Anatomy and Physiology / Unit 9: The Respiratory and Digestive Systems
9.2 Mechanics of Breathing
Notes
Pulmonary ventilation (breathing) depends on pressure changes explained by Boyle's law: at constant temperature, volume and pressure are inversely related. Air moves from high pressure to low pressure.
- Inspiration (active): the diaphragm contracts and flattens and the external intercostal muscles lift the ribs. Thoracic volume increases, so lung pressure falls below atmospheric pressure and air flows in.
- Expiration (quiet breathing is passive): muscles relax, lungs recoil, thoracic volume decreases, pressure rises above atmospheric, and air flows out. Forced expiration uses internal intercostals and abdominal muscles.
- A pneumothorax (air in the pleural cavity) breaks the pleural seal and the lung collapses.
- Lung volumes: tidal volume (about 500 mL per quiet breath), inspiratory reserve volume, expiratory reserve volume and residual volume (air that stays so lungs do not collapse). Vital capacity is the maximum amount that can be exhaled after a maximal inhalation.
- Asthma: airways narrow (bronchoconstriction) and inflame. COPD (emphysema, chronic bronchitis) is usually from smoking.
Practice
During inspiration, the diaphragm:
A flatter diaphragm increases chest volume, which lowers the pressure.
According to Boyle's law, when the volume of the chest cavity increases, the pressure inside the lungs:
Pressure and volume are inversely related.
A stab wound lets air into the pleural space and one lung collapses. This condition is:
Air in the pleural cavity destroys the pressure difference that holds the lung expanded.
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