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29.3
The Newborn
    LEARNING OUTCOMES

    After reading this section, you should be able to:

  1. Discuss the respiratory, cardiovascular, and digestive changes that occur in the newborn.
  2. Explain the significance of the Apgar score.
  3. List causes of congenital disorders.

The newborn baby, or neonate, immediately experiences several dramatic changes due to being separated from the maternal circulation and transferred from a fluid to a gaseous environment.

Respiratory and Cardiovascular Changes

The large, forced gasps of air taken in when the infant cries at the time of delivery help inflate the lungs. This initial inflation of the lungs causes important changes in the cardiovascular system (figure 29.20). Before birth, very little blood flows through the fetus's pulmonary arteries to the lungs and back to the heart through the pulmonary veins. As a result, the left atrium has little blood and very low blood pressure. Therefore, blood flows from the right atrium, through the foramen ovale, and into the left atrium.

 

FUNDAMENTAL Figure

PROCESS FIGURE 29.20
Circulatory Changes at Birth(a) Circulatory conditions in the fetus. (b) Circulatory changes that occur at birth.

Expansion of the lungs reduces the resistance to blood flow through the lungs, resulting in increased blood flow through the pulmonary arteries. Consequently, more blood flows from the right atrium to the right ventricle and into the pulmonary arteries, and less blood flows from the right atrium through the foramen ovale to the left atrium. In addition, more blood returns from the lungs through the pulmonary veins to the left atrium, which increases the pressure in the left atrium. The increased left atrial pressure and decreased right atrial pressure, resulting from decreased pulmonary resistance, force blood against the septum primum, causing the foramen ovale to close. This action functionally completes the separation of the heart into two pumps: the right side of the heart and the left side of the heart. The closed foramen ovale becomes the fossa ovalis.

Within 1 or 2 days after birth, the ductus arteriosus, which connects the pulmonary trunk to the aorta and allows blood to flow from the pulmonary trunk to the systemic circulation, closes off. This closure occurs because of the sphincterlike constriction of the artery and is probably stimulated by local changes in blood pressure and blood oxygen. Once closed, the ductus arteriosus is replaced by connective tissue and is known as the ligamentum arteriosum. If the ductus arteriosus does not close completely, it is said to be patent. This is a serious birth defect, resulting in marked elevation in pulmonary blood pressure because blood flows from the left ventricle to the aorta, through the ductus arteriosus to the pulmonary arteries. If not corrected, it can lead to irreversible degenerative changes in the heart and lungs.

Before birth, the fetal blood passes to the placenta through umbilical arteries from the internal iliac arteries and returns through an umbilical vein. The blood passes through the liver via the ductus venosus, which joins the inferior vena cava. At birth, when the umbilical cord is tied and cut, no more blood flows through the umbilical vein and arteries, and they degenerate. The remnant of the umbilical vein becomes the ligamentum teres, or round ligament, of the liver, and the ductus venosus becomes the ligamentum venosum. The remnants of the umbilical arteries become the cords of the umbilical arteries.

Digestive Changes

When a baby is born, it is suddenly separated from its source of nutrients, the maternal circulation. Because of this separation and the stress of birth and new life, the neonate usually loses 5–10% of its total body weight during the first few days of life.

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Although the digestive system of the fetus becomes somewhat functional late in development, it is still very immature, compared with that of an adult. Late in gestation, the fetus swallows amniotic fluid from time to time. Shortly after birth, this swallowed fluid plus intestinal cells, intestinal mucus, and bile pass from the digestive tract as a greenish anal discharge called meconium (mē-kō′nē-ŭm).

The pH of the stomach at birth is nearly neutral because of the swallowed basic amniotic fluid. Within the first 8 hours of life, gastric acid secretion increases, causing the stomach pH to decrease. Maximum acidity is reached at 4–10 days, and the pH gradually increases for the next 10–30 days.

The neonatal liver is also functionally immature. It lacks adequate amounts of the enzyme required to produce bilirubin. This enzyme system usually develops within 2 weeks after birth in a healthy neonate; however, because it is not fully developed at birth, some full-term babies temporarily develop jaundice, characterized by elevated blood levels of bilirubin and a slightly yellowish cast to the skin or whites of the eye. Jaundice is also common in premature babies.

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The newborn digestive system is capable of digesting lactose (milk sugar) from the time of birth. The pancreatic secretions are sufficiently mature for a milk diet, but the digestive system only gradually develops the ability to digest more solid foods over the first year or two; therefore, new foods should be introduced gradually during the first 2 years. Parents are also advised to introduce only one new food at a time so that, if an allergic reaction occurs, the cause is more easily determined.

Amylase secretion by the salivary glands and the pancreas remains low until after the first year. Lactase activity in the small intestine is high at birth but declines during infancy, although the levels still exceed those in adults. In many adults, lactase activity is lost, and an intolerance for milk develops (see chapter 24).

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Apgar Scores

A newborn baby is usually evaluated soon after birth using a physiological assessment tool known as the Apgar score. The Apgar test, named for Virginia Apgar, the physician who developed it, evaluates the neonate's appearance, pulse, grimace, activity, and respiratory effort. Each of these characteristics is rated on a scale of 0–2, with 2 denoting normal function; 1, reduced function; and 0, seriously impaired function. The total Apgar score is the sum of the scores from the five characteristics, ranging from 0 to 10 (table 29.3). A total Apgar score of 8–10 at 1–5 minutes after birth is considered normal. Other systems that assess neonatal growth and development, including general external appearance and neurological development, also exist.

TABLE 29.3
Apgar Rating Scales
Physiological Conditions 0 1 2
Appearance (skin color) White or blue Limbs blue, body pink Pink
Pulse (rate) No pulse 100 bpm >100 bpm
Grimace (reflexive grimace initiated by stimulating the plantar surface of the foot) No response Facial grimaces, slight body movement Facial grimaces, extensive body movement
Activity (muscle tone) No movement, muscles flaccid Limbs partially flexed, little movement, poor muscle tone Active movement, good muscle tone
Respiratory effort (amount of breathing) No breathing Slow, irregular breathing Good, regular breathing; strong cry
Congenital Disorders

The term congenital means “present at birth,” and congenital disorders are abnormalities commonly referred to as birth defects. Of all congenital disorders, approximately 70% are of unknown cause, 15% have a known genetic cause, and the remaining 15% result from environmental causes or a combination of environmental and genetic factors. Environmental causes damage the fetus during development. Environmental agents that cause congenital disorders are called teratogens (ter′ă-tō-jenz). For example, fetal alcohol syndrome results when a pregnant woman drinks alcohol, which crosses the placenta and damages the fetus. The baby is born with a smaller-than-normal head, mental retardation, and possibly other defects. Researchers are working to identify various teratogens so that women can avoid known teratogens and reduce the risk for congenital disorders.

ASSESS YOUR PROGRESS
  1. What changes occur in the newborn's cardiovascular system shortly after birth? What do each of the following fetal structures become: foramen ovale, ductus arteriosus, umbilical vein, and ductus venosus?

  2. What changes take place in the newborn's digestive system shortly after birth?

  3. What does the Apgar score measure?

  4. What are congenital disorders? What are some causes of these disorders?

  5. What is a teratogen? Give an example.