1. Chapter 19 Inspiration and Expiration

2. Ventilation Breathing – Movement of air from outside the body into the bronchial tree and alveoli and then back out – Accomplished through inspiration (inhalation) and expiration (exhalation)

3. Inspiration Inspiration occurs when air is forced into the lungs due to changes in air pressure – When the respiratory muscles are at rest: Air pressure on the inside of the lungs and aveoli are about the same as the air pressure on the outside of the thoracic wall and no air movement occurs – However, when the diaphragm moves downwards: The thoracic cavity is enlarged which causes the interior pressure (called the intra-alveolar pressure) to fall below the outside atmospheric pressure forcing air into the lungs

4. Inspiration Expansion of thoracic cavity: – Diaphragm Consists of skeletal muscle fibers – When muscles are stimulated to contract by phrenic nerves the diaphragm moves down and the thoracic cavity enlarges – The thoracic cavity can be enlarged further when the external intercostal muscles (along with other thoracic muscles) contract which raises the ribs and elevates the sternum

5. Inspiration Expansion of thoracic cavity: – External intercostal muscles Located between the ribs When contracted (along with contraction of other thoracic muscles) the ribs are raised and the sternum is elevated which further enlarges the thoracic cavity – Number of muscles involved and the force of contraction determines the degree to which the thoracic cavity is expanded

6. Inspiration Expansion of lungs – Pleural membranes Parietal pleura – covers inner wall of the thoracic cavity Visceral pleura – covers the surface of the lung Held together by: – decreasing pressure in the intrapleural space (decreased pressure is caused by separation of the membranes) – Attraction of water molecules in the serous fluid of the intrapleural space to the membranes themselves So, when the parietal pleura moves upward due to expansion of thoracic cavity, the visceral pleura follows helping to expand the lungs in all directions

7. Inspiration Expansion of lungs – Surfactant Mixture of lipoproteins that is continuously secreted into alveolar air spaces by specific alveolar cells Decreases the tendency of the aveoli to collapse due to surface tension created by the attraction of water molecules within the alveoli to themselves Makes it easier for inflation of alveoli to occur

8. Inspiration Compliance – A measure of the ease at which the lungs can expand due to changes in pressure – In a normal lung, compliance decreases as lung volume increases – Compliance may also be decreased by conditions such as obstruction of air passages or destroyed lung tissue

9. Expiration Normal resting expiration – Passive process caused by an increase in intra- alveolar pressure due to: Recoil of elastic tissues within the lungs when the external intercostal muscles and diaphragm relax (this returns the lungs to their original shape and reduces intrapleural pressure) Recoil of abdominal organs that had been compressed by the contraction of the diaphragm (this pushes the diaphragm up) Shrinking alveoli due to increased surface tension

10. Expiration Active expiration – Contraction of internal intercostal muscles pul the ribs and the sternum downward and inward (which increases intra-alveolar pressure) – Abdominal wall muscles can increase pressure in the abdominal cavity forcing the diaphragm higher (further decreases intra-alveolar pressure)