Question
Describe the mechanism of breathing in humans. Explain the roles of the diaphragm, intercostal muscles, and pressure changes during inspiration and expiration.
(NCERT Class 11, frequently asked in NEET)
Solution — Step by Step
Breathing is driven by pressure differences between the atmosphere and the lungs. Air moves from high pressure to low pressure. The body creates this pressure gradient by changing the volume of the thoracic cavity.
Boyle’s Law is the physics behind it: when volume increases, pressure decreases (and vice versa).
During inspiration:
- The diaphragm contracts and flattens (moves downward), increasing the vertical dimension of the thoracic cavity.
- The external intercostal muscles contract, pulling the ribs upward and outward, increasing the anteroposterior and lateral dimensions.
- This increases thoracic volume, which increases pulmonary volume.
- Increased volume causes intra-pulmonary pressure to drop below atmospheric pressure (by about 2 mmHg).
- Air rushes into the lungs down the pressure gradient.
During normal (quiet) expiration:
- The diaphragm relaxes and domes upward, reducing thoracic volume.
- The external intercostal muscles relax, allowing the ribs to move downward and inward due to gravity and elastic recoil.
- Thoracic volume decreases, compressing the lungs.
- Intra-pulmonary pressure rises above atmospheric pressure (by about 2 mmHg).
- Air is pushed out of the lungs.
Normal expiration is largely a passive process — it relies on elastic recoil of lung tissue and relaxation of muscles. Forced expiration additionally involves contraction of internal intercostal muscles and abdominal muscles.
The pleural cavity (between the two pleural membranes surrounding the lungs) contains a thin film of fluid and maintains a negative pressure (sub-atmospheric). This negative intrapleural pressure prevents the lungs from collapsing and keeps them expanded against the thoracic wall.
During inspiration, intrapleural pressure becomes even more negative (about mmHg), helping lung expansion.
Why This Works
The entire mechanism is an application of Boyle’s Law ( at constant temperature). The respiratory muscles do not directly squeeze or inflate the lungs — they change the thoracic cavity volume, and the lungs passively follow because they are attached to the thoracic wall via the pleural membranes.
This is why a pneumothorax (air entering the pleural cavity) is dangerous — it breaks the negative pressure seal, and the lung collapses because it can no longer follow chest wall movements.
NEET loves questions on the pressure changes during breathing. Remember: during inspiration, intra-pulmonary pressure is less than atmospheric; during expiration, it is more than atmospheric. The intrapleural pressure is ALWAYS negative (sub-atmospheric) during normal breathing.
Common Mistake
Students frequently write that “the lungs expand and pull air in.” This reverses the cause and effect. The lungs are passive — they do not have muscles. It is the diaphragm and intercostal muscles that expand the thoracic cavity, and the lungs are pulled along due to the adhesive force of pleural fluid. The pressure drop happens as a consequence of lung expansion, not the other way around.
Also, do not confuse internal and external intercostal muscles. External intercostals are active during inspiration (they lift the ribs), while internal intercostals assist in forced expiration (they pull ribs downward).