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)