A) Mechanics of inspiration:1. The thoracic cavity increases in alldirections:a) Vertically: by contraction of the diaphragmand its descent 1.5 cm.b) Transversely: by elevation of the ribs bycontraction of external intercostal musclesc) Antero-posteriorly: by elevation of the ribs,
2. Expansion of the thoracic cavityleads to ⇒ increases the negativity ofthe intrapleural pressure leading todistension of the lungs ⇒cause rushof 500 c.c. of atmospheric air into the
B) Mechanics of expiration:Quiet expiration is a passive process, it isdone by:1) Relaxation of inspiratory muscles(diaphragm and external intercostalmuscles).2) Drop of the thoracic cage by its weight .3) Elastic recoil of the lung .This pushes 500 c.c. of air out of thelung.
Definition:It is the pressure inside the pleuralsac.►It is always negative (subatmospheric)during normal resting respiratory cycle .
a) Over growth of thethoracic cage than the lung.b) Elastic recoil of the lungagainst chest wall.
a) During normal expiration ⇒ I.P.P.is -3 mmHg.b) During normal inspiration ⇒ I.P.P.is -6 mmHg.c) During forced inspiration ⇒ I.P.P.is -30 mmHg.d) During forced expiration ⇒theI.P.P. is + 40 mmHg.
1) It helps the expansion of the lungsduring inspiration.2) It helps the venous return from theextrathoracic veins.3) It helps flow of lymph fromextrathoracic lymph vessels.4) It helps flow of blood through thepulmonary vessels.5) It is a measurement of elastic recoil ofthe lung.