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Information

Author(s): RupengLi, Chang A.Liu

  1. Organogenesis is virtually complete after the 12th gestational week.
  2. Respiratory development
    1. Anatomic
      1. The lungs begin as a bud on the embryonic gut in the fourth week of gestation. Failure of separation of the lung bud from the gut later results in the formation of a tracheoesophageal fistula (TEF).
      2. The diaphragm forms during the 4th through 10th week of gestation, dividing the abdominal and thoracic cavities.
        1. If the diaphragm is not completely formed when the midgut reenters the abdomen from the umbilical pouch, the abdominal contents can enter the thorax.
        2. The presence of abdominal contents within the thorax is associated with arrested lung growth.
        3. The lungs from patients with congenital diaphragmatic hernia (CDH) have a decreased number of arterioles in the hypoplastic lung. In addition, the pulmonary arteries of both lungs are abnormally thick and reactive, resulting in increased pulmonary vascular resistance.
    2. Physiologic
      1. Lung development is generally insufficient for survival at less than the 23rd week of gestation, prior to the saccular stage of lung development when thinning of the pulmonary interstitium due to decreased collagen fiber deposition, increased cellular differentiation, and capillary development begins the capacity for gas exchange.
      2. Secretion of surfactant, which reduces alveolar wall surface tension and promotes alveolar aeration, is often inadequate until the last month of gestation.
        1. Birth before 32 weeks of gestation is associated with respiratory distress syndrome (RDS).
        2. Because glucose metabolism affects lung surfactant maturation, infants of diabetic mothers are at increased risk of RDS when prematurely born at later stages of gestation.
        3. Antenatal treatment with steroids is associated with a decrease in the incidence of RDS in prematurely born infants.
      3. After birth, the onset of breathing is stimulated by hypoxemia, hypercarbia, tactile stimulation, and a decrease in plasma prostaglandin E2. After aeration and distention of the lung, the pulmonary vascular resistance decreases, and pulmonary blood flow increases nearly 10-fold. Failure of the reduction of pulmonary vascular resistance after birth is associated with extrapulmonary shunting of blood and severe hypoxemia and is called persistent pulmonary hypertension of the newborn (PPHN).
  3. Cardiovascular development
    1. Anatomic
      1. The cardiovascular system is the first organ system to function in utero. Its formation consists of three developmental stages including tube formation, looping, and septation. Heart formation is complete by approximately 8 weeks of gestation.
      2. The primitive cardiac tube consists of the sinoatrium, the ventricle, the bulbus cordis (primitive right ventricle), and the truncus (primitive main pulmonary artery). During the second month of gestation, a heart with two parallel pumping systems develops out of this initially tubular system. During this process, various structures divide and migrate. Failure of structural maturation at this stage of development causes numerous cardiac malformations. For example:
        1. Failure of division of the sinoatrium into the two atria results in a single atrium. Improper closure results in an atrial septal defect.
        2. Failure of migration of the ventricular septum and atrioventricular valve between the primitive ventricle and the bulbus cordis results in a double-outlet left ventricle (single ventricle). Minor migrational defects result in ventriculoseptal defects.
        3. Failure of division of the truncus into the pulmonary artery and the aorta results in truncus arteriosus.
      3. The aortic arch system initially consists of six pairs of arches.
        1. The sixth arches produce the pulmonary arteries. The ductus arteriosus develops from the distal portion of the right sixth arch. Although the left proximal sixth arch usually degenerates, it can persist and form an aberrant left ductus arteriosus.
        2. Failure of regression of various portions of the aorta and arch system also can result in aberrant vessels and vascular rings. For example, failure of regression causes a double aortic arch. Regression of the left-sided but not the right-sided arches can result in a right-sided aortic arch.
    2. Physiologic
      1. Fetal circulation: After the 12th week, the circulatory system is in its final form. Oxygenated blood from the placenta passes through the umbilical vein and the ductus venosus and returns to the heart. Subsequently, 85% to 95% of fetal cardiac output bypasses the pulmonary circulation by flowing right to left through the foramen ovale and the ductus arteriosus into the aorta.
      2. At birth, umbilical placental circulation ceases with the clamping of the umbilical cord, and blood flow through the ductus venosus ceases. However, the ductus venosus often remains patent for up to a week. Also, the interruption of umbilical blood flow at birth reduces right atrial pressure and causes functional closure of the foramen ovale. Moreover, pulmonary resistance decreases as the lungs are distended and ventilated at birth, while systemic resistance increases with removal of the high-capacitance placental circulation. Constriction of the ductus arteriosus occurs with increasing PaO2. Cessation of ductus arteriosus blood flow often occurs within several hours to days in term infants but may be delayed in prematurely born or sick infants.
  4. Body composition
    1. Extracellular fluid (ECF) and total body water decrease as the fetus grows, while intracellular fluid increases with gestational age. ECF is 90% of total body weight at 28 weeks, 80% at 36 weeks, and 75% at term.
    2. After birth, a physiologic diuresis occurs, with the term infant losing 5% to 10% of ECF in the first few days of life. Premature infants may lose up to 15% of ECF.
    3. Before 32 weeks of gestation, the neonatal kidney is immature and has a relatively low glomerular filtration rate and altered tubular function. This leads to difficulties in excreting water loads and diminished capacity to reabsorb sodium and water and thereby concentrate urine. In part, this is due to incomplete glomerular development, tubule insensitivity to vasopressin, loops of Henle that have not yet penetrated into the medulla, low osmolality in the medullary interstitium, and low serum urea levels. Renal tubular function increases with postnatal age, and the concentrating ability of the kidney reaches adult levels at 6 to 12 months postnatal age.