During labour, as soon as the placental circulation ceases, the respiratory system must function to take over the activity of gaseous exchange from the placenta.

During labour, the oxygen tension of the fetal blood is lowered, carbon dioxide tension rises and the blood becomes acidotic.

There appear to be two phases in the respiratory efforts. The first phase requires considerable exertion to achieve expansion of the lungs with air. During this phase, the baby exhibits strong gasping movements.

The second phase is the establishment of rhythmic breathing movements. This is usually followed quickly by a lusty cry. A normal baby establishes rhythmic breathing movements within one minute after delivery.

The respiratory centre in the medulla oblongata responds to a number of subliminal stimuli, such as:

1. 
   

   ◦ stimulation through chemoreceptor reflexes from the aortic and carotid bodies due to acidosis after tying of umbilical cord

   
   
2. 
   

   ◦ cutaneous stimulation and exposure to cold

The amniotic fluid present within the respiratory tree before birth is expelled through the mouth in vertex deliveries by compression of the thorax by the birth canal. Any residual fluid is expelled through the pulmonary lymphatics.

The interruption of umbilical flow when the cord is tied results in immediate fall in blood pressure in the inferior vena cava.

The closure of the ductus arteriosus by muscular contraction of its wall increases the blood flow through the lung vessels, with subsequent increase in pressure in the left atrium.

Due to the above changes, the foramen ovale is functionally closed by apposition of the septum primum with the septum secundum as the neonate takes its first breath. Permanent closure occurs in about one year.

The diminished pulmonary vascular resistance associated with inflation of the lungs causes the direction of blood flow from left to right. Later, the ductus becomes fibrosed to form the ligamentum arteriosum.

Closure of the umbilical arteries is caused by thermal and mechanical stimuli, and change in oxygen tension. The distal part of the umbilical arteries form median umbilical ligaments and proximal portions remain open as superior vesical arteries.

Closure of the umbilical vein and ductus venosus occurs shortly after that of the umbilical arteries. After obliteration, they form ligamentum teres hepatis and ligamentum venosum respectively. See Figure 6.1.