Cry after birth is not really the first breath of life because around 20 weeks of gestation, fetus making relatively rapid (80 to 120/min) and ineffective respiratory movement .
Fetal lung is filled with its own production. The lung fluid appears to be an ultrafiltrate of plasma , it is acidic and higher chloride content than amniotic fluid .
At birth, some of the fluid is squeezed out of the upper airways during the passage through the pelvic canal. The rest is rapidly reabsorbed through lymphatics .
Hypoxia , acidosis , cord occlusion and thermal changes played an important role for the first functional breath after birth . “From a well cushioned , dark , worm and comfortable aqueous milieu of uterus , the infant emerges out into a hostile physical environment providing a volley of sensory stimuli in the form of light , sound , cold and air currents . It is no wonder that most babies cry and yell! ” ( Meharban Singh)
As a consequence of hypoxia( biochemical stimuli) –> stimulate carotid and aortic chemoreceptors .
Clamping of cord –> sudden rise of blood pressure –> stimulate aortic baroreceptor and sympathetic nervous system .
Sudden cooling after birth –> provide respiratory drive –> receptor mostly located in the fascial skin .
By the direct effect of increase O2 and decrease Co2 –> decrease pulmonary vascular resistance –> expansion of lung volume .
“When the normal infant starts to breathe there are at least three important changes in the lungs :- they acquire a stable alveolar gas volume, lose the liquid with which they were filled in the foetal state, and increase their blood flow by five or ten times.
The formation of the stable alveolar volume depends on the properties of the surface film (surfactant) which forms in the alveoli. The factors concerned in the uptake of liquid are still uncertain. The increase
in blood flow is largely due to a decrease in pulmonary vasomotor tone brought about by an increase in
Po2 and a decrease in Pco2.
Deficiency or inactivation of surfactant leads to atelectasis and all the abnormalities encountered in
the respiratory distress syndrome. Maturity is one important factor in determining the amount of surfactant in the lungs at birth.
Underventilation for any reason causes a rise in pulmonary vascular resistance and a tendency to reverse the changes in the foetal circulation at birth,
and hence right-to-left shunting is a common complication of respiratory failure in infants.” (L. B. STRANG)
Gradually established fetal to adult type of circulation although foramen ovale and ducts arteriosus may remain open for varying period of time .