Newborn has to contend with sudden transition from fetal life to extrauterine existence at the time of birth. In childhood, the lungs have to keep pace with the ordered growth of the body. During fetal life, the placenta helps in the gas exchanges. Therefore the fetal oxygen tension remains constant, independent of the maternal levels of oxygen. As the lungs are fluid filled, there is no air fluid interface.

By the 24th week of gestation, the surfactant precursors appear as inclusion bodies in the alveolar lining cells. Prior to 28-32 weeks of gestation the lungs have an inherent tendency to collapse. They are unable to refain any air. The ‘surfactant, a protein in the alveolar lining layer decreases the alveolar surface tension and inrparts finite elasticity to the interface. As a result less pressure is needed to distend the lungs. When the lung is inflated from a small or negligible volume such as in atelectasis or from a situation when the alveoli are filled with liquid as happens during the first breath at birth, lesser force is required to open up the alveoli if there is low surface tension at the air liquid interface and if the radius of the terminal units of the lung is adequate.

Although the surfactant can be detected in the lung exudate from the human fetuses as early as 24th week, the quantity increases greatly towards the end of the term. Deficiency of the surfactant leads to respiratory distress syndrome.

Gas transport in fetal life

Carbon dioxide tension falls from35_:5mmHg at the 10th -week of gestation to 28 mm Hg at the full term. The difference in the oxygen dissociation curves and increase in the hemoglobin concentration enable the fetus to carry out effective oxygenation of the tissues. Recently it has been found that 2,3 diphosphoglycerate (2,3 DPG) in the surrounding medium seems to increase its affinity for oxygen. Fetal hemoglobin appears to be insensitive to this 2,3 DPG, hence its affinity for oxygen is not affected by the presence of 2,3 DPG. The uptake of C0₂ also shifts the oxygen dissociation curve to the right and therefore adequate 0₂ delivery is ensured by the high tissue levels of CO_z.

Onset of respiration

The mechanisms of the onset of respiration at birth are multifactorial. Hypoxia, hypercapnia (more CO_z in blood) and sudden increase in the sensitivity of chemoreceptor due to increased sympathetic activity after the cord is clamped,

Respiratory function in the newborn

Before birth, the lungs are filled with fluid. This has to be replaced by air. Some of the fluid is extruded from the mouth and some is absorbed by the lymphatics.

Intrapleural negative pressure required for the first breath is 40 to 100 cm of water. This pressure is higher initially, because of the low compliance of the newborn lung (1.5 ml/cm H₂0 at birth). The compliance of the lungs increases in the first few hours to 6 ml/cm H₂0 and resistance to the air flow decreases. The tidal volume of a 3 kg infant is . approximately! 6 ml at about 28 breaths per minute. Resting lung volume also gradually increases in the first few hours and reaches a maximum of 80 ml within 24 hours.

Gas exchange in newborn

The normal newborn requires about 7 ml of oxygen/ minute/kg. This is almost double the 0₂ requirements of an adult on a relative weight basis.

The uptake of oxygen is a complex process involving transport across the alveolar capillary membrane. This process known as diffusion is virtually the same as in the adult in relation to the surface area. The dead space of the newborn is about 2 ml/kg with the resting tidal volume being 20 ml; 35 percent of the breath is wasted as compared to 30 percent in the adults. Persisting fetal channels and the ventilation perfusion problems in a newborn lead to increase in the right to left shunt.

Gas transport

Relative hypoxia in a newborn is corrected in 5 minutes, hypercapnia by 20 minutes and acidosis in 24 hours. The initial acidosis is partly metabolic in origin, due to the elevated blood lactate levels. The higher hemoglobin concentration in the newborn, and the shift to the left of the 0₂ dissociation curve allow the newborn baby to carry higher concentration of oxygen than in the adult.

Mechanical function throughout childhood

The total lung capacity in a newborn is 150 ml as compared to that in an adult which is around 5000 ml. With the growth of the lungs, there is multiplication of the alveoli and increase in the size of alveoli and airways. Pores of Cohn or interalveolar communications also develop, with increasing age. There is a large increase in compliance and a fall in resistance. The reciprocal of resistance, i.e., conductance rises in proportion to the increase in the lung volumes.  Increase in  minute ventilation reflects the increase in the metabolic rate. The dead space, the tidal volume and the frequency changes reflect changes in the mechanics of the lungs.

Gas transport in childhood

The rise of pH and pC02 together means that the buffer base of the blood also increases. Bicarbonate rises from 19 mEq/L at the age of 2 years to 24 mEq/L at 16 years of age. Arterial p02 is about 75 mm Hg in the newborn period and around the age of 5 years it reaches the adult level of 95 mm Hg.