1. Muscles Involved In Ventilation MuscleOriginInsertionAction Diaphragm Xiphoid process, inferior margin of rib cage and lumbar vertebrae Central Tendon Forms floor of thoracic cavity; pulls central tendon inferiorly during inspiration – this increases vertical length of thorax External intercostals Inferior border of superior rib Superior border of inferior rib Elevates ribs during inspiration – increasing dimensions of thorax Internal intercostals Superior border of inferior rib Inferior border of superior rib Draws adjacent ribs together during forced expiration – decreasing dimensions of thorax

2. Boyle’s Law The pressure of a gas is inversely proportional to its volume. Increasing volume of a container – decreases pressure Decreasing volume of a container – increases pressure

3. Diaphragm Central Tendon Diaphragm When relaxed, the diaphragm is dome shaped and forms the floor of the thoracic cavity When it contracts, it flattens out and pulls the central tendon inferiorly This increases the volume of the thoracic cavity and therefore reduces the pressure (Boyle’s law). This draws air into the lungs – air moves from high pressure to low pressure When the diaphragm relaxes, the central tendon recoils, bringing the diaphragm back to its resting dome shape – no antagonist muscle This decreases the volume of the thoracic cavity, which increases the pressure and pushes the air passively out of the lungs.

4. AIR

5. External Intercostal Muscles Superior Rib Inferior Rib Superior Border Inferior Border Origin: Inferior border of superior rib Insertion: Superior border of inferior rib When a muscle contracts, the point of insertion moves towards the point of origin. During inhalation, this elevates the ribs – drawing them up and out. This increases the volume of the thoracic cavity, which lowers the pressure and draws air into the lungs

6. Internal Intercostal Muscles Superior Rib Inferior Rib Superior Border Inferior Border Origin: Superior border of inferior rib Insertion: Inferior border of superior rib When a muscle contracts, the point of insertion moves towards the point of origin. During forced exhalation, this depresses the ribs – drawing them down and in. This decreases the volume of the thoracic cavity, which increases the pressure and forces air out of the lungs Normal exhalation is a passive process – the ribs fall back into place