Air always moves from higher to lower pressure. Breathing works by changing the size of the chest, which changes the pressure inside the lungs.
Breathing in
The diaphragm contracts and moves down, and the external intercostal muscles lift the ribs up and out. The chest gets bigger, pressure in the lungs falls below air pressure and air flows in.
Breathing out
At rest, breathing out is passive: the muscles relax and the elastic lungs recoil. During exercise or an asthma attack, the abdominal and internal intercostal muscles push air out actively.
Accessory muscles
When breathing is hard, neck muscles such as the sternocleidomastoid and scalenes help lift the chest. Their use is a visible sign of respiratory distress.
Compliance and resistance
Compliance is how easily the lungs stretch; stiff lungs, as in fibrosis or ARDS, take more effort to fill. Resistance depends mainly on airway width, so narrow airways in asthma or COPD make airflow harder.
At the bedside
Count the respiratory rate for a full 30 to 60 seconds and watch the work of breathing: accessory muscle use, retractions and whether the patient can speak in full sentences. With a bag-valve mask you push air in with positive pressure instead, so give each breath gently over about 1 second.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).