GCSE Biology: Inspiration and Expiration

Inspiration

When breathing in, the muscles of the diaphragm contract. This pulls the diaphragm downwards, which increases the volume in the thorax. As the same time the external intercostal muscles contract. This pulls the rib cage upwards and outwards. Together, these movements increase the volume of the thorax. As the volume of the thorax increases, the pressure inside it falls below atmospheric pressure. Extra space has been made and something must come into fill it up. Air therefore rushes in along the trachea and bronchi to the lungs.

Expiration

When breathing out the muscles of the diaphragm relax. The diaphragm springs back up into its diamond shape because it is made of elastic tissue. This decreases the volume of the thorax. The external intercostal muscles also relax. The rib cage drops down again into its normal position. This also decreases the volume of the thorax. As the volume of the thorax decreases, the pressure inside it increases. Air is squeezed out through the trachea into the nose and mouth, and on out of the body.