Breathing

• Air is warmed, moistened and filtered as it travels through the mouth and nasal passages.
• It then passes through the trachea and one of the two bronchi [Bronchi: Singular 'bronchus'. ] into one of the lungs.
• After passing into the many bronchioles, it finally arrives into some of the millions of tiny sacs called alveoli.
• This is where gas exchange takes place - oxygen passes out of the air into the blood, and carbon dioxide passes out of the blood into the air in the alveoli.

When you inhale [Inhale: Breathe in. ]:

• the intercostal muscles contract, expanding the ribcage.
• the diaphragm contracts, pulling downwards to increase the volume of the chest.
• pressure inside the chest is lowered and air is sucked into the lungs.

When you exhale [Exhale: Breathe out. ]:

• the intercostal muscles [Intercostal muscles: Muscles between the ribs that aid breathing. ] relax, the ribcage drops inwards and downwards
• the diaphragm [Diaphragm: Sheet of muscle below the lungs. ] relaxes, moving back upwards, decreasing the volume of the chest.
• pressure inside the chest increases and air is forced out.

See the 'Exercise and Training' revision bite for effects of exercise on the respiratory system.