main muscle of quiet breathing

• Intrapleural pressure is the pressure of the fluid in the narrow space between the the parietal pleura and the visceral pleura 
•  Intrapulmonary pressure is the pressure inside the lung alveoli . 
• When the glottis is open and no air is flowing into or out of the lungs , the pressure in all parts of the respiratory tree , all the way to the alveoli are all equal to atmospheric pressure . 
• Inrapleural pressure is sub atmospheric in the normal person and is usually about 2.5 mmHg ( relative to atmospheric ) at the base of lung . 
• At the beginning of inspiration because of the expansion of the chest wall and increase of the intrathoracic volume it drops to around -6 mm Hg. 
•  As the chest wall moves out it takes with it the lung and so they are also pulled into a more expanded position 
• This causes the intrapulmonary pressure to become slightly negative and air flows into the lung down a pressure gradient until the intrapulmonary pressure is equal to atmospheric pressure . 
• At the end of inspiration , the lung recoil begins to pull the chest back to the expiratory position.where the recoil pressures of the lung and the chest wall balance . The intrapulmonary pressure becomes slightly positive and air flows out of the lungs until the pressure inside the lungs becomes equal to that outside the lungs 
• (Expiration during the quiet breathing is passive in the sense that no muscle , which decrease intrathoracic volume , contract . However there is some contraction of the inspiratory muscle in the early part of expiration. This contraction exerts a braking action on the recoil forces and slows expiration) 
• During deep breathing intrapeulral pressure is reduced to values as low as – 30 mmHg.producing corresponding greater degrees of lung inflation. When the ventilation is increased , the extent of the deflation is also increased by active contraction of expiratory muscle that decrease intrathoracic volume. 

Labels: Physiology SEQ, RS, RS Revision, RS SEQs