have no skeletal muscle. What actually happens is that the thoracic cavity expands, intrapulmonary pressure drops, and the lungs (which are stuck to the thoracic wall via pleural fluid) passively expand as atmospheric air pushes in. The pressure gradient does the work, not any direct muscular action on lung tissue.
This distinction appears directly in NEET questions phrased as “The expansion of lungs during inspiration is due to…”