6.4.5 Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostals muscles, the diaphragm and abdominal muscles

Ventilation consists of inhalation and exhalation.

Inhalation:

When a person breathes in, or inhales, the external intercostals muscles contract, making ribcage move up and out. The diaphragm contracts and flattens. This makes the thorax increase in volume; hence the pressure decreases inside the thorax. The volume of the lungs and alveoli also increase. Pressure eventually drops below atmospheric pressure, so air flows into the body through the mouth or nose, trachea, bronchi and bronchioles.

Exhalation:

When a person breathes out, or exhales, the intercostals muscles contract, making the ribcage move down and in. The abdominal muscles contract, pushing the diaphragm down and back to its original shape. This makes the volume of the thorax decrease; hence the pressure increases inside the thorax. The volume of the lungs and alveoli also decrease. Pressure eventually rises above atmospheric pressure, so air flows out of the lungs through nose or mouth to go out of the body.

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