Mechanism of Breathing: Inhalation and Exhalation Steps

The mechanism of breathing is the physical process by which air is drawn into the lungs and then pushed out. Breathing itself does not exchange gases; it simply moves air so that the alveoli always contain fresh oxygen-rich air.

This mechanism matters because it works automatically about 12 to 16 times per minute at rest, yet you can also control it to speak, sing or swim. Understanding inhalation and exhalation explains why chest injuries and diaphragm problems can be life-threatening.

Key Concepts

Inhalation (inspiration) is the active intake of air. Exhalation (expiration) is the release of air, usually passive at rest. The diaphragm is the sheet muscle separating chest and abdomen. The external intercostal muscles lift the ribs. According to Boyle law, when the volume of a closed space increases, its pressure falls. The thoracic cavity changes volume, air pressure inside the lungs falls below atmospheric pressure, and air rushes in.

• Tidal breathing: quiet normal breathing at rest.
• Atmospheric pressure: about 760 mmHg at sea level, the reference for lung pressures.
• Intra-pulmonary pressure: pressure inside alveoli, changing by 1 to 3 mmHg during quiet breathing.

How It Works

1. For inhalation, the diaphragm contracts and flattens downward.
2. External intercostal muscles contract, raising ribs upward and outward.
3. Thoracic volume increases in all directions.
4. Lung pressure falls slightly below atmospheric pressure.
5. Air flows in through nose and trachea until pressures equalise, filling alveoli.
6. For exhalation, the diaphragm and external intercostals relax.
7. Ribs fall, thoracic volume decreases, lung pressure rises slightly above atmospheric.
8. Air is pushed out passively by elastic recoil of lungs.
9. During forceful exhalation, as in coughing, abdominal and internal intercostal muscles contract actively.

Worked Example

At rest, each quiet breath moves about 500 mL of air, called tidal volume. At 14 breaths per minute, minute ventilation is 500 multiplied by 14, which equals 7,000 mL or 7 litres per minute. Not all reaches alveoli; about 150 mL stays in the conducting tract as dead space. So alveolar ventilation is (500 minus 150) multiplied by 14, which equals 4,900 mL per minute. During running, tidal volume may rise to 2,000 mL and rate to 30 per minute, giving 60 litres per minute to meet muscle demand. These numbers show how changing volume and rate controls air supply.

Common Misconceptions

• Misconception: Lungs pull air in by themselves. Correction: Lungs have no muscle; chest muscles change volume and air follows the pressure difference.
• Misconception: Exhalation always needs effort. Correction: Quiet exhalation is passive, powered by elastic recoil when inspiratory muscles relax.
• Misconception: We empty our lungs completely each breath. Correction: A large residual volume always remains, preventing alveolar collapse.

Real-World Applications

Ventilators in hospitals mimic this pressure mechanism for patients who cannot breathe. CPR uses chest compressions to move air and blood. Singers and athletes train diaphragmatic breathing for better control and endurance. Doctors measure breathing rate as a vital sign because abnormal patterns signal lung or brain problems.

Breathing is a pump driven by the diaphragm and ribs: enlarging the chest draws air in, and shrinking it pushes air out, repeating tirelessly to keep alveolar air fresh.