1.       Driving pressure- the difference in pressure from one point in a tube to another.

·         This pressure difference will produce flow so long as the tube is open or patent.

·         If mouth pressure is 760 mm Hg and alveolar pressure is 757 mm, then there is a driving pressure of 3 mm Hg and gas will be inspired or inhaled into the lungs.

 

2.       Transairway or Transrespiratory pressure (pta)

·         The difference in pressure between  the alveoli (PA) and the mouth

·         P ta = P A - P m (text calls pressure at mouth the airway opening pressure or Pao) 

·         During normal quiet breathing:

-          3 mm Hg = 757 mm Hg - 760 mm Hg (during inspiration)

·         3 mm Hg = 763 mm Hg - 760 mm Hg (during exhalation)

·         3 and -3 represent positive and negative pressures (pressures more or less than atmospheric)

·         These pressure changes in the alveoli occur as a result of diaphragmatic movement. (Boyles Law--increasing volume decreases pressure and vice versa)

 

3.       Transpulmonary Pressure (PL)

·         The difference in pressure between the alveolus and the pleural space

·         PL  = PA - Ppl.

·         -5 mm Hg = 758 - 753 (during quiet inspiration)

·         -5 mm Hg = 763 - 758 (during quiet expiration)

·         Notice that pleural pressure is always negative during the entire normal respiratory cycle

·         The transpulmonary pressure is caused by the tendency of the lung to compress (like a balloon which is partially inflated)

 

4.       Transthoracic pressure (Pw)

·         The difference between atmospheric pressure at chest wall and pleural pressure

·         Pw  = Patm - Ppl.

·         7 mm Hg = 760 - 753 (during quiet inspiration)

·         2 =  760 - 758 ( during quiet expiration)

·         Pleural pressure only greater than atmospheric pressure during a forced exhalation or cough or with mechanical ventilation

·         Opening from the chest wall to the pleural space will cause air to enter pleural space (pneumothorax)