Respiratory Physiology: Mechanics of Breathing

Respiratory Physiology: Mechanics of Breathing Week 4.6  Overview Breathing (Pulmonary Ventilation) consists of two phases ○ Inspiration - air flows into the lungs ○ Expiration - air flows out of the lungs Respiratory pressure is always given rela...

Respiratory Physiology: Mechanics of Breathing Week 4.6  Overview Breathing (Pulmonary Ventilation) consists of two phases ○ Inspiration - air flows into the lungs ○ Expiration - air flows out of the lungs Respiratory pressure is always given relative to atmospheric pressure ○ 760 mmHg at sea level Intrapulmonary Pressure - pressure within the alveoli, rises and falls with the phases of breathing ○ ALWAYS equalizes itself with atmospheric pressure outside the body Intrapleural Pressure - pressure within the pleural cavity ○ ALWAYS 4 mmHg less the Intrapulmonary Pressure  Boyle’s Law Volume changes lead to pressure changes which leads to the flow of gases to equalize the pressure Under a constant temperature this relationship is seen in the formula… P1V1=P2V2 ○ P is the pressure of the gas in mmHg ○ V is the volume in mm3 If the volume is LARGE, gas molecules will be far apart and pressure will be LOW If the volume is SMALL, gas molecules will be close together and pressure will be HIGH  Inspiration Diaphragm and external intercostal muscles become activated ○ Diaphragm contracts moving inferiorly and flattens, vertical diameter of thoracic cage increases ○ External intercostals contract moving sternum forward, ribs are moved outward expanding diameter of thorax Intrapulmonary pressure drops relative to atmospheric pressure and air rushes into the lungs until the two pressures are equalized  Expiration Passive process that depends on the lungs natural elasticity to occur Relaxation of diaphragm and intercostals causes lungs to recoil dropping lung volume Alveoli compress causing intrapulmonary pressure to rise above atmospheric pressure Gas flows out of the lungs to match the change in pressure  Lung Compliance The ease in which lungs can be expanded (distensibility) Can be diminished if… ○ Reduction of the natural resilience of the lungs, fibrosis for example ○ Blockage of the bronchi or any part of the respiratory pathway ○ Impairment of the flexibility of the thoracic cage Assessed by measuring the increase in intrapulmonary pressure ○ Greater the volume INCREASE, the greater the compliance  Respiratory (Lung) Volume Tidal Volume (TV) - The air that flows into and out of the lungs during normal breathing ○ About 500 ml in normal humans Inspiratory Reserve Volume (IRV) - amount of air that can be inspired forcibly beyond the tidal volume ○ 2100 - 3200 ml in normal humans Expiratory Reserve Volume (ERV) - amount of air that can evacuated from the lungs beyond tidal volume ○ 1000 - 1200 ml in normal humans Residual Volume (RV) - amount of air that remains in the lungs preventing collapse ○ 1200ml in normal humans  Respiratory Capacities Inspiratory Capacity (IC) - total amount of air that can be inspired after a tidal expiration ○ TV + IRV Functional Residual Capacity (FRC) - amount of air remaining in the lungs after tidal expiration ○ RV + ERV Vital Capacity (VC) - total amount of exchangeable air ○ TV + IRV + ERV Total Lung Capacity (TLC) - complete capacity of the lungs normally around 6000 ml ○ IRV + TV + ERV + Rv  Dead Space Anatomical Dead Space - Inspired air that fills the conducting respiratory passageway and never contributes to gas exchange in the alveoli ○ 150ml in average adults Alveolar Dead Space - alveoli become nonfunctional and cease to act in gas exchange Total Dead Space = Anatomical Dead Space + Alveolar Dead Space  Pulmonary Function Tests Minute Respiratory Volume (MRV) - total amount of gas flow into and out of the respiratory tract in 1 min ○ Avg. 6 L/min or 12 breaths per minute Spirometers are common tools used to measure expiration and inspiration ○ Helps identify restrictive vs obstructive respiratory disorders  Nonrespiratory Air Movements Cough - Deep inspiration, glottis closes, and air forced superiorly from lungs against glottis for sudden opening and dislodgement from respiratory track Sneeze - Same as cough but air forced through nasal cavities Cry/Laugh - Inspiration followed by release of air in short bursts, emotionally induced Hiccup -Sudden inspiration resulting in spasms of the diaphragm Yawn - Deep inspiration with open jaws ventilating all alveoli