MECHANICS AND MECHANISM OF RESPIRATION
The respiratory muscles are the skeletal muscles and do not possess any inherent rhythmicity Their contraction and relaxtion depend on the motor impulses that are being discharged from the respiratory centres situated in the medulla -oblongata of the brain
There are two centres in medulla oblongata inspiratory centre and expiratory centre inspiratory centres rhythmically discharge the impulses which are carried by phrenic and intercostal nerves They result in contraction of their respective muscles Expiratory centre inhibits inhibitory centres and thereby bring about the relaxation of the respiratory muscles
Lungs of the foetus before birth in the mothers womb are solid The pleural membrane is almost attached to parietal membrane and the pressure in the pleural cavity (intrapleural pressure ) is zero Immediately after the start of parturition i.e the moment the head of the baby comes out of the mothers womb the change in temperature is felt in the medulla oblongata inspiratory centres of medulla oblongata then send impulses to the diaphragm and intercostal muscles through phrenic nerves and intercostal nerves respectively As a result there occurs contraction of diaphragm and intercostal muscles and the space in the pleural cavity suddenly increases this causes a negative pressure in the pleural cavity Because of this negative intrapleural pressure the air is sucked through the mouth and nose and that is how the baby takes its first breath
Expiratory Centres then act to inhibit the inhibitory centre The diaphragm and the intercostal muscles get relaxed This brings the intrapleural pressure back to zero and the air is expelled out Normal rate of respiration in adults is 14-18 per min in children it is higher and in infants it is still higher
The inspiration and expiration continue by the contraction and relaxation of respiratory muscles respectivity During normal inspiration the pressure falls to about -2mmHg to -5mmHg However during forced inspiration pressure falls to -40 mmHg to -50 mmHg the fall in intrapleural pressure causes an equivalent rise in intrapulmonary pressure because of the suction of air from the atmosphere
Besides respiratory muscles in the forced inspiration sternocleidomastoid scalenei mylohyoid and platysma muscles also contract similarly in forced expiration abdominal muscles also contract to increase the abdominal pressure and thereby pushing diaphragm upwards it is worth noting that woman s respiration is thoracico abdominal as the foetus has to occupy a part of the abdomen whereas in males respiration is abdomino thoracic
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