MAVO Year 4 · Gas exchange and excretion · Section 14.4

Breathing in and out

Ventilation

Ventilation renews air in the lungs. The lungs do not actively inflate themselves like a contracting muscle. They follow movements of the chest wall and diaphragm, coupled through the pleural surfaces. Changes in thoracic volume alter pressure in the lungs, causing air to flow in or out.

Rib-cage breathing

During inhalation, the external intercostal muscles help raise the ribs and move the sternum upwards and outwards. Chest volume increases, pressure within the lungs falls below atmospheric pressure and air enters.

During quiet exhalation, inspiratory muscles relax and elastic recoil helps reduce lung and chest volume. Pressure rises and air flows out. Deeper breathing can recruit additional muscles, including neck muscles during strong inhalation and some internal intercostal muscles during forced exhalation.

Diaphragmatic breathing

When the diaphragm contracts, it moves downwards and becomes flatter. Thoracic volume increases, drawing air in. The abdominal contents are displaced and the abdominal wall moves outwards.

When the diaphragm relaxes, it rises as the lungs and surrounding tissues recoil, helping air leave. During forced exhalation, abdominal muscles contract and push the abdominal contents and diaphragm upwards. Air enters the lungs, not the abdomen; abdominal movement reflects the diaphragm's action.

Ordinary breathing

Normal breathing combines diaphragmatic movement with rib-cage movement. Their relative contribution changes with posture, activity and breathing depth. Quiet exhalation is mainly passive, while stronger breathing requires more muscular work.