Mechanism Of Breathing
The Basic Idea: Suction and Squeeze
Your lungs cannot pull air in by themselves — they have no muscles of their own and behave like passive elastic bags. Think of a syringe with no needle: pulling the plunger back increases the volume inside the barrel, the pressure inside drops below the pressure outside, and air rushes in to equalise it. Pushing the plunger in does the reverse. Your chest cavity works the same way — the diaphragm and the muscles between the ribs act as the plunger, and the lungs are the balloon pulled along with them.
Volume Change Drives Pressure Change
Breathing follows a simple physical rule: for a fixed amount of gas at a constant temperature, pressure and volume are inversely related — increase the volume of the thoracic cavity and the pressure inside falls; decrease it and the pressure rises. Air always flows from higher pressure to lower pressure.
Inspiration (Inhalation)
Inspiration is an active process. Two sets of muscles contract together:
- The diaphragm, a dome-shaped sheet of muscle separating the chest from the abdomen, flattens and moves downward.
- The external intercostal muscles, between the ribs, pull the rib cage upward and outward.
Together these increase the volume of the thoracic cavity in every direction.
The lungs are held against the inner chest wall by a thin layer of pleural fluid, so as the thoracic cavity expands, the lungs are pulled open and expand passively along with it.
As the cavity expands, pressure inside the alveoli drops to roughly 1 mmHg below atmospheric pressure, and air flows in until the pressures equalise.
Expiration (Exhalation)
Quiet, normal expiration is passive — the diaphragm and external intercostals simply relax. The diaphragm domes back upward, the ribs fall back under gravity and elastic recoil, thoracic volume decreases, alveolar pressure rises to about 1 mmHg above atmospheric, and air flows out.
During forced expiration — blowing out a candle, coughing, or heavy exercise — the internal intercostal muscles and abdominal muscles contract actively to push the diaphragm up harder and pull the ribs down faster.
A Common Misconception
Many students imagine the lungs actively expand to suck air in. It is the opposite: the expanding chest wall stretches the lungs open. If the chest wall is punctured (a pneumothorax), the lung collapses, because the negative pressure that was holding it open is lost — the lung itself has no power to inflate on its own.
Do not confuse the pressure in the space between lung and chest wall (always negative during normal breathing) with the pressure inside the alveoli, which oscillates between slightly negative during inspiration and slightly positive during expiration. The first keeps the lungs stuck to the chest wall; the second is what actually drives airflow.
One-line summary: breathing is a pressure-driven flow caused by volume changes of the thoracic cavity, produced by the diaphragm and intercostal muscles — the lungs themselves are passive passengers.
Students searching for "Mechanism Of Breathing class 11 biology" or "Mechanism Of Breathing: definition and examples" will find this topic sits squarely inside the Breathing and Exchange of Gases unit of the NCERT/CBSE Class 11 Biology syllabus. It is also a recurring favourite in "Mechanism Of Breathing important questions" lists compiled for NEET and state-level medical/CET aspirants, since Breathing and Exchange of Gases carries real weightage in competitive Biology papers.