The respiratory system is crucial in determining whether a baby is mature enough to survive. The fetus is like a deep sea diver, immersed in fluid, and completely dependent on diffusion of maternal oxygen across the placenta. Its developing lungs which are filled with liquid are not needed during prenatal life and there is very little blood passing through them-enough only to keep them alive and developing.
However, even a nonfunctioning system has to be exercised in order to develop and it has been observed by ultrasound that the fetus makes breathing movements as early as four months (Niswander, 1976). Most pregnant women have also felt a rhythmic tapping which means that the fetus has hiccups. This probably helps to exercise the diaphragm and chest muscles so they will be functional at birth (Crelin, 1973).
The respiratory system starts developing at about five weeks in the embryo. The trachea forms first, then the bronchi and finally during the last trimester the tiny alveoli begin to develop . By twenty-six to twenty eight weeks, capillaries have reached out to the alveoli and the structural development of the lungs is practically complete.
One more thing is needed though before the baby can breathe on its own. At the sixth or seventh month, so-called Type II cells in the alveoli start producing a chemical called lecithin whose function is to keep the thin walls of the alveoli from collapsing after each breath. The reason they tend to cave in is because of surface tension pulling on the film of fluid which coats the inside of each alveolus. Lecithin is a surfactant (surface acting agent) which behaves like a detergent in reducing the surface tension of this fluid so our alveoli won’t collapse when we exhale. It also reduces the muscular effort needed to draw air into our lungs.
(I would recommend obtaining the article from Scientific American called ”The Lung of the Newborn Infant” by Mary Ellen Avery, Nai-San Wang and H. William Taeusch, Jr., April, 1973. It has good drawings and descriptions of the alveoli and Type II cells.)
When a baby is born, vaginal pressure squeezes much of the fluid out of its lungs and some air moves in to take its place. (The rest of the fluid is absorbed by the blood vessels and lymphatics in the lungs.) The baby’s first few breaths are the hardest it will ever take in its life. They require a pressure that is ten to fifteen times that of later breaths because the baby has to inflate all of its alveoli at once (Avery et al., 1973).
With subsequent breaths forty percent of the air remains as residual air in the open alveoli and breathing becomes much easier. A premature baby who does not have enough surfactant in its lungs will continue to have to exert enormous effort with later breaths because all its alveoli will continue to collapse with each exhalation and must be totally opened again with each new breath.
This condition called Respiratory Distress Syndrome (RDS) is, of course, very serious; mortality rates may vary from thirty to sixty percent (Evans and Glass, 1976). It is also known as Hyaline Membrane Disease because serum and cell debris leak into the alveoli and bronchioles and coat them with a glassy covering which makes breathing even harder. The longer a baby can survive, the more of a chance it has of making surfactant although there is always the probability of brain damage due to lack of oxygen in the brain (hypoxia).
The best cure is prevention of premature birth if possible. Doctors now have a test which enables them to tell if the fetus’s lungs are mature enough to breathe air. By amniocentesis they take a sample of amniotic fluid and test it for both the presence of lecithin and another chemical called sphingomyelin. If the L/S ratio is at least 2.0 it is a good indication that the lungs are producing enough lecithin for survival. If the ratio is lower, then everything possible is done to ensure that the mother will not give birth yet.
The respiratory system does not finish growing at birth in contrast, for instance, to the skeletal muscular system where all the fibers we will have as adults have been produced by the fourth fetal month (Crelin, 1973). At birth each of our lungs has about 25 million alveoli and this increases to about 300 million by the time we are adults (Crelin, 1973). Flattening out all our alveoli and bronchial tubes (if we were so inclined) would give a surface area of about eighty square meters, the same as a tennis court (Clements, 1962).