It must be particulalry forceful because the newborns lungs are collapsed and the airways are small, offering consideralbe resistance to air movement. also, surface tension tends to hold the moist membranes of the lungs together. however, the lungs of a full-term fetus continuously secrete surfactant, which reduces surface tension. after the first powerful breath begins to expand the lungs, breathing eases. it is stimulated by increasing concentration of carbon dioxide, decreasing ph, low oxygen concentration, drop in body temperature, and mechanical stimulation during and after birth. also, in response to the stress the fetus experiences during birth, blood concentrations of epinephrine and norepinephrine rise significantly. these hormones promote normal breathing by increasing the secretion of surfactant and dilating the airways.
