| Chapter 5: | Fetal Breathing and Lung Fluid |   |
The prenatal lungs do not function as gas exchange organs, but they do serve important purposes:
- The lung is a primary source of amniotic fluids.
- Lungs act as reservoirs of carbohydrates needed for the organism's energy.
- The produce surfactant beginning at about 24 weeks
Fetal breathing movements, which appear to be essential for normal development, begin around 12 weeks gestation. Factors that can affect fetal breathing movements include:
- maternal cigarette smoking, which stops fetal breathing for many hours.
- maternal consumption of alcohol or drugs
- declines in 02, C02, or glucose all decrease fetal breathing as do prostaglandins stress;
- stress
- prostoglandins
Arousal of the fetus appears to be more related to fetal breathing movements than central nervous system or chemoreceptor stimulation. When fetal breathing movements are absent the cause is related to chromosomal and other abnormalities, or death. The fetus has breathing movements approximately 30% of the time during the last 10 weeks of gestation.
In utero, the lungs are filled with fluid, and fetal breathing movements exchange this fluid with amniotic fluid. Diagnostic testing of lung maturation via amniocentesis is made possible as a result of this fluid exchange. When Type II pneumocytes mature, they secrete surfactant into the lung and amniotic fluid.
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Normal fetal development requires the presence of adequate amounts of lung and amniotic fluid. A diminished amount of either can result in a hypoplastic lung. Hypoplasia is a decrease in either lung weight or volume at birth. A decrease in alveolar number or an increase in alveolar size also can occur. An absence of fetal breathing movements or a lack of adequate space for lung growth may also cause hypoplasia.
The presence and chemical composition of surfactant can be tested in the amniotic fluid samples. Caution needs to be taken while obtaining these samples via amniocentesis because at 14-17 weeks, amniocentesis may reduce birth weight and lung volume; while at 22-25 weeks, amniocentesis may cause a decrease in the size and number of alveolar number.
Lung fluid, which contains glucose and other carbohydrates, acts as a storage reservoir that can be utilized after delivery. When the lung fluid is absorbed at birth, the materials are made available for use by the newborn's entire body. After delivery, the flow of fluid from lungs into the interstitium and lymphatics is facilitated by increased alveolar pore size.
During lung development, collagen is the dominant connective tissue in airways, blood vessels, and nonrespiratory components of the lung. While collagen fibers appear disorderly in actively branching airways, they are orderly in formed airways. Elastin, not collagen, is the dominant connective tissue in lung parenchyma.
Elastin first appears at 20 to 25 weeks gestation. Neither collagen nor elastin are particularly prominent at birth. However, the amount of elastin increases rapidly within 6 months after birth, helping to explain the easy rupture and decreased elastic recoil of the newborn. As the amount of elastin increases, the lung becomes more elastic.
A hypoplastic lung, one that has defective or incomplete development, is generally smaller than normal. Lung size is assessed via lung weight, volume, or DNA. However, lung weight is considered a poor indicator of development because most problems increase lung weight. When a lung weight-body ratio is used for assessment, a ratio <0.015 in a fetus less than 28 weeks old or a ratio <0.012 in an older fetus, is considered hypoplastic.
A better indicator of lung size is a measurement of inflated lung volume, because lung volume is unaffected by most fetal lung diseases. The correlation between crown-rump length and lung volume over the last half of gestation is excellent. Lung volume can therefore be predicted from crown-rump length, with lungs less than 69% of predicted volume being considered hypoplastic. Using lung DNA to reveal the number of cells present is another technique for assessing lung size. If lung DNA is <100 mg/kg body weight, the lungs are considered hypoplastic.